Answers
Assignment No – 4
Acids, Bases and Salts

Ans 31. Mixing of water and acid is a highly exothermic process. When water is added to acid, due to small amount of water, evolved heat will change it to vapour and acid will spill out and may cause injury. So, it is advised to mix acid to water and not water to acid.

Ans 32. Baking powder is a mixture of sodium bicarbonate and tartaric acid. This acid neutralizes sodium carbonate formed on decomposition of baking soda. If tartaric acid is not present in baking powder, the food materials such as cake will taste bitter due to the presence of sodium carbonate.

Ans 33. The syrupy liquid was concentrated sulphuric acid. On adding water, large amount of heat is released. For this reason, the tube cracked and the acid produced blisters on the skin.

Ans 34. The compound X is baking soda, a constituent of baking powder. Its chemical name is sodium hydrogen carbonate and its formula is NaHCO3.  On heating this compound gives CO2 gas. 2 NaHCO3 (s) → Na2CO3 + H2O + CO2

Ans 35. In the dry state, hydrogen chloride (HCl) does not release H+ ions. Therefore, it cannot behave as an acid. When dissolved in water, it forms hydrochloric acid. It dissociates to give H+ ion in solution and behaves as an acid.
HCl + H2O → H3O+ (aq) + Cl- (aq)

    
Ans 36. Curd and other sour substances contain acids. Therefore, when they are kept in brass and copper vessels, the metal reacts with the acid to liberate hydrogen gas and harmful products, thereby spoiling the food.


 Ans 37. Hydrogen gas is usually liberated when an acid reacts with a metal.


Take few pieces of zinc granules and add 5 ml of dilute H2SO4. Shake it and pass the gas produced into a soap solution. The bubbles of the soap solution are formed. These soap bubbles contain hydrogen gas.
Zn + H2SO4 → ZnSO4 + H2 ↑
We can test the evolved hydrogen gas by its burning with a pop sound when a candle is brought near the soap bubbles.

Ans 38. CaCO3  +  2 HCl  →  CaCl2  +  CO2  +  H2O

Ans 39. The concentration of hydroxide ions (OH−) would increase when excess base is dissolved in a solution of sodium hydroxide.


Ans 40. If the soil is acidic and improper for cultivation, then to increase the basicity of soil, the farmer would treat the soil with quick lime or slaked lime or chalk.