A. What are reversible and irreversible changes? Give one example for each. [2]
B. What do you observe when a little amount of curd is added to a bowl containing warm milk and kept for five hours? [2]
C. Why is evaporation faster in the case of a china dish than a test tube? [3]
D. A burning candle shows both physical and chemical changes. Explain. [3]
B. What do you observe when a little amount of curd is added to a bowl containing warm milk and kept for five hours? [2]
C. Why is evaporation faster in the case of a china dish than a test tube? [3]
D. A burning candle shows both physical and chemical changes. Explain. [3]
A. Reversible change : When a change in a substance can be reversed by changing the conditions, it is said to be a reversible change. Example : Melting of ghee or wax.
Irreversible change : When a substance can not be brought back to its original state after a change, it is said to be an irreversible change. Example : Souring of milk.
B. When a little amount of curd is added to a bowl containing warm milk and kept for five hours, a permanent change will occur. The milk will change to curd.
C. The phenomenon of change of a liquid into vapours at any temperature below its boiling point is called evaporation. It is a surface phenomena i.e. it takes place from the surface only. China dish have a greater surface area than the test tube. So, the rate of evaporation would be higher in it.
D.
When we burn a candle, the melting of solid wax is a physical change as melting involves just change in state and no new substance is formed. At the same time wax on burning produces water vapours and carbon dioxide which escape into the atmosphere which are new substances with new properties. It is a chemical change.
Hence, burning of a candle shows both physical and chemical changes.
Irreversible change : When a substance can not be brought back to its original state after a change, it is said to be an irreversible change. Example : Souring of milk.
B. When a little amount of curd is added to a bowl containing warm milk and kept for five hours, a permanent change will occur. The milk will change to curd.
C. The phenomenon of change of a liquid into vapours at any temperature below its boiling point is called evaporation. It is a surface phenomena i.e. it takes place from the surface only. China dish have a greater surface area than the test tube. So, the rate of evaporation would be higher in it.
D.
When we burn a candle, the melting of solid wax is a physical change as melting involves just change in state and no new substance is formed. At the same time wax on burning produces water vapours and carbon dioxide which escape into the atmosphere which are new substances with new properties. It is a chemical change.
Hence, burning of a candle shows both physical and chemical changes.
