Chapter 1: Chemical Reactions and Equations

• Most of the substance around us undergoes various changes. Some of these changes are temporary with no new substance being formed. They are called physical changes.

Example: Water changes to steam on boiling but no new substance is formed(Even though steam and water look different when they are made to react with a piece of Na, they react the same way and give the exact same products). This involves only a change in state (liquid to vapour).

• A substance is said to undergo a chemical change when the chemical properties of a substance alter. As a result, there is either formation or breaking of atomic bonds at the molecular level. Some characteristics of a chemical change are:
• New substances are produced during a chemical reaction.
• Changes in energy are involved.
• During the reaction, there occurs a change in mass.
• There is a permanent alteration.

Examples:

• Cooking of food
• Rusting of iron
• Heating of Lead nitrate
• Souring of milk
• Ripening of fruit.

Chemical Equation

Writing Chemical Equations

• In a chemical reaction, the reactants are written on the left hand side and the products on the right hand side of the equation.
• An arrow (→) pointing towards the products is inserted between the reactants and the products. It also represents the direction of the reaction.
• A single arrow (→) indicates the direction in which the reaction proceeds.
• A double arrow indicates a reversible reaction, i.e. the products recombine to form the reactants.
• A plus sign (+) is inserted between two or more reactants or products formed.
• If reactions are carried out under specific conditions of temperature, pressure, catalyst etc., then these conditions are mentioned on the arrow.
• The chemical equation can be made more informative by mentioning the physical states of the reactants and products.
• If gas is liberated as a product, then it is represented by an arrow pointing upwards (↑). If the product formed is in the form of a precipitate, it is represented by an arrow pointing downwards (↓).

Balancing the Chemical Equations

• In a balanced chemical equation, the total number of atoms of each element of the reactants on the left hand side of the equation is equal to the number of atoms of the products formed on the right hand side of the equation.
• The total mass of the reactants is equal to the total mass of the products or the number of atoms of each element before the reaction and after the reaction is equal.

Steps Involved in Balancing a Chemical Equation

Consider the chemical reaction between magnesium and oxygen to understand the steps involved in balancing a chemical equation.

Step 1

Let us first write the word equation for this reaction.

Magnesium + Oxygen → Magnesium oxide

Step 2

Write the chemical equation for the reaction between magnesium and oxygen.

Mg + O₂ → MgO

Step 3

Count the number atoms of an element occurring on both L.H.S. and R.H.S. in this equation.

Mg + O2 → MgO

Component	Reactant	Product
Magnesium	1	1
Oxygen	2	1

This is an unbalanced equation.

Step 4

To balance a chemical equation, first draw boxes around each formula. Do not change anything inside the boxes while balancing the equation.

• Choose a reactant or a product which has the maximum number of atoms in it. In that compound, select the element which has the maximum number of atoms. In this equation we shall select MgO i.e. magnesium oxide and the element oxygen in it.

To balance the oxygen atoms, let us multiply magnesium oxide molecule by 2 on the right hand side. The equation can now be expressed as,

Component	Reactant	Product
Magnesium	1	1 × 2 = 2
Oxygen	2	1 × 2 = 2

Step 5

There are two oxygen atoms on either side of the equation but one magnesium atom on the reactant’s side and two on the product’s side. Therefore, multiply the magnesium atom by 2 on the left hand side.

Component	Reactant	Product
Magnesium	1 × 2 = 2	2
Oxygen	2	2

Balanced equation is,

∴ The number of atoms of each element of reactants = The number of atoms of each element of products

Step 6

Writing Specific Conditions on the Arrow

The reaction is carried out in the presence of ‘Heat’. On heating, magnesium combines with oxygen present in air to form magnesium oxide.

Step 7

Writing Symbols of Physical States

Using these steps, you can balance any chemical equation.

Types of Chemical Reactions

1.    Combination Reaction

When two or more substances combine to form a single product, the reaction is known as a combination reaction.

For example:

In the laboratory, iron sulphide is prepared by mixing iron and sulphur.

Fe_(s) + S_(s) → FeS(s)

Formation of slaked lime by the reaction of calcium oxide with water

• Endothermic Reaction: The reactions accompanied by the absorption of heat are called endothermic reactions.
• Exothermic Reaction: The reactions accompanied by the evolution of heat are called endothermic reactions.

2.    Decomposition Reaction

A chemical reaction in which a single compound splits into two or more simple substances is called a decomposition reaction.

Correct way of heating the boiling tube containing crystals of ferrous sulphate and of smelling the odour

For example:

When mercuric oxide is heated in a crucible, the orange-red powder begins to darken and a silver mirror begins to deposit on the cooler parts of the crucible.

• Thermal Decomposition Reaction: The decomposition reactions carried out by heating are known as thermal decomposition reactions.
• Photochemical reaction: The chemical reactions which proceed with the absorption of light energy are called photochemical reactions.

3.   Displacement Reaction

Reactions in which the more reactive element displaces the less reactive element from its compound are called displacement reactions.

Iron nails dipped in copper sulphate solution

For example:

Zinc displaces copper in copper sulphate to form zinc sulphate.

4.   Double Displacement Reaction

Reactions in which ions of the reactants exchange places to form two new compounds, are called double displacement reactions.

For example: Sodium hydroxide reacts with hydrochloric acid to form sodium chloride and water.

NaOH_(aq)        +          HCl_(aq) →       NaCl_(aq)          +          H2O_(l)

Types of Double Displacement Reactions:

1. Precipitation

In all of the following processes, a white material is generated that is insoluble in water. A precipitate is the insoluble substance that forms. A precipitation reaction is a reaction that results in the formation of a precipitate.

• Neutralization

The reactants are a base and an acid, and the products are salt and water. Neutralization is a form of double displacement reaction in which the reactants are a base and an acid, and the products are salt and water. The positive charge of the acid’s hydrogen ion and the negative charge of the base’s hydroxyl ions or oxide ions lose their electrical charge and form covalent water molecules.

Na⁺OH^– + H⁺Cl^–                  →            Na⁺Cl^– + H₂O

Base     Acid                Salt    Water

Zn₂+O₂– + 2H+NO₃^– → Zn₂⁺(NO₃)₂^– + H₂O

A neutralization reaction is basically a reaction between H+ and OHions i.e.,

H⁺_(aq) + OH^–_(aq) → H₂O_(l)

5.   Oxidation – Reduction Reactions/Redox Reactions

Oxidation and reduction take place simultaneously.

Reactions which involve the addition of oxygen, or the removal of hydrogen are called oxidation reactions. For example:

C_(s) + 2H_{2 (g)} → CH_{4 (g)}

Chemical reactions in which the reactants gain hydrogen are reduction reactions. For example:

·       Oxidation: Substance loses electrons or gains oxygen or loses hydrogen.

·       Reduction: Substance gains electrons or loses oxygen or gains hydrogen.

·       Oxidising agent: a substance that oxidises another substance and self-gets reduced.

·       Reducing agent: a substance that reduces another substance and self-gets oxidised.

Redox Reaction

The chemical reaction in which oxidation and reduction takes place simultaneously is known as a redox reaction.

Corrosion

The slow process of decay and destruction of metals due to the action of air, moisture or acids is called corrosion.

For example:

Iron combines with oxygen present in the air, in the presence of water, to form a red-brown flaky substance called rust. This process is commonly called the rusting of iron.

The chemical formula of rust is Fe₂O₃. × H₂O.

Prevention of Corrosion

• Corrosion damages buildings, bridges, ships, automobiles and other articles made of iron. Hence, prevention of corrosion is necessary. This will not only save money but can also prevent the occurrence of accidents.
• It can be prevented by processes like galvanising and electroplating with other metals.

Rancidity

• Oils and fats react with oxygen and get oxidised or turn rancid. This process is called rancidity.
• Rancidity can be prevented by keeping food in air tight containers or by using antioxidants.
• Antioxidants are used to prevent oxidation of food containing fats and oils.
• Storage of food in air tight containers also decelerates oxidation.
  

Important Questions

• Multiple Choice Questions:

1. Which of the following is a displacement reaction?

2. Magnesium ribbon is rubbed before burning because it has a coating of

(a) basic magnesium carbonate

(b) basic magnesium oxide

(c) basic magnesium sulphide

(d) basic magnesium chloride

3. Which of the following statements about the given reaction are correct?

3Fe (s) + 4H₂O (g) → Fe₃O₄ (s) + 4 H₂ (g)

(i) Iron metal is getting oxidized

(ii) Water is getting reduced

(iii) Water is acting as reducing agent

(iv) Water is acting as oxidizing agent

(a) (i), (zi) and (iii)

(b) (in) and (iv)

(c) (i), (ii) and (iv)

(d) (ii) and (iv)

4. Which of the following are exothermic processes?

(i) Reaction of water with quick lime

(ii) Dilution of an acid

(iii) Evaporation of water

(iv) Sublimation of camphor (crystals)

(a) (i) and (ii)

(b) (ii) and (iii)

(c) (i) and (iv)

(d) (ii) and (iv)

5. Oxidation is a process which involves

(a) addition of oxygen

(b) addition of hydrogen

(c) removal of oxygen

(d) removal of hydrogen

6. The process of reduction involves

(a) addition of oxygen

(b) addition of hydrogen

(c) removal of oxygen

(d) removal of hydrogen

7. Three beakers labelled as A, B and C each containing 25 ml of water were taken. A small amount of NaOH, anhydrous CuSO₄ and NaCl were added to the beakers A, B and C respectively. It was observed that there was an increase in the temperature of the solution contained in beakers A and B, whereas in case of beaker C, the temperature of the solution falls. Which one of the following statement(s) is (are) correct?

(i) In beakers A and B, exothermic process has occurred.

(ii) In beakers A and B, endothermic process has occurred.

(iii) In beaker C exothermic process has occurred.

(iv) In beaker C endothermic process has occurred.

(a) (i) only

(b) (ii) only

(c) (i) and (iv)

(d) (iv), (ii) and (iii)

8. Give the ratio in which hydrogen and oxygen are present in water by volume.

(a) 1:2

(b) 1:1

(c) 2:1

(d) 1:8

9. Which among the following statement(s) is (are) true?

Exposure of silver chloride to sunlight for a long duration turns grey due to

(i) the formation of silver by decomposition of silver chloride

(ii) sublimation of silver chloride

(iii decomposition of chlorine gas from silver chloride

(iv) oxidation of silver chloride

(a) (i) only

(b) (i) and (iii)

(c) (ii) and (iii)

(d) (iv) only

10. MnO₂ + 4HCl → 2 + 2H₂O + Cl₂

Identify the substance oxidized in the above equation.

(a) MnCl₂

(b) HCl

(c) H₂O

(d) MnO₂

• Very Short Question:
• How does the food become rancid?
• A student burnt a metal A found in the form of ribbon. The ribbon burnt with a dazzling flame and a white powder B was formed which was basic in nature. Identify A and B. Write the balanced chemical equation.
• What is a balanced chemical equation?
• Write a balanced equation for a chemical reaction that can be characterized as precipitation.
• What is rust?
• A zinc rod is left for nearly 20 minutes in a copper sulphate solution. What change would you observe in the zinc rod?
• Name two salts that are used in black and white photography.
• Which chemical process is used for obtaining a metal from its oxide?
• If you collect silver coins and copper coins you may have seen that after some days a black coating forms on silver coins and a green coating on copper coins. Which chemical phenomenon is responsible for these coatings? Write the chemical name of the black and green coatings.
• When carbon dioxide is passed through lime water, it turns milky, why?
• Short Questions:

1. You are given the following materials

(i) Marble chips (ii) dilute hydrochloric acid (iii) Zinc granules

Identify the type of reaction when marble chips and zinc granules are added separately to acid taken in two test tubes.

2. What do you understand by precipitation reaction? Explain with suitable examples.

3. What happens when aqueous solutions of sodium sulphate and barium chloride are mixed? What type of reaction is it?

4. Explain the following terms with suitable examples.

(a) Oxidation

(b) Reduction

5. Complete the missing components/variables given as x and y in the following reactions. 

(a) Pb(NO₃)₂ (aq) + 2Kl (aq) → PbI₂ (x) + 2KNO₃ (y)

(b) Cu (s) + 2AgNO₃ (aq) → Cu(NO₃)₂ (aq) + x (s)

(c) Zn (s) + H₂SO₄ (aq) → ZnSO₄ (x) + H₂ (y)

6. An iron knife kept dipped in a blue copper sulphate solution turns the blue solution light green. Why?

7. A, B and C are three elements which undergo chemical reactions in the following way.

A₂O₃ + 2B → B₂O₃ + 2A

3CSO₄ + 2B → B₂(SO₄)₃ + 3C

3CO + 2A → A₂O₃ + 3C

Answer the following:

(a) Which element is most reactive?

(b) Which element is least reactive?

8. Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.

(a) Nitrogen gas is treated with hydrogen gas in the presence of a catalyst at 773 K to form ammonia gas.

(b) Sodium hydroxide solution is treated with acetic acid to form sodium acetate and water.

(c) Ethanol is warmed with ethanoic acid to form ethyl acetate in the presence of concentrated H₂SO₄.

(d) Ethene is burnt in the presence of oxygen to form carbon dioxide, water and releases heat and light.

• Long Questions:

1. Balance the following equations:

(a) Bacl₂ + H₂SO₄ → BaSO₄ + HCl

(b) CH₄ + O₂ → CO₂ + H₂O

2. On heating blue coloured powder of copper (II) nitrate in a boiling tube, copper oxide (black), oxygen gas and a brown gas X is formed.

(а) Write a balanced chemical equation of the reaction.

(b) Identify the brown gas X evolved.

(c) Identify the type of reaction.

(d) What could be the pH range of aqueous solution of the gas X?

3. (A) Name the type of chemical reaction represented by the following equation:

(c) Zn(s) + H₂SO₄(ag) → ZnSO₄(aq) + H₂(g)

(B) “A solution of potassium chloride when mixed with silver nitrate solution, and an insoluble white substance is formed”.

(i) Translate the above statement into a chemical equation.

(ii) State two types for the classification of this reaction.

• Assertion Reason Questions:
• For two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below:
• Both A and R are true, and R is correct explanation of the assertion.
• Both A and R are true, but R is not the correct explanation of the assertion.
• A is true, but R is false.
• A is false, but R is true.

Assertion: Silver articles become black after sometime when exposed to sunlight.    

Reason: It is because silver reacts with carbonates present in the air.

• For two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below:
• Both A and R are true, and R is correct explanation of the assertion.
• Both A and R are true, but R is not the correct explanation of the assertion.
• A is true, but R is false.
• A is false, but R is true.

Assertion: A lead nitrate on thermal decomposition gives lead oxide, brown coloured nitrogen dioxide and oxygen gas.

Reason: Lead nitrate reacts with potassium iodide to form yellow ppt. of lead iodide and the reaction is double displacement as well as precipitation reaction.

• Case Study Questions:
• Read the following and answer any four questions from (i) to (v).

Oxidation has damaging effect on metals as well as on food. The damaging effect of oxidation on metal is studied as corrosion and that on food is studied as rancidity. The phenomenon due to which metals are slowly eaten away by the reaction of air, water and chemicals present in atmosphere, is called corrosion. For example, iron articles are shiny when new, but get coated with a reddish-brown powder when left for some time. This process is known as rusting of iron. Rancidity is the process of slow oxidation of oil and fat (which are volatile in nature) present in the food materials resulting in the change of smell and taste in them.

1. Rancidity can be prevented by:
2. Adding antioxidants.
3. Packaging oily food in nitrogen gas.
4. Both (a) and (b).
5. None of these.
6. Combination of phosphorus and oxygen is an example of:
7. Oxidation.
8. Reduction.
9. Rancidity.
10. None of these.
11. A science teacher wrote the following statements about rancidity:
12. When fats and oils are reduced, they become rancid.
13. ln chips packet, rancidity is prevented by oxygen.
14. Rancidity is prevented by adding antioxidants.

Select the correct option.

1. (I) only
2. (II) and (III) only
3. (III) only
4. (I), (II) and (III)
5. Two statements are given below regarding rusting of iron.
6. The rusting of iron is a redox reaction and reaction occurs as, 4Fe + 3O₂ → 4Fe³⁺ + 6O^2-
7. The metallic iron is oxidised to Fe²⁺ and O₂ is reduced to O^2-.

Select the correct statement(s).

1. I only.
2. II only.
3. Both I and II.
4. None of these.
5. Which of the following measures can be adopted to prevent or slow down rancidity?
6. Food materials should be packed in airtight container.
7. Food should be refrigerated.
8. Food materials and cooked food should be kept away from direct sunlight.
9. Only II and III.
10. Only I and III.
11. Only II and III.
12. I, II and III.
13. Read the following and answer any four questions from (i) to (v).

Chemical equation is a method of representing a chemical reaction with the help of symbols and formulae of the substances involved in it. ln a chemical equation, the substances which combine or react are called reactants and new substances produced are called products. A chemical equation is a shorthand method of representing a chemical reaction. A balanced chemical equation has equal number of atoms of different elements in the reactants and products side. An unbalanced chemical equation has unequal number of atoms of one or more elements in reactants and products. Formulae of elements and compounds are not changed to balance an equation.

1. Consider the following reaction:

pMg₃N₂ + qH₂O → rMg(OH)₂ + sNH₃

When the equation is balanced, the coefficients p, q, r, s respectively are:

1. 1, 3, 3, 2
2. 1, 6, 3, 2
3. 1, 2, 3, 2
4. 2, 3, 6, 2
5. Which of the following information is not conveyed by a balanced chemical equation?
6. Physical states of reactants and products.
7. Symbols and formulae of all the substances involved in a particular reaction.
8. Number of atoms/ molecules of the reactants and products formed.
9. Whether a particular reaction is actually feasible or not.
10. The balancing of chemical equations is in accordance with:
11. law of combining volumes.
12. law of constant proportions.
13. law of conservation of mass.
14. both (b) and (c).
15. Which of the following chemical equations is an unbalanced one?
16. 2NaHCO₃ → Na2CO₃ + H₂O + CO₂
17. 2C₄H₁₀ + 12O₂ → 8CO₂ + 10H₂O
18. 2AI + 6H₂O → 2Al(OH)₃ + 3H₂
19. 4NH₃ + 5O₂ → 4NO + 6H₂O
20. Which of the following statements is/ are correct?
21. A chemical equation tells us about the substances involved in a reaction.
22. A chemical equation informs us about the symbols and formulae of the substances involved in a reaction.
23. A chemical equation tells us about the atoms or molecules of the reactants and products involved in a reaction.
24. All the above.
25. Answer Key-
26. Multiple Choice Answers:
27. (b)
28. (a) basic magnesium carbonate
29. (c) (i), (ii) and (iv)
30. (a) (i) and (ii)
31. (a) addition of oxygen
32. (b) addition of hydrogen
33. (c) (i) and (iv)
34. (a) 1 : 2
35. (a) (i) only
36. (d) MnO₂
37. Very Short Answers:
38. Answer: Food becomes rancid when fat and oils present in the food are oxidized.
39. Answer: X = Mg, Y = MgO, Mg + O₂ → 2MgO
40. Answer: An equation that has equal number of atoms of each element on both the sides of the equation is called a balanced chemical equation, i.e., mass of the reactants is equal to mass of the products.
41. Answer: BaCl₂(aq) + Na₂SO₄ (aq) → BaSO₄(s) + 2NaCl(aq)
42. Answer: It is a brown mass known as hydrated ferric oxide. Its formula is Fe₂O₃. xH₂O.
43. Answer: The zinc rod will change into zinc sulphate.
44. Answer: Both silver chloride and silver bromide are used in black and white photography.
45. Answer: The process is known as the reduction of metal oxide.
46. Answer: Corrosion is responsible for the formation of this coating. Black coating is due to formation of Ag₂S and green coating is due to formation of CuCO₃.Cu(OH)₂.
47. Answer: Lime water (calcium hydroxide) combines with carbon dioxide to form a suspension of calcium carbonate which makes lime water milky.

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

• Short Answer:
• Answer:

(i) Marble chips react with dilute hydrochloric acid to form calcium chloride and carbon dioxide. It is a double displacement reaction.

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

(ii) Zinc granules react with dilute hydrochloric acid to give hydrogen gas. It is a displacement reaction.

Zn (s) + 2HCl → ZnCl₂ (aq) + H₂ (g)

• Answer:

The reaction in which two compounds in their aqueous state react to form an insoluble compound. When two reactants react and product formed remains insoluble and settles as a solid it is substance (precipitate) is called a precipitation reaction.

For example,

(i) When aqueous solution of sodium sulphate is mixed with an aqueous solution or barium chloride, barium sulphate is obtained as a white precipitate.

Na₂SO₄ (aq) + BaCl₂ (aq) → BaSO₄ (s) + 2NaCl (ag)

(ii) When aqueous solution of sodium chloride is mixed with an aqueous solution of silver nitrate, silver chloride is obtained as a white precipitate.

• Answer:

On mixing the solutions of sodium sulphate and barium chloride, a white precipitate of barium sulphate is obtained.

It is a double displacement reaction.

• Answer:

(a) Oxidation is a process of addition of oxygen to a substance or removal of hydrogen from a substance, for example,

Chemical Reactions and Equations Class 10 Extra Questions with Answers Science Chapter 1, 3

Copper is oxidized to CuO, as oxygen is added to copper.

(b) It is the process of removal of oxygen from a substance or addition of hydrogen to a substance, for example,

Copper oxide is reduced to copper as it involves removal of oxygen.

• Answer:

(a) x = (s), y = (aq)

(b) x = 2Ag

(c) x = (aq); y = (g)

(d) x = heat

• Answer:

We know that iron is more reactive than copper, so it displaces copper from copper sulphate solution and forms ferrous sulphate which is of light green colour.

• Answer:

(a) The most reactive element is ‘B’. It has displaced both ‘A’ and ‘C’ from their compounds.

(b) The least reactive element is ‘C’ as it has been displaced by both ‘A’ and ‘B’.

• Answer:

• Long Answer:
• Answer:

(a) Bacl₂ + H₂SO₄ → BaSO₄ + 2HCl

(b) CH₄ + 2O₂ → CO₂ + 2H₂O

• Answer:

(b) Brown gas X is nitrogen dioxide (NO₂).

(c) It is a thermal decomposition reaction.

(d) The gas (NO₂) is an oxide of a non-metal. Hence, its aqueous solution will be acidic, i.e., pH range would be between 0 and 7.

• Answer:

(A) (a) Decomposition reaction

(b) Combination reaction

(c) Displacement reaction.

(B) (i) KCl (aq) + AgNO₃ (aq) → AgCl (s) + KNO₃ (aq)

(ii) It is a double displacement reaction also called precipitation reaction.

• Assertion Reason Answer:
• Answer:
• A is true, but R is false.

Silver reacts with sulphur present in the air and form the layer of silver sulphide therefore silver articles get tarnished.

•  Answer:
• Both A and R are true, but R is not the correct explanation of the assertion.

Explanation:

Decomposition reaction is a reaction in which a compound breaks down into two or more simpler substances.

2Pb(NO3)2 → 2Pb0 + 4NO2 + O2

• Case Study Answer:
• Answer:
• (c) Both (a) and (b).

Explanation:

Antioxidants and nitrogen gas prevent oxidation of food.

1. Oxidation.

Explanation:

4P + 30₂ → 2P₂O₃ 

4P + 5O₂ → 2P₂O₅

1. (c) (III) only

Explanation:

The oils and fats are slowly oxidised to certain bad smelling compounds, which release foul smell. This is known as rancidity. Rancidity is prevented by filling nitrogen gas in chips packets.

1. Only II and III.
2. (d) I, II and III.
3. Answer:
4. (b) 1, 6, 3, 2

Explanation:

Mg₃N₂ + 6H₂O- 3Mg(OH)₂ + 2NH₃

1. (d) Whether a particular reaction is actually feasible or not.
2. (c) law of conservation of mass.

Explanation:

ln a balanced chemical equation, total mass of reactants must be equal to the total mass of products. This is the statement of law of conservation of mass.

1. (b) 2C₄H₁₀ + 12O₂ → 8CO₂ + 10H₂O
2. (d) All the above.

Chapter 2: Acids, Bases & Salts

Three different theories have been put forth in order to define acids and bases.

●      The Arrhenius theory of acids and bases states that “an acid generates H+ ions in a solution whereas a base produces an OH– ion in its solution”.

●      The Bronsted-Lowry theory defines “an acid as a proton donor and a base as a proton acceptor”.

●      Finally, the Lewis definition of acids and bases describes “acids as electron-pair acceptors and bases as electron-pair donors”.

Acids and Bases in the Laboratory

Indicators

An indicator tells us whether a substance is acidic or basic in nature, by the change in colour.

Common Indicators

• An acid turns blue litmus red and a base turns red litmus blue.
• Methyl orange indicator gives a red colour in an acidic solution and gives a yellow colour in a basic solution.
• Phenolphthalein is colourless in an acidic solution and gives a pink colour in a basic solution.

Olfactory Indicators

• Those substances whose odour changes in acidic or basic media are called olfactory indicators. For example: onion, vanilla and clove oil.
• On adding sodium hydroxide solution to a cloth strip treated with onion, the smell of the onion is not detected. An acidic solution does not eliminate the smell of the onion.

Reaction of Acids & Bases with Metals

Acids react with metals to produce salt by displacing hydrogen.

For Example:

1. When dilute sulphuric acid reacts with the metal zinc, zinc sulphate is formed with the evolution of hydrogen gas.

Zn                   +    H₂SO₄             →                 ZnSO₄         +            H₂

• Zinc is the only metal which reacts with sodium hydroxide to form sodium zincate with the release of hydrogen gas.

Zn                   +    2NaOH          →                Na₂ZnO₂      +            H₂

Reaction of zinc granules with dilute sulphuric acid and testing hydrogen gas by burning

Reaction of Metal Carbonates & Bicarbonates with Acids

Acids react with metal carbonates or bicarbonates to form salt and water with the evolution of carbon dioxide gas.

For Example:

1. Hydrochloric acid reacts with sodium carbonate to form sodium chloride and water with the release of carbon dioxide gas.

Na₂CO_3(s)                   +   2 HCl_(aq) →   2NaCl_(aq) + CO_2(g) + H₂O_(l)

• Similarly, sodium bicarbonate also reacts with hydrochloric acid to form sodium chloride and  water with the release of carbon dioxide gas.

NaHCO_3(s)                   +    HCl _(aq) →   NaCl _(aq) + CO_2(g) + H₂O_(l)

Passing carbon dioxide gas through calcium hydroxide solution

Acids and Bases React with each other

The reaction between an acid and a base to form salt and water is called a neutralisation reaction.

For example:

Hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water.

NaOH_(aq) + HCl_(aq) → NaCl_(aq) + H₂O_(l)

In general, a neutralisation reaction can be written as:

Base + Acid → Salt + Water

Reaction of Metallic Oxides with Acids

Acids react with metallic oxides to form salt and water.

For Example:

Copper oxide (II), a black metal oxide reacts with dilute hydrochloric acid to form a blue-green coloured copper chloride (II) solution.

CuO                     +        2HCl    →     CuCl_2(aq)     +     H₂O

Reaction of Non-Metallic Oxides with Base

Bases react with non-metallic oxides to form salt and water.

For Example:

Calcium hydroxide reacts with non-metallic oxides like carbon dioxide to form calcium carbonate salt and water.

Ca(OH)₂    +                   CO₂    →        CaCO₃        +            H₂O

Acids and Bases in Water

Acids

An acid is a substance which dissociates (or ionises) when dissolved in water to release hydrogen ions.

For Example:

An aqueous solution of hydrochloric acid dissociates to form hydrogen ions. Since hydrogen ions do not exist as H⁺ in solution, they combine with polar water molecules to form hydronium ions [H₃O⁺].

HCl _(aq)   →        H⁺ _(aq)        +       Cl^– _(aq)

H⁺           +          H₂O         →       H₃O⁺

The presence of hydrogen ions [H⁺] in hydrochloric acid solution makes it behave like an acid.

Bases

A base is a substance which dissolves in water to produce hydroxide ions [OH- ions]. Bases which are soluble in water are called alkalis.

For Example:

Sodium hydroxide dissolves in water to produce hydroxide and sodium ions.

NaOH _(aq)       →       Na⁺_(aq) +          OH^– _(aq)

The presence of hydroxide ions [OH^–] in sodium hydroxide solution makes it behave like a base.

pH Scale

• pH of a solution: pH of a solution is the negative logarithm to the base 10 of the hydrogen ion concentration expressed in mole per litre.

pH = –log₁₀ (H⁺)

p H = 7 – Neutral	[H+] = [OH–]
pH less than 7 – Acidic	[H+] more than [OH–]
pH more than 7 – Basic	[OH–] more than [H+]

Universal Indicator

In case of a colourless liquid, the accurate pH can be obtained by adding a universal indicator.

It is a mixture of several indicators and shows different colours at different concentration of hydrogen ions in a solution.

Variation of pH with the change in concentration of H+ (aq) and OH– (aq) ions

For Example:

1. A universal indicator produces green colour in a neutral solution, pH = 7.
2. The colour changes from blue to violet as pH increases from 7 to 14.
3. The colour changes from yellow to pink and then to red as pH decreases from 7 to 1.

pH of some common substances shown on a pH paper (colours are only a rough guide)

Importance of pH in everyday life

pH change and survival of animals

• Our body works well within a narrow pH range of 7.0 to 7.8.
• When the pH of rain water is less than 5.6, it is known as acid rain.
• When this acid rain flows into rivers, it lowers the pH of the river water making the survival of aquatic life difficult.

pH in our digestive system

• Our stomach produces hydrochloric acid which helps in the digestion of food without harming the stomach.
• Sometimes excess acid is produced in the stomach which causes indigestion.
• To get rid of this pain, bases called antacids are used.
• Antacids are a group of mild bases which react with the excess acid and neutralise it.
• Commonly used antacids are magnesium hydroxide [Mg(OH)₂] & sodium bicarbonate [NaHCO₃]

pH change – Cause of tooth decay

• Tooth decay starts when the pH in the mouth falls below 5.5.
• Tooth enamel is made up of calcium phosphate which is the hardest substance in the body.
• It is insoluble in water but gets corroded when the pH in the mouth falls below 5.5.
• The bacteria present in the mouth produce acids due to the degradation of sugar and food particles after eating.
• Hence, to prevent tooth decay, the mouth should be rinsed after eating food and toothpastes which are basic should be used cleaning teeth to neutralise the excess acid.

Soil of pH and plant growth

Most of the plants have a healthy growth when the soil has a specific pH (close to 7) range which should be neither alkaline nor highly acidic. Therefore,

• Compound ‘X’ is Sodium hydroxide (NaOH).
• Compound ‘A’ is Zinc sulphate (ZnSO4).
• Compound ‘B’ is Sodium chloride (NaCl).
• Compound ‘C’ is Sodium acetate (CH3COONa)

More about Salts

A salt is a combination of an anion of an acid and a cation of a base.

Examples: KCl, NaNO₃ ,CaSO₄, etc.

Salts are usually prepared by the neutralisation reaction of an acid and a base.

Family of salts

Salts having same positive ions (or same negative ions) are said to belong to a family of salts. For example, NaCl, KCl, LiCl.

pH of Salts

• Salts of strong acid and a strong base are neutral, with a pH value of 7.

For Example: NaCl, Na₂SO₄

• Salts of strong acid and weak base are acidic, with a pH value less than 7.

For Example: Ammonium chloride solution has pH value of 6.

• Salts of weak acid and strong base are basic, with a pH value more than 7.

For Example: Sodium carbonate solution has a pH value of 9.

Common Salt

• Common salt is a neutral salt and can be prepared in the laboratory by the reaction of sodium hydroxide and hydrochloric acid.

NaOH _(aq)                  + HCl _(aq)     →       NaCl _(aq) + H2O _(aq)

• It is an important raw material for products of daily use such as NaOH, baking soda, washing soda and bleaching powder.

Sodium Hydroxide

●      Sodium hydroxide is produced by the electrolysis of an aqueous solution of sodium chloride (called brine).

• The process is called the chlor-alkali process because of the products formed, i.e. ‘chlor’ for chlorine and ‘alkali’ for sodium hydroxide.

2NaCl_(aq)                         + 2H₂O_(aq)       →       2NaOH_(aq)    +    H_2(g) + Cl_2(g)

Important products from the chlor-alkali process

Bleaching Powder

• It is produced by the action of chlorine on dry slaked lime [Ca(OH)₂].

Ca(OH)₂    +                Cl₂                     →       CaOCl₂      +    H₂O

• It is represented as CaOCl₂

Uses

• for bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in laundry;
• as an oxidising agent in many chemical industries; and
• to make drinking water free from germs.

Baking Soda

• Chemical formula: NaHCO₃
• It is produced on a large scale by treating cold and concentrated solution of sodium chloride (brine) with ammonia and carbon dioxide.

NaCl + H2O + CO2 + NH3	→	NH4Cl	+	NaHCO3
		(Ammonium chloride)		(Sodium hydrogen carbonate)

• On heating, it decomposes to give sodium carbonate with the evolution of carbon dioxide.

2NaHCO3		Na2CO3	+ H2O + CO2
(Sodium hydrogen carbonate)		(Sodium carbonate)

Uses

• For making baking powder, which is a mixture of baking soda (sodium hydrogen carbonate) and a mild edible acid such as tartaric acid. When baking powder is heated or mixed in water, the following reaction takes place:

NaHCO₃ + H⁺ → CO₂ + H₂O + Sodium salt of acid

Carbon dioxide produced during the reaction can cause bread or cake to rise making them soft and spongy.

• Sodium hydrogencarbonate is also an ingredient in antacids. Being alkaline, it neutralises excess acid in the stomach and provides relief.
• It is also used in soda-acid fire extinguishers.

Washing Soda

• Chemical formula: Na₂CO₃.10H₂O
• Sodium hydrogen carbonate, on heating decomposes to give sodium carbonate with the release of hydrogen gas. Re-crystallisation of sodium carbonate produces washing soda.

2NaHCO₃ Na₂CO₃ + H₂O + CO₂

Na2CO3	+ 10H2O	→	Na2CO3.10H2O
(Sodium carbonate)

Uses

• Sodium carbonate (washing soda) is used in glass, soap and paper industries.
• It is used in the manufacture of sodium compounds such as borax.
• Sodium carbonate can be used as a cleaning agent for domestic purposes.
• It is used for removing permanent hardness of water.

Water Of Crystallisation

• Water molecules which form a part of the structure of a crystal are called water of crystallisation.
• The salts which contain water of crystallisation are called hydrated salts.
• Every hydrated salt has a fixed number of molecules of crystallisation in its one formula unit.

For Example: CuSO₄.5H₂O, Na₂CO₃.10H₂O, CaSO₄.5H₂O, and FeSO₄.7H₂O

• Copper sulphate crystals (CuSO₄.5H₂O) are blue in colour, and on heating strongly they lose all the water of crystallisation and form anhydrous copper sulphate, which is white. On adding water to anhydrous copper sulphate, it gets hydrated and turns blue.

CuSO₄.5H₂O                              CuSO₄ + 5H₂O

CuSO₄ + 5H₂O                 →                  CuSO₄.5H₂O

Plaster of Paris

Plaster of Paris is prepared by heating gypsum at 373 K. On heating, it loses water molecules and becomes calcium sulphate hemihydrate (CaSO₄.H₂O) which is called Plaster of Paris.

CaSO4.2H2O		CaSO4.  H2O + 1  H2O
(Gypsum)		(Plaster of Paris)

Uses

• Used in hospitals as plaster for supporting fractured bones in the right position.
• Used as a fire-proofing material.
  

Important Questions

• Multiple Choice Questions:

1. What happens when a solution of an acid is mixed with a solution of a base in a test tube?

(i) Temperature of the solution decreases

(ii) Temperature of the solution increases

(in) Temperature of the solution remains the same

(iv) Salt formation takes place

(a) (i) and (iv)

(b) (i) and (iii)

(c) (ii) only

(d) (ii) and (iv)

2. When hydrogen chloride gas is prepared on a humid day, the gas is usually passed through the guard tube containing calcium chloride. The role of calcium chloride taken in the guard tube is to

(a) absorb the evolved gas

(b) moisten the gas

(c) absorb moisture from the gas

(d) absorb Cl– ions from the evolved gas

3. Which one of the following salts does not con-tain water of crystallisation?

(a) Blue vitriol

(b) Baking soda

(c) Washing soda

(d) Gypsum

4. In terms of acidic strength, which one of the following is in the correct increasing order?

(a) Water < Acetic acid < Hydrochloric acid

(b) Water < Hydrochloric acid < Acetic acid

(c) Acetic acid < Water < Hydrochloric acid

(d) Hydrochloric acid < Water < Acetic acid

5. What is formed when zinc reacts with sodium hydroxide?

(a) Zinc hydroxide and sodium

(b) Sodium zincate and hydrogen gas

(c) Sodium zinc-oxide and hydrogen gas

(d) Sodium zincate and water

6. Tomato is a natural source of which acid?

(a) Acetic acid

(b) Citric acid

(c) Tartaric acid

(d) Oxalic acid

7. Brine is an

(a) aqueous solution of sodium hydroxide

(b) aqueous solution of sodium carbonate

(c) aqueous solution of sodium chloride

(d) aqueous solution of sodium bicarbonate

8. Na2CO3. 10H2O is

(a) washing soda

(b) baking soda

(c) bleaching powder

(d) tartaric acid

9. At what temperature is gypsum heated to form Plaster of Paris?

(a) 90°C

(b) 100°C

(c) 110°C

(d) 120°C

10. How many water molecules does hydrated calcium sulphate contain?

(a) 5

(b) 10

(c) 7

(d) 2

• Very Short Question:

1. Write a balanced chemical equation for the reaction between sodium carbonate and hydrochloric acid indicating the physical state of the reactants and products.

2. During summer season, a milkman usually adds a small amount of baking soda to fresh milk. Give reason.

3. What is the difference between slaked lime and lime water?

4. Which acid is present in sour milk or curd?

5. Why is potassium iodide added into common salt to use it as table salt?

6. What are the pH values of distilled water and common salt solution

7. A dry pellet of a common base B, when kept in open absorbs moisture and turns sticky. The compound is also a by-product of chloralkali process. Identify B. What type of reaction occurs when B is treated with an acidic oxide? Write a balanced chemical equation for one such solution.

8. Which bases are called alkalies? Give an example of an alkali.

9. A knife, which is used to cut a fruit, was immediately dipped into water containing drops of blue litmus solution. If the colour of the solution is changed to red, what inference can be drawn about the nature of the fruit and why?

10. How do H⁺ ions exist in water?

• Short Questions:

1. How will you find pH of lemon juice?

2. A sample of bleaching powder was kept in an air tight container. After a month, it lost some of its chlorine content. How will you account for it?

3. An aqueous solution of sodium carbonate is basic and not acidic. Assign reason.

4. An old person complained of acute pain in the stomach. Doctor gave him a small antacid tablet and he got immediate relief. What actually happened?

5. A milkman adds very small amount of baking soda to fresh milk. What happens to its pW?

6. A few drops of phenolphthalein indicator were added to an unknown solution A. It acquired pink colour. Now another unknown solution B was added to it dropwise and the solution ultimately became colourless. Predict the nature of the solutions A and B.

7. A compound which is prepared from gypsum has the property of hardening when mixed with proper quantity of water. Identify the compound. Write chemical equation to prepare the compound. Mention one important use of the compound.

8. The oxide of a metal M was water soluble. When a blue litmus strip was dipped in this solution, it did not undergo any change in colour. Predict the nature of the oxide.

• Long Questions:

1. (a) A solution has a pH of 7. Explain how you would you:

(i) increase its pH

(ii) decrease its pH

(b) If a solution changes the colour of the litmus from red to blue, what can you say about its pH?

(c) What can you say about the pH of a solution that liberates carbon dioxide from sodium carbonate?

2. Explain why:

(i) Common salt becomes sticky during the rainy season

(ii) Blue vitriol changes to white upon heating

(iii) If bottle full of concentrated sulphuric acid is left open in the atmosphere by accident, the acid starts flowing out of the bottle of its own.

3. (a) Name the raw materials used in the manufacture of sodium carbonate by Solvay process.

(b) How is sodium hydrogen carbonate formed during Solvay process separated from a mixture of NH₄Cl and NaHCO₃?

(c) How is sodium carbonate obtained from sodium hydrogen carbonate?

• Assertion Reason Question:
• For question two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below:
• Both A and R are true, and R is correct explanation of the assertion.
• Both A and R are true, but R is not the correct explanation of the assertion.
• A is true, but R is false.
• A is false, but R is true.

Assertion: The process of dissolving an acid or a base in water is highly exothermic reaction.

Reason: Water must always be added slowly to acid with constant stirring.

• For question two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below:
• Both A and R are true, and R is correct explanation of the assertion.
• Both A and R are true, but R is not the correct explanation of the assertion.
• A is true, but R is false.
• A is false, but R is true.

Assertion: Higher the H+ ion concentration, lower is the pH value.

Reason: The pH of a neutral solution = 7, that of a basic solution< 7 and that of an acidic solution> 7.

• Case Study Questions:
• Read the following and answer any four questions from (i) to (v).

Bleaching powder is also known as chloride of lime. It is a solid and yellowish white in colour. Bleaching powder can be easily identified by the strong smell of chlorine. When calcium hydroxide (slaked lime) reacts with chlorine, it gives calcium oxychloride (bleaching powder) and water is formed. Aqueous solution of bleaching powder is basic in nature. The material to be bleached is first passed through solution of Na OH to remove greasy matter. Then it is passed through aqueous solution of bleaching powder and very dil. HCl solution. HCl reacts with bleaching powder to liberate nascent oxygen which bleaches material.

1. Bleaching powder is used as:
2. Bleaching agent in textile, paper and jute industry.
3. Disinfectant for water to make water free of germs.
4. Oxidising agent in many industries.
5. All of these.
6. Bleaching powder is also known as:
7. Calcium oxychloride.
8. Calcium hypochlorite.
9. Chloride of lime.
10. All of these.
11. Bleaching powder gives smell of chlorine because it.
12. Is unstable.
13. Gives chlorine on exposure to atmosphere.
14. Is a mixture of chlorine and slaked lime.
15. Contains excess of chlorine.
16. Select the correct statement (s) regarding bleaching powder.
17. It is pale yellow powder having smell of chlorine.
18. It is sparingly soluble in water and gives milky suspension when dissolved in water.
19. As bleaching powder gives nascent oxygen, it shows bleaching property.
20. All of these.
21. Identify the product ‘X’ in the given reaction.

Ca (OH)₂ + Cl₂ → X + H₂O

1. CaOCl₂
2. CaCl₂
3. Ca(ClO₃)₂
4. CaCO₃
5. Read the following and answer any four questions from (i) to (v).

Baking powder produces carbon dioxide on heating, so it is used in cooking to make the batter spongy. Although, baking soda also produces CO₂ on heating, but it is not used in cooking because on heating, baking soda produces sodium carbonate along with carbon dioxide. Sodium carbonate, thus, produced, makes the taste bitter. Baking powder is the mixture of baking soda and a mild edible acid. Generally, tartaric acid is mixed with baking soda to make baking powder. When baking powder is heated, NaHCO₃ decomposes to give CO₂ which makes bread and cake fluffy. Tartaric acid helps to remove bitter taste due to formation of sodium tartrate.

1. On passing excess CO₂ gas in aqueous solution of sodium carbonate, the substance obtained is:
2. NaOH
3. NaHCO₃
4. Na₂CO_3. I0H₂O
5. Na₂CO_3. H₂O
6. When sodium hydrogen carbonate is added to acetic acid, it evolves a gas. Which of the following statements are true about the gas evolved?
7. It turns lime water milky.
8. It extinguishes a burning splinter.
9. It turns lime water milky.
10. It has a pungent odour.
11. (I) and (II).
12. (I), (II) and (III).
13. (II), (III) and (IV).
14. (I) and (IV).
15. Select the correct statement regarding sodium hydrogen carbonate.
16. CO and CO₂ are produced during the heating of NaHCO₃.
17. It is insoluble in water.
18. It is used in soda-acid fire extinguishers.
19. All of these.
20. Acetic acid was added to a solid X kept in a test tube. A colourless and odourless gas was evolved. The gas was passed through lime water which turned milky. It was concluded that,
21. Solid X is sodium hydroxide and the gas evolved is CO₂.
22. Solid X is sodium bicarbonate and the gas evolved is CO₂.
23. Solid X is sodium acetate and the gas evolved is CO₂.
24. Solid X is sodium chloride and the gas evolved is CO₂.
25. Which of the following statements are correct regarding baking soda?
26. Baking soda is sodium hydrogen carbonate.
27. On heating, baking soda gives sodium carbonate.
28. It is used for manufacture of soap.
29. It is an ingredient of baking powder.
30. I and IV only.
31. I, II and III only.
32. I, II and IV only.
33. I, II, III and IV.

Answer Key-

• Multiple Choice Answers:
• (d) (ii) and (iv)
• (c) absorb moisture from the gas
• (b) Baking soda
• (a) Water < Acetic acid < Hydrochloric acid
• (b) Sodium zincate and hydrogen gas
• (d) Oxalic acid
• (c) aqueous solution of sodium chloride
• (a) washing soda
• (b) 100°C
• (d) 2
• Very Short Answers:
• Answer: Dilute Hydrochloric(HCl) acid reacts with sodium carbonate(Na₂CO₃) to produce sodium chloride (NaCl), carbon dioxide(CO₂), and water(H₂O). Brisk effervescence is seen due to the formation of carbon dioxide gas.

The balanced chemical reaction is as follows:

2HCl(aq)+Na2CO3(aq)⟶2NaCl(aq)+CO2(g)+H2O(l)

• Answer: Baking soda acts as a bit like preservative. It makes milk a bit alkaline so as to reduce the pace of bacteria acting on it ,as they add acids (e.g. lactic acid) to the milk and turn it sour. this keeps the milk from becoming sour for a long time.
• Answer: Slaked lime is hydrated calcium hydroxide whereas lime water is a saturated solution of calcium hydroxide. Furthermore, slaked lime has calcium hydroxide in its unsaturated form while lime water has calcium hydroxide in its saturated form in the chemical nature of each these solutions. Similarly, we produce slaked lime from calcium oxide whereas we produce lime water from calcium hydroxide. Below inforgraphic presents more details on the difference between slaked lime and lime water in tabular form.
• Answer: Lactic Acid or milk acid is an organic acid with chemical formula C₃H₆O₃. When milk sugar or lactose undergoes fermentation, the product obtained is lactic acid. It is found in cottage cheese, leban, sour milk, yogurt, and Koumiss.
• Answer: Potassium iodide and potassium iodate are commonly added to table salt to prevent iodine deficiency and associated thyroid disease.
• Answer: Normal distilled water has a pH of less than 7.0 and acidic because it dissolves carbon dioxide from the air. pH value of sodium chloride (common salt) in a solution (i.e) common salt solution is about equal to 7.
• Answer: Sodium hydroxide is a byproduct of chloralkali process. When it is kept in open, it absorbs moisture and turns sticky.

When sodium hydroxide is treated with carbon dioxide, it gives sodium carbonate. It is important to remember that carbon dioxide is an acidic oxide.

2NaOH+CO_2​→Na₂​CO₃​+H₂​O

Since this reaction is between a basic compound and an acidic compound, hence it is a neutralization reaction.

B is sodium hydroxide.

• Answer: Alkalis can be defined as bases that are soluble in water. All alkalis are bases, but all bases are not Alkalis. Example: Sodium hydroxide. They are recognized to comprise of high pH value, i.e., above 7.
• Answer: A knife, which is used to cut a fruit, was immediately dipped into water containing drops of blue litmus solution. If the colour of the solution is changed to red, what inference can be drawn about the nature of the fruit and why?
• Answer: H+ ions do not exist independently, it gains the unshared electron pairs on the oxygen in the water molecule to form a hydronium ion.

H⁺ + H₂O ⟶ H₃O⁺

Hence H⁺ ions in water are found as hydronium ions.

• Short Answer:
• Answer:

(a) Take about 5mL of the given sample of lemon juice in a test tube.

(b) Dip a strip of the universal pH paper in the tube.

(c) Take out the strip and note its colour. It will acquire a orange red colour.

(d) On comparison with pH paper chart, the pH of the solution falls in the range between 2 and 3.

• Answer: Bleaching powder if kept even in an airtight container, will slowly decompose of its own and form calcium chlorate and calcium chloride. The reaction is called auto-oxidation. This will result in decrease in its chlorine contents.

• Answer: Sodium carbonate reacts with water to form sodium hydroxide and carbonic acid.

Since the base is strong while acid is weak, the solution is basic and not acidic.

• Answer: The old person was suffering from acute acidity. Antacid tablet contains sodium hydrogen carbonate (NaHCO₃). It reacts with the acid (HCl) formed because of acidity and neutralizes its effect. That is how the old person got relief.
• Answer: The pH of fresh milk is nearly 6. Baking soda is sodium hydrogen carbonate (NaHCO₃). On adding it to fresh milk, the medium becomes alkaline and its pH therefore, increases.
• Answer: The solution A is basic in nature and phenolphthalein has imparted pink colour to it. The solution B is of an acid which has ultimately made solution A colourless by neutralising its basic effect.
• Answer: The compound is Plaster of Paris (CaSO₄.½ H₂O). It is formed from Gypsum (CaSO₂.2H₂O) upon heating to a temperature of 373 K and also changes back to Gypsum on adding water. Plaster of Paris is used for setting fractured bones.

• Answer: The metal oxide (MO) is of basic in nature. It dissolves in water to form metal hydroxide as follows:

MO + H₂O → M(OH)₂

A blue litmus does not undergo any change in colour in the basic medium.

• Long Answer:
• Answer:

(a) The solution with pH 7 is neutral. Its pH can be increased by adding a small amount of base like sodium hydroxide. Basic solutions have pH more than 7. Similarly, pH can be decreased by adding small amount of acid like hydrochloric acid. Acidic solutions have pH less than 7.

(b) The change in colour of litmus from red to blue indicates that the solution is of basic nature with pH more than 7.

(c) Carbon dioxide can be liberated by reacting sodium carbonate solution with acid like dilute hydrochloric acid. This shows that the solution is of acidic nature with pH less than 7.

• Answer:

(i) Common salt contains the impurity of magnesium chloride (MgCl₂) which is of deliquescent nature. When exposed to atmosphere, it becomes moist. Therefore, common salt becomes sticky during the rainy reason.

(ii) Blue vitriol (CUSO₄.5H₂O) upon heating changes to anhydrous copper sulphate (CUSO₄) which is white in colour.

(iii) Concentrated sulphuric acid is highly hygroscopic. It absorbs moisture from air and gets diluted. Since the volume increases, the acid starts flowing out of the bottle.

• Answer:

(a) The raw materials used are: NaCl, lime stone or CaCO₃ and NH₃.

(b) Sodium hydrogen carbonate (NaHCO₃) is sparingly soluble or less soluble in water and gets separated as a preciptate while NH₄Cl remains in solution. The precipitate is removed by filtration.

(c) Sodium hydrogen carbonate is converted to sodium carbonate upon heating.

• Assertion Reason
• (c) A is true, but R is false.

Explanation:

The process of dissolving an acid or a base in water is highly exothermic reaction. Acid must always be added slowly to water with constant stirring.

• (c) A is true, but R is false.

Explanation:

Higher the H⁺ ion concentration, lower is the pH value. The pH value less than 7 represents an acidic solution and value more than 7 represents a basic solution.

• Case Study Answer:
•  
• (d) All of these.
• (d) All of these.
• (b) Gives chlorine on exposure to atmosphere.

Explanation:

Bleaching powder gives chlorine on exposure to air by reacting with CO₂.

1. (d) All of these.
2. (a) CaOCl₂

 Explanation:

•