NCERT 6th Class (CBSE) Science: Changes Around Us

Question: One way changes are classified is reversible and irreversible Explain with examples.

Answer: Reversible:

1. A change that can be carried out in the reverse direction is known as reversible change.
2. The internal structure of the substances involved do not change in reversible changes.
3. They can easily be reversed.
4. For Eg: (i) The shape of the balloon come back to it original size when we stop squeezing.
   (ii) The molten wax becomes solid again when cooled.

Irreversible:

1. A change that can be carried out in the reverse direction is known as irreversible change.
2. The internal structure of the substances involved change in irreversible changes.
3. They cannot be reversed easily.
4. For Eg: (i) Bursting of a balloon cannot be reversed.
   (ii) Cooking of food or burning of wax cannot be reversed.

Question: When a candle burns, both physical and chemical changes occur. Explain.

Answer: When a candle burns the wax is solid, wax. When solid wax is converted into liquid wax and the liquid wax is converted into wax vapour it is physical change. But when the wax vapour is converted into Carbon dioxide + water + energy (Co₂ + H₂o + energy)

Question: What is a chemical reaction?

Answer: When the molecules of the substances change to from a new substance chemical reaction takes place. Before reaction they are known as reactants and after reaction they are known as products. For Eg- vinegar reacts salt farm carbon dioxide, iron reacts with water to from rust.

Question: Give one example to that when substance are mixed, it may result in a chemical change or a physical change depending on their conditions.

Answer: It we mix iron and sulfur, no new substance has formed. They can be separated by a magnet and if we heat them new substance, iron sulphaite is formed.

Question: Tearing of paper is irreversible. It is therefore a chemical change. Do you agree? Give reasons.

Answer: Yes tearing of paper is irreversible but not a chemical change as we cannot get the original paper back in its original for and a new substance is not formed.

Question: A potato remains a potato even after cooked. Therefore, Cooking  is a physical change. Do you agree? give reasons.

Answer: No, it is not a physical change as when it is cooked changes take pace in molecules. Then the colour, softness, taste changes it is a chemical change.