Students are required to pay attention dealing with the following topics from this chapter:
- Assessing the chemical properties of acids and bases
- What is the reaction between acids, bases, and metals?
- What is the result when you mix acids and bases?
- The reaction of metallic oxides with acids
- What is common to all acids and all bases?
- In a water solution, what happens to an acid or a base?
Class 10 Science Important Questions Chapter 2 – Acids, Bases and Salts
Q1. To a solution of sodium hydroxide in a test tube, two drops of phenolphthalein are added.
(i) State the colour change observed.
(ii) If dil HCl is added dropwise to the solution, what will be the colour change?
(iii) On adding a few drops of NaOH solution to the above mixture the colour of the solution reappears. Why?
(i) On adding phenolphthalein to NaOH solution, the colour becomes pink.
(ii) On adding dilute HCl solution dropwise to the same test tube, the pink colour disappears and the solution again becomes colourless.
(iii) On again adding NaOH to the above mixture, pink colour reappears because the medium becomes basic again.
Q2. 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 the 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.
Onion juice is an olfactory indicator. Olfactory indicators give one type of odour in acidic medium and a different odour in basic medium. As the liquid ‘X’ turns blue litmus red, hence it is an acidic solution.
(a) Acids react with active metals such as zinc, magnesium etc. and evolve hydrogen gas, for example,
Zn(s) dil.H2SO4(aq) → ZnSO4H2(g)
(b) Acids react with metal carbonates to give carbon dioxide with brisk effervescence.
For example, Na2CO3 + H2SO4 → Na2SO4 + CO2 + H2O
Q3. On diluting an acid, it is advised to add acid to water and not water to acid. Explain why it is so advised?
Draw a labelled diagram to show the preparation of hydrogen chloride gas in the 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 for exhibiting acidic character by dry HCl gas/HCl solution.
Diluting a concentrated acid with water is a highly exothermic process. So, when water is added to concentrated acid, large amounts of heat is liberated which changes some water to steam explosively which can splash the acid and even the glass apparatus may break due to excessive heating.
Q4. Complete and balance the following chemical equations :
(i) NaOH(aq) + Zn(s) →
(ii) CaCO3(s) + H2O(l) + CO2(g) →
(iii) HCl(aq) + H2O(l) →
(i) 2NaOH(aq) + Zn(s) → Na2ZnO2(aq) + H2(g)
(ii) CaCO3(s) + CO2(g) + H2Ol → Ca(HCO3)2(aq)
(iii) HCl(aq) + H2Ol > H3O+ Cl–(aq)
Q5. How do the following substances dissociate to produce ions in their solutions?
(i) Hydrochloric acid
(ii) Nitric acid
(iii) Sulphuric acid
(iv) Sodium hydroxide
(v) Potassium hydroxide
(vi) Magnesium hydroxide
Dissociation of various substances to produce ions in their solutions are :
(i) Hydrochloric acid (HCl):
HCl(aq) ⇌ H+(aq) + Cl–(aq)
(ii) Nitric acid (HNO3
HNO3(aq) ⇌ + H+aq + NO–3(aq)
(iii) Sulphuric acid (H2SO4):
H2SO4(aq) ⇌ 2H+(aq) + SO2-4(aq)
(iv) Sodium hydroxide (NaOH):
NaOH(aq) ⇌ Na+(aq) + OH–(aq)
(v) Potassium hydroxide (KOH) :
KOH(aq)⇌ K+(aq) + OH–(aq)
(vi) Magnesium hydroxide [Mg(OH)2] :
Mg(OH)2(aq) ⇌ Mg2++(aq) + 2OH– (aq)
Q6. An aqueous solution ‘A’ turns phenolphthalein solution pink. On addition of an aqueous solution ‘B’ to ‘A’ the pink colour disappears. The following statement is true for solution ‘A’ and ‘B’:
(a) A is strongly basic and B is a weak base.
(b) A is strongly acidic and B is a weak acid.
(c) A has pH greater than 7 and B has pH less than 7.
(d) A has pH less than 7 and B has pH greater than 7
(c) As the aqueous solution of A turns phenolphthalein solution pink, hence A is basic in nature. On adding an acidic solution, the pink colour will disappear. Hence, B is an acid.
Q7. What happens when nitric acid is added to the eggshells?
Eggshells contain calcium carbonate. Calcium carbonate reacts with nitric acid to form calcium nitrate and carbon dioxide gas.
CaCO3 (s) + HNO3 (aq) → CaNO3 (aq) + CO2 (g) + H2O (l)
Q8. How would you distinguish between baking powder and washing soda by heating?
Baking soda (NaHCO3) liberates carbon dioxide gas on heating, confirmed by passing it in lime water. Whereas on heating washing soda Na2CO3.10H2O water of crystallisation is given out, the salt becomes anhydrous.
2 NaHCO3 → Na2CO3 + H2O + CO2
Na2CO3.10H2O → Na2CO3 + 10 H2O
Q9. Salt A is commonly used in bakery products on heating gets converted into another salt B, which is used to remove the hardness of water, and a gas C is evolved. The gas C, when passed through lime water, turns it milky. Identify A, B and C.
Baking powder is the salt used in bakery products that give sodium carbonate, and carbon dioxide gas on heating.
2 NaHCO3 → Na2CO3 + H2O + CO2
Sodium carbonate is used to remove the hardness of the water.
Carbon dioxide turns lime water milky due to the formation of insoluble calcium carbonate.
CO2 + Ca(OH)2 → CaCO3 + H2O.
Thus, A is sodium bicarbonate (Baking powder), B is sodium carbonate, and C is carbon dioxide gas.
Q10. In one of the industrial processes used to manufacture sodium hydroxide, a gas X is formed as a byproduct. The gas X reacts with lime water to give a compound Y used as a bleaching agent in the chemical industry. Identify X and Y giving the chemical equation of the reactions involved.
Sodium chloride is used to manufacture sodium hydroxide, called the chloralkali process. In this process, chlorine and hydrogen gas are formed as byproducts and sodium hydroxide.
2 NaCl (aq) + 2H2O (l) → 2 NaOH (aq) + Cl2 (g) + H2 (g)
Chlorine gas gives bleaching power when it reacts with lime water and is used as a bleaching agent in chemical industries.
Ca(OH)2 + Cl2 ⟶ CaOCl2 + H2O.
Therefore, the gas X is chlorine. Compound Y is calcium oxychloride, commonly known as bleaching powder and is used as a bleaching agent in chemical industries.