Chemical Reactions and Equations

NCERT InText Solution

Question: 1: Why should a magnesium ribbon be cleaned before burning in air?

Answer: Mangesium ribbon is cleaned before burning to remove the layer of magnesium oxide which is formed over magnesium ribbon while stored. This magnesium oxide prevents further reaction of magnesium with oxygen.

Question: 2: Write the balanced equation for the following chemical reactions.

(i) Hydrogen + Chlorine → Hydrogen chloride

Answer:

(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride

Answer:

(iii) Sodium + Water → Sodium hydroxide + Hydrogen

Answer:

Question: 3: Write a balanced chemical equation with state of symbols for the following

(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.

Answer:

(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Answer:

Question: 4: A solution of a substance ‘X’ is used for white washing.

(i) Name the substance ‘X’ and write its formula

Answer: Name of substance ‘X’ : Quick lime (Calcium Oxide)

Formula of substance ‘X’: CaO

(ii) Write the reaction of the substance ‘X’ named in (i) above with water.

Answer:

Question: 5: Why is the amount of gas collected in one of the test tubes in Activity 1.7 double the amount collected in the other? Name the gas.

Answer: (In activity 1.7 given in the text book of NCERT, hydrolysis of water done.)

When water is put under hydrolysis, i.e. if electric current passed through water, water dissociates into hydrogen and oxygen which are constituents of water (H₂O).

In the given activity, hydrogen is collected in one tube and oxygen is collected in another tube. Hydrogen collected in one tube has double amount than oxygen collected in another tube. Because in water hydrogen and oxygen are present in the ration of 2:1 i.e. one water molecule contains 2 molecule of hydrogen and one molecule of oxygen.

The name of gas is hydrogen which is present in double amount.

Question: 6: Why does the colour of copper sulphate solution change when and iron nail is dipped in it?

Answer: When iron nail is dipped in the solution of copper sulphate, copper is displaced by iron and forms ferrous sulphate. Thus, the blue colour of copper sulphate changes into green because of formation of ferrous sulphate.

Question: 7: Give an example of a double displacement reaction other than the one given in Activity 1.10.

Answer: When silver nitrate reacts with sodium chloride it gives silver chloride and sodium nitrate.

In this reaction, nitrate ion of silver nitrate exchanges chloride ion from sodium chloride and produce silver chloride and sodium nitrate. This is an example of double displacement reaction.

Question: 8: Identify the substances that are oxidized and the substances that are reduced in the following reactions.

Answer: In this reaction Sodium gains oxygen and forms sodium oxide. Because of gain of oxygen sodium is oxidized.

On the other hand oxygen is reduced here.

Answer: In this reaction copper oxide (CuO) loses oxygen and forms copper, hence copper oxide reduced to copper.

On the other hand hydrogen gains oxygen and forms water (H₂O), hence hydrogen oxidized to water.