Acids Bases and Salts NCERT Exemplar Problems

Short Answer Questions

Part 2

Question 37: Salt A commonly used in bakery products on heating gets converted into another salt B which itself is used for removal of 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.

Answer: Baking power which is a salt is used as bakery product which gives sodium carbonate and carbon dioxide gas on heating. Sodium carbonate is used to remove the hardness of water. Carbon dioxide turns lime water milky.

2NaHCO₃ + Heat ⇨ Na₂CO₃ + CO₂ + H₂O

Therefore

Salt A is sodium bicarbonate and used as baking powder.
Salt B is sodium carbonate, which is used to remove the hardness of water.
The C is carbon dioxide gas which turns lime water milky.

Question 38: How would you distinguish between baking powder and washing soda by heating?

Answer: The production of carbon dioxide is main characteristic of baking soda that makes it suitable for baking.

Baking soda gives carbon dioxide and water vapour on heating at even low temperature such as 100°C. The gas so formed turns lime water milky, which confirms the presence of carbon dioxide gas.

2NaHCO₃ + Heat ⇨ Na₂CO₃ + CO₂ + H₂O

When Washing soda (Na₂CO₃) is heated it does not produce carbon dioxide even at high temperature, such as 200°C or 300°C. However sodium carbonate gives sodium oxide and carbon dioxide when heated at about 1000°C.

Na₂CO₃.10H₂O + Heat ⇨ Na₂CO₃ + 10H₂O

The gas produced by heating of the samples can be checked by passing through lime water. If the lime water turns milky then the evolved gas is carbon dioxide and the heated substance is baking powder.

Question 39: In one of the industrial processes used for manufacture of sodium hydroxide, a gas X is formed as byproduct. The gas X reacts with lime water to give a compound Y which is used as a bleaching agent in chemical industry. Identify X and Y giving the chemical equation of the reactions involved.

Answer: Sodium chloride is used for manufacture of sodium hydroxide which is called Chlor-Alkali Process. In this process chlorine and hydrogen gas are formed as byproducts along with sodium hydroxide. Chlorine gas gives bleaching power when reacts with lime water and used as bleaching agent in chemical industries.

Ca(OH)₂ + Cl₂ ⇨ CaOCl₂ + H₂O

Therefore

The gas ‘X’ is chlorine.
Compound ‘Y’ is calcium oxychloride which is commonly known as bleaching powder and used as bleaching agent in chemical industries.

Question 40: Fill in the missing data in the following table

Answer:

Name of salt	Formula	Salt obtained from
Name of salt	Formula	Base	Acid
Ammonium chloride	NH₄Cl	NH₄OH	HCl
Copper sulphate	CuSO₄	Cu(OH)₂	H₂SO₄
Sodium chloride	NaCl	NaOH	HCl
Magnesium nitrate	Mg(NO₃ )₂	Mg(OH)₂	HNO₃
Potassium sulphate	K₂SO₄	KOH	H₂SO₄
Calcium nitrate	Ca(NO₃ )₂	Ca(OH)₂	HNO₃

Question 41: What are strong and weak acids? In the following list of acids, separate strong acids from weak acids. (Hydrochloric acid, citric acid, acetic acid, nitric acid, formic acid, sulphuric acid.)

Answer: Strong Acid – Strong acids ionize completely in their aqueous solution and produce maximum number of hydrogen ions. Mineral acids are generally strong acids. Examples: Hydrochloric acid, Nitric Acid, Sulphuric Acid

Weak Acid – Weak acids do not ionize completely in their aqueous solution. Organic acids are generally weak acids. Examples: Citric Acid, Acetic Acid, Formic Acid

Question 42: When zinc metal is treated with a dilute solution of a strong acid, a gas is evolved, which is utilised in the hydrogenation of oil. Name the gas evolved. Write the chemical equation of the reaction involved and also write a test to detect the gas formed.

Answer: Zinc metal gives hydrogen gas when it is treated with dilute sulphuric acid. Hydrogen gas is utilized in hydrogenation of oil. Therefore, the gas evolved is hydrogen.

Zn + H₂SO₄ ⇨ ZnSO₄ + H₂

Test for hydrogen gas: When a burning candle is brought near the hydrogen gas, it burns with pop sound which confirms the presence of hydrogen gas.