Solutions of Acids, Bases and Salts (Page No - 100) - Chemistry Lakhmir Singh, Class 10 | Extra Documents, Videos & Tests for Class 10 PDF Download

Question 69:
P and Q are aqueous solutions of sodium chloride and sodium hydroxide, respectively. Which of these will turn :
(a) blue litmus red ?
(b) red litmus blue ?
Solution :
(a) No solution will turn blue litmus to red.
(b) Solution Q (sodium hydroxide) will turn red litmus blue.

Question 70:
The metal salt A is blue in colour. When salt A is heated strongly over a burner, then a substance B is eliminated and a white powder C is left behind. When a few drops of a liquid D are added to powder C, it becomes blue again. What could be A, B, C and D ?
Solution :
A is copper sulphate pentahydrate, CuSO₄.5H₂O
B is water, H₂O
C is anhydrous copper sulphate, CuSO₄
D is water, H₂O

Question 71:
When the concentrated aqueous solution of substance X is electrolysed, then NaOH, Cl₂ and H₂ are produced. Name the substance X. What is the special name of this process ?
Solution :
X is Sodium chloride. The process is called Chlor-alkali process.

Question 72:
Consider the following substances :
NaCl, Ca(OH)₂, NaHCO₃, NH₃, Na₂CO₃, H₂O, Cl₂, CO₂, CaSO₄.2H₂O, 2CaSO₄.H₂O, CaOCl₂
(a) Which two substances combine to form bleaching powder ?
(b) Which four substances are utilised in the production of washing soda ?
(c) Which compound represents plaster of Paris ?
(d) Which compound is a part of baking powder ?
(e) Which compound is used as an antacid ?
Solution :
(a) Ca(OH)₂ and Cl₂
(b) NaCl, NH₃, H₂O and CO₂
(c) 2CaSO₄.H₂O
(d) NaHCO₃
(e) NaHCO₃

Question 73:
Give one example each of a salt which gives an aqueous solution having :
(a) pH less than 7
(b) pH equal to 7
(c) pH more than 7
Solution :
(a) Ammonium chloride, NH₄Cl
(b) Sodium chloride, NaCl
(c) Sodium carbonate, Na₂CO₃

Question 74:
A compound X which is prepared from gypsum has the property of hardening when mixed with a proper quantity of water.
(a) Identify the compound X ,
(b) Write the chemical equation for its preparation
(c) For what purpose is it used in hospitals ?
Solution :
(a) Plaster of Paris.

(c) POP is used in hospitals for setting fractured bones in the right position to ensure correct healing.

Question 75:
Consider the following salts :
Na₂CO₃, NaCl, NH₄Cl, CH₃COONa, K₂SO₄, (NH₄)₂SO₄ Which of these salts will give :
(a) acidic solutions ?
(b) neutral solutions ?
(c) basic solutions (or alkaline solutions) ?
Solution :

Question 76:
A white powdery substance having strong smell of chlorine is used for disinfecting drinking water supply at waterworks. Identify the substance. Give its chemical name and write the chemical reaction for its preparation.
Solution :
Bleaching powder, CaOCl₂.

Question 77:
A salt X when dissolved in distilled water gives a clear solution which turns red litmus blue. Explain the phenomenon.
Solution :
Salt X is like sodium carbonate, Na₂CO₃, which is made from
a strong base and a weak acid. On dissolving in water, salt X gets hydrolysed
to form some strong base and some weak acid. The strong base thus formed
makes the solution alkaline which turns red litmus blue.

Question 78:
A person found that the cake prepared by him is hard and small in size. Which ingredient has he forgotten to add that would have caused the cake to rise and become light ? Explain your answer.
Solution :
Baking
powder; When baking powder mixes with water, then sodium hydrogen carbonate
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 79:
A white chemical compound becomes hard on mixing with proper quantity of water. It is also used in surgery to maintain joints in a fixed position. Name the chemical compound.
Solution :
Plaster of Paris.

Question 80:
When chlorine and sodium hydroxide being produced during the electrolysis of brine are allowed to mix, a new chemical is formed. Name this chemical and write its uses.
Solution :
Sodium hypochlorite, NaClO; used in making household bleaches and for bleaching fabrics.

Question 81:
Write the name and formula of one salt each which contains :
(a) two molecules of water of crystallisation
(b) five molecules of water of crystallisation
(c) ten molecules of water of crystallisation
Solution :
(a) Gypsum – CaSO₄.2H₂O
(b) Copper sulphate crystals – CuSO₄.5H₂O
(c) Sodium carbonate crystals – Na₂CO₃.10H₂O

Question 82:
How many molecules of water of crystallisation (per formula unit) are present in :
(a) copper sulphate crystals ?
(b) washing soda ?
(c) gypsum ?
Solution :
(a) 5.
(b) 10.
(c) 2.