NCERT Class X Science Solutions: Chapter 1 – Chemical Reactions and Equations Part 4 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Question 17:

A shiny brown-coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.


‘X’ is copper () and the black-coloured compound formed is copper oxide () . The equation of the reaction involved on heating copper is given below.

Question 18:

Why do we apply paint on iron articles?


Iron articles are painted because it prevents them from rusting. When painted, the contact of iron articles from moisture and air is cut off. Hence, rusting is prevented their presence is essential for rusting to take place.

Question 19:

Oil and fat containing food items are flushed with nitrogen. Why?


Nitrogen is an inert gas and does not easily react with these substances. On the other hand, oxygen reacts with food substances and makes them rancid. Thus, bags used in packing food items are flushed with nitrogen gas to remove oxygen inside the pack. When oxygen is not present inside the pack, rancidity of oil and fat containing food items is avoided.

Question 20:

Explain the following terms with one example each.

  1. Corrosion
  2. Rancidity


  1. Corrosion:

Corrosion is defined as a process where materials, usually metals, deteriorate as a result of a chemical reaction with air, moisture, chemicals, etc. For example, iron, in the presence of moisture, reacts with oxygen to form hydrated iron oxide.

This hydrated iron oxide is rust.

This Diagram Shows Hydrated Iron Oxide is Rust
  1. Rancidity:

The process of oxidation of fats and oils that can be easily noticed by the change in taste and smell is known as rancidity.

For example, the taste and smell of butter changes when kept for long. Rancidity can be avoided by:

  1. Storing food in air tight containers
  2. Storing food in refrigerators
  3. Adding antioxidants
  4. Storing food in an environment of nitrogen

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