Solutions of Acids, Bases and Salts (Page No - 98) - Chemistry Lakhmir Singh, Class 10 | Extra Documents, Videos & Tests for Class 10 PDF Download

Question 37:
(a) What is meant by “water of crystallisation” in a substance ? Explain with an example.
(b) How would you show that blue copper sulphate crystals contain water of crystallisation ?
(c) Explain how anhydrous copper sulphate can be used to detect the presence of moisture (water) in a liquid.
Solution :
(a) The water molecules which form part of the structure of a crystal are called water of crystallization.
Example: CuSO₄.5H₂O
(b) The blue copper sulphate crystals contain water of crystallization as it is blue in colour.
(c) Anhydrous copper sulphate turns blue on adding water. This property of anhydrous copper sulphate is used to detect the presence of moisture in a liquid.

Question 38:
(a) What is the common name of sodium hydrogencarbonate ?
(b) What happens when a solution of sodium hydrogencarbonate is heated ? Write equation of the reaction involved.
(c) Explain why, sodium hydrogencarbonate is used as an antacid.
Solution :
(a) Baking soda.
(b) When a solution of sodium hydrogencarbonate is heated, then it decomposes to give sodium carbonate with the evolution of carbon dioxide gas.

(c) Sodium hydrogencarbonate is used as an antacid because it neutralises the excess acid present in the stomach and relieves indigestion.

Question 39:
(a) What will happen if heating is not controlled while preparing plaster of Paris ?
(b) Write an equation to show the reaction between plaster of Paris and water.
Solution :
(a) If heating is not controlled while preparing POP, then all the water of crystallisation of gypsum is eliminated and it turns into a dead burnt plaster.

Question 40:
(a) What happens when copper sulphate crystals are heated strongly ? Explain with the help of an equation.
(b) What happens when a few drops of water are added to anhydrous copper sulphate ? Explain with the
help of an equation.
Solution :
(a) On strong heating, blue copper sulphate crystals turn white.

(b) When water is added to anhydrous copper sulphate, it gets hydrated and turns blue. CuS0₄ + 5H₂O → CuS0₄.5H₂0

Question 41:
(a) Name two constituents of baking powder.
(b) How does baking powder differ from baking soda ?
(c) Explain the action of baking powder in the making of cake (or bread). Write equation of the reaction involved.
Solution :
(a)Sodium hydrogencarbonate and tartaric acid.
(b)Baking powder is a mixture of baking soda and tartaric acid whereas baking soda is only sodium hydrogencarbonate.
(c)When baking powder mixes with water, then sodium hydrogencarbonate reacts with tartaric acid to evolve carbon dioxide gas which gets trapped in the wet dough and bubbles out slowly making the cake soft and spongy.

Question 42:
(a) What is the chemical name of bleaching powder ?
(b) What is the chemical formula of bleaching powder ?
(c) What are the materials used for the preparation of bleaching powder ?
(d) State one use of bleaching powder (other than bleaching).
Solution :
(a) Calcium oxychloride.
(b) CaOCl₂
(c) Calcium hydroxide and chlorine.
(d) It is used for disinfecting drinking water supply.

Question 43:
What does a soda-acid type fire extinguisher contain ? How does it work ? Explain the working of a soda- acid fire extinguisher with the help of a labelled diagram.
Solution :
Working:
A soda-acid type fire extinguisher contains a solution of sodium hydrogencarbonate and sulphuric acid in separate containers in separate containers inside them. When the knob of the fire extinguisher is pressed, then sulphuric acid mixes with sodium hydrogencarbonate solution to produce carbon dioxide gas which forms a blanket around the burning substance and cuts off the supply of air to burning substance; this stops the process of burning and fire gets extinguished.

Question 44:
(a) Name a sodium compound used for softening hard water.
(b) Which compound of calcium is used for disinfecting drinking water supply ?
(c) Name a metal compound which has detergent properties (cleansing properties).
(d) Name one compound of calcium which is used for removing the colour of a coloured cloth.
(e) State a peculiar (or remarkable) property of plaster of Paris.
(f) Name the substance obtained by the action of chlorine on solid (dry) slaked lime.
Solution :
(a) Sodium
carbonate.
(b) Bleaching
powder.
(c) Sodium carbonate.
(d) Bleaching
powder.
(e) It sets into a
hard mass on mixing with proper quantity of water.
(f) Bleaching
powder.

Question 45:
(a) What is gypsum ? What happens when gypsum is heated to 100°C (373 K) ?
(b) Name a sodium compound which is used for making borax and glass.
(c) Name the compound which is used in hospitals for setting fractured bones.
(d) Which is the real bleaching agent present in bleaching powder ?
Solution :
(a) Gypsum is calcium sulphate dihydrate, CaSO₄.2H₂O. When gypsum is heated to a temperature of 100?C, it loses 3/4^th of its water of crystallisation and forms plaster of Paris.
(b) Sodium carbonate.
(c) Plaster of Paris.
(d) Chlorine.

Question 46:
(a) What is “baking powder”? How does it make the cake soft and spongy ?
(b) In addition to sodium hydrogencarbonate, baking powders contain a substance X. Name the substance X. What is the role of substance X in the baking powder ?
Solution :
(a) Baking powder is a mixture of baking soda and tartaric acid. When baking powder mixes with water, then sodium hydrogencarbonate reacts with tartaric acid to evolve carbon dioxide gas which gets trapped in the wet dough and bubbles out slowly making the cake soft and spongy.
(b) Substance X is tartaric acid. It can react with any sodium carbonate formed and neutralise it otherwise cakes and bread will taste bitter.

Question 47:
State two uses each of the following compounds :
(a) Sodium hydroxide
(b) Chlorine
(c) Hydrogen
(d) Hydrochloric acid
Solution :
(a) Sodium hydroxide:
(i) It is used for making soaps and detergents.
(ii) It is used in the manufacture of paper.
(b) Chlorine:
(i) It is used in the production of bleaching powder.
(ii) It is used in the production of hydrochloric acid.
(c) Hydrogen:
(i) It is used in the production of hydrochloric acid.
(ii) It is used in the hydrogenation of oils.
(d) Hydrochloric acid:
(i) It is used in medicines and cosmetics.
(ii) It is used in textile/dyeing and tanning industries.

Question 48:
(a) What is the common name of the compound CaOCl₂ ?
(b) Name the raw material used for the preparation of plaster of Paris.
(c) Which property of plaster of Paris is utilised in making casts for broken limbs in hospitals ?
(d) Explain why chlorine is used for sterilising drinking water supply.
Solution :
(a) Bleaching powder.
(b) Gypsum.
(c) It sets into a hard mass in about 30 mins.
(d) Chlorine is used for sterilising drinking water supply because it is a disinfectant which kills germs or bacteria.

Question 49:
(a) What happens when a concentrated solution of sodium chloride (brine) is electrolysed ? Write the equation
of the reaction involved.
(b) Why is the electrolysis of a concentrated solution of sodium chloride known as chlor-alkali process ?
(c) Name three products of the chlor-alkali process. State two uses of each of these products.
Solution :
(a) When a concentrated solution of sodium chloride is electrolysed, it decomposes to form sodium hydroxide, chlorine and hydrogen.

(b) Because of the products formed: Chlor for chlorine and alkali for sodium hydroxide.
(c) Sodium hydroxide, chlorine and hydrogen.
Uses of Sodium hydroxide:
(i) It is used for making soaps and detergents.
(ii) It is used in the manufacture of paper.
Uses of chlorine:
(i) It is used in the production of bleaching powder.
(ii) It is used in the production of hydrochloric acid.
Uses of hydrogen:
(i) It is used in the production of hydrochloric acid.
(ii) It is used in the hydrogenation of oils.

Question 50:
(a) Describe how washing soda is produced starting from sodium chloride (common salt). Write equations
of all the reactions involved.
(b) State whether an aqueous solution of washing soda is acidic or alkaline ? Give reason for your answer.
(c) What is meant by saying that washing soda has detergent properties ?
(d) Give two important uses of washing soda (or sodium carbonate).
Solution :
(a) Production of washing soda: Washing soda is produced from sodium chloride (or common salt) in the following three steps:
(i) A cold and concentrated solution of sodium chloride (called brine) is reacted with ammonia and carbon dioxide to obtain sodium hydrogen carbonate :

Sodium hydrogen carbonate formed is only slightly soluble in water, so it precipitates out as a solid.
(ii) Sodium hydrogen carbonate is separated by filtration, dried and heated. On heating, sodium hydrogen carbonate decomposes to form sodium carbonate:

The anhydrous sodium carbonate obtained here is called soda ash.
(iii) Anhydrous sodium carbonate (soda ash) is dissolved in water and recrystallised to get washing soda crystals containing 10 molecules of water of crystallisation :

(b) An aqueous solution of washing soda is alkaline because it turns red litmus to blue.
(c) Washing soda has detergent properties because it can remove dirt and grease from dirty clothes.
(d) (i) It is used as cleansing agent for domestic purposes.
(ii) It is used for removing permanent hardness of water.