NCERT Class X Science Class: Chapter – 1. Chemical Reactions and Equations – Part-8 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 37:

Zinc liberates hydrogen gas when reacted with dilute hydrochloric acid, whereas copper does not. Explain why?


When a metal reacts with an acid it liberates hydrogen gas. In the activity series of metals, the metals which are above hydrogen can displace it, while the metals which are below hydrogen cannot.

Zinc is above hydrogen, so it displaces hydrogen from hydrochloric acid to form zinc chloride and hydrogen gas.

Copper is below hydrogen so it cannot displace it from hydrochloric acid.

Question 38:

A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.

  1. Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved
  2. Name the black substance formed and give its chemical formula


  1. Metals such as silver when attacked by substances around it such as moisture, acids, gases etc. , are said to corrode and this phenomenon is called corrosion
  2. The black substance is formed because silver reacts with present in air. It forms thin black coating of silver sulphide

Long Answer Questions

Question 39:

On heating blue coloured powder of copper (II) nitrate in a boiling tube, copper oxide (black) , oxygen gas and a brown gas X is formed

  1. Write a balanced chemical equation of the reaction.
  2. Identity the brown gas X evolved.
  3. Identity the type of reaction.
  4. What could be the pH range of aqueous solution of the gas X?


  1. Balanced chemical equation:

  1. The brown gas X evolved is nitrogen dioxide
  2. This is a decomposition reaction
  3. Nitrogen dioxide dissolves in water to form acidic solution because it is an oxide of non-metal. Therefore, pH of this solution is less than 7

Question 40:

Give the characteristic tests for the following gases


  1. Carbon dioxide gas turns limewater milky when passed through it due to the formation of insoluble calcium carbonate.

  1. Sulphur dioxide gas when passed through acidic potassium permanganate solution (purple in colour) turns it colour less because is a strong reducing agent

Sulphur dioxide gas when passed through acidic dichromate solution (orange in colour) turns it to green because sulphur dioxide is a strong reducing agent.

  1. The evolution of oxygen gas during a reaction can be confirmed by bringing a burning candle near the mouth of the test tube containing the reaction mixture. The intensity of the flame increases because oxygen supports burning.
  2. Hydrogen gas burns with a pop sound when a burning candle is brought near it.