Science previous year paper solutions class 10

Previous year CBSE Class 10^th questions with solutions from Acids, Bases and salts

This article contains questions that were asked in the CBSE exam of Class 10^th in the year 2020. These questions are combined from multiple sets of question paper from 2020. Try solving them on your own first. 

Questions from 1-5 carry 1 Mark each

1. Baking powder is a mixture of :                                                      

a) Sodium carbonate and acetic acid 

b) Sodium carbonate and tartaric acid 

c) Sodium hydrogen carbonate and tartaric acid 

d) Sodium hydrogen carbonate and acetic acid 

Answer- c) Sodium hydrogen carbonate and tartaric acid

2NaHCO₃  + C₄H₆O₆ ⟶ 2CO₂ + 2H₂O + Na₂C₄H₄O₆

Baking soda + Tartaric acid                    Sodium tartrate 

2. The chemical formula for plaster of Paris is

a) CaSO4.2H2O

b) CaSO4. H2O

c) CaSO4. ½ H2O

d) 2 CaSO4. H2O

Answer- c) CaSO4. ½ H2O (Calcium sulfate hemihydrate)

3. When sodium hydrogen carbonate is added to ethanoic acid a gas 
evolves. Consider the following statements about the gas evolved?

(A) It turns lime water milky. 

(B) It is evolved with a brisk effervescence. 

(C) It has a smell of burning Sulphur. 

(D) It is also a by-product of respiration. 

The correct statements are:

a) A and B only

b) B and D only 

c) A, C and D

d) A, B and D 

Answer- d) A, B and D.

NaHCO3  +  CH3COOH ⟶ CH3COONa + H2O + CO2

When sodium hydrogen carbonate reacts with ethanoic acid/acetic acid it forms CO2, 
which turns lime water- Ca(OH)2 milky. CO2 also gives brisk effervescence 
(bubbles formed in liquid or fizz). Also, during respiration glucose and oxygen react 
to form ATP, while CO2 and H2O are released as byproducts (secondary products).

4. When a small amount of acid is added to water, the phenomena which occur are:

(A) Dilution

(B) Neutralization

(C) Formation of H₃O⁺ ions

(D) Salt formation 

The correct statements are:

(a) (A) and (C)

(b) (B) and (D)

(c) (A) and (B)

(d) (C) and (D)

Answer- (a) (A) and (C) 

5. A visually challenged student, has to perform a lab test to

detect the presence of acid in a given solution. The acid-base

indicator preferred by him will be:

(a) Blue litmus

(b) Clove oil

(c) Red cabbage extract

(d) Hibiscus extract

Answer- (b) Clove oil

Question number 6-9 carry 3 Marks

6. List the important products of the Chlor-alkali process. Write one 

important use of each.

Answer- Important products of the Chlor-alkali process are-

1. Sodium hydroxide (NaOH)- Used in making soaps and detergents.

2. Chlorine gas (Cl2)- Used in treatment of water to kill germs.

3. Hydrogen gas (H2)- Used as a fuel for rockets and spacecrafts.

7. How is washing soda prepared from sodium carbonate? Give its chemical equation. State the type of this salt. Name the type of hardness of water that can be removed by it?
Answer- Rehydration of Na₂CO₃ with ten water molecules results in the formation of washing soda.

Na₂CO₃ + 10H₂O ⟶ Na₂CO₃.10H₂O

Sodium carbonate      Washing soda

Type of salt- Basic salt, because its parents are Sodium hydroxide (NaOH) (which is a strong base) and Carbonic acid (H₂CO₃) (which is a weak acid). And we know that when a strong base reacts with a weak acid, the salt formed because of neutralization reaction is also basic. 

2NaOH + H₂CO₃ ⟶ Na₂CO₃ + 2H₂O

Washing soda is used for removing permanent hardness of the water. The permanent hardness of water is due to the presence of calcium sulphate and magnesium sulphate in it. These, cannot be removed by boiling water, therefore washing soda is useful for their removal.

8. Give reasons for the following:

(i) Only one half of water molecule is shown in the formula of Plaster of Paris.

(ii) Sodium hydrogen carbonate is used as an antacid.

(iii) On strong heating, blue coloured copper sulphate crystals turn white.

Answer- (i) The formula CaSO₄.1/2H₂O (Plaster of Paris) is written in this form because two formula units of CaSO₄ share one molecule of water. So, when we have only one CaSO₄ then we write it as having only 1/2 H₂O (CaSO₄.1/2 H₂O). On the other hand, when we will have two CaSO₄ then we will write it as having 1H₂O (2CaSO₄.H₂O).

Single CaSO₄ has only 1/2H₂O

(ii) Sodium hydrogen carbonate or baking soda is a basic salt, because its parents are Sodium hydroxide (NaOH) (which is a strong base) and Carbonic acid (H₂CO₃) (which is a weak acid). And we know that when a strong base reacts with a weak acid, the salt formed because of neutralization reaction is also basic. 

NaOH + H₂CO₃ ⟶ NaHCO₃ + H₂O

So, it is used as an antacid to neutralize the effect of extra acid (HCl) produced in our stomach during acidity. When sodium hydrogen carbonate which is a base, reacts with acid, it causes neutralization reaction to form salt and water.

(iii) Copper sulphate crystals look dry, but contain water of crystallization. That means they have water in the crystals, which we cannot see. When we heat the crystals, this water is removed and the salt turns white.

If you again moisten the crystals with water, you will find that blue color of the crystals reappears.

CuSO4.5H2O (Blue) + Heat ⟶ CuSO4 (White)

9. (i) Draw a labelled diagram to show the preparation of hydrogen chloride gas in laboratory.

(ii) Test the gas evolved first with dry and then with wet litmus paper.

In which of the two cases, does the litmus paper show change in colour?

(iii) State the reason of exhibiting acidic character by dry HCl gas / HCl solution.

Answer- 

(i) H₂SO₄ + 2NaCl ⟶ 2HCl + Na₂SO₄         

Experiment for preparation of HCl gas 

(ii) Change in litmus paper color from blue to red is observed when a wet litmus paper is used while dry litmus paper shows no color change. This is because while dry litmus paper cannot.

(iii) We know that acids form H⁺ ions in aqueous solutions.

Dry HCl gas does not show acidic character when a dry litmus paper is brought close to it, because dry HCl gas cannot form H⁺ ions by reacting with a dry litmus paper.

However, wet litmus paper can easily react with gaseous HCl and forms H⁺ ions. Therefore, it shows acidic character.

Question number 10 and 11 carry 5 Marks

10. A cloth strip dipped in onion juice is used for testing a liquid “X”. The liquid “X” changes its odour. Which type of an indicator is onion juice. The liquid “X” turns blue litmus red. List observations the liquid “X” will show on reacting with the following.

a) Zinc granules

b) Solid sodium carbonate

Write the chemical equations for the reactions involved.

Answer- Onion juice is an olfactory indicator.

The liquid “X” turns blue litmus red, therefore it must be an acid.

a) Reaction of liquid “X” with zinc granules- Let us assume “X” is HCl. And we know that Zinc is a metal. We have read this that reaction of acid with metals results in the production of hydrogen gas. Let us see this by writing the reaction.

HCl (aq) + Zn (s) ⟶ ZnCl₂ + H₂

Thus, when liquid “X” reacts with zinc granules it will form hydrogen gas.

b) Reaction of liquid “X” with solid sodium carbonate. We know that sodium is a metal and it has carbonate with it. So it is a metal carbonate. And we have studied that when acid reacts with metal carbonates they produce carbon dioxide and water along with a salt.

2HCl (aq) + Na₂CO₃ (s) ⟶ 2NaCl + H₂O + CO₂

11. Define water of crystallization. Give the chemical formula for two compounds as examples. How can it be proved that the water of crystallization makes a difference in the state and color of the compounds?

Answer- Water of crystallization is the fixed number of water molecules present

in one formula unit of a salt.

Example- Copper sulphate (CuSO₄. 5H₂O) and Gypsum (CaSO₄ .2H₂O)

Effect of water of crystallization on state of a compound-

CaSO₄.1/2H₂O + 1.5H₂O ⟶ CaSO₄ .2H₂O

Plaster of paris (powder)       Gypsum (solid)

Plaster of Paris is a white powder and on mixing with water, it changes to gypsum which has a hard solid mass. Thus, addition of water of crystallization results in changing the state of a compound from powder to solid form.

Effect of water of crystallization on color of a compound-

Copper sulphate crystals look dry, but contain water of crystallization. That means they have water in the crystals, which we cannot see. When we heat the crystals, this water is removed and the salt turns white.

If you again moisten the crystals with water, you will find that blue color of the crystals reappears.

CuSO4.5H2O (Blue) + Heat ⟶ CuSO4 (white)