Chapter 3: Physical and Chemical Changes

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Q
Physical and Chemical Changes

Question:

Classify the changes involved in the following processes as physical or chemical changes:

(a) Photosynthesis

(b) Dissolving sugar in water

(c) Burning of coal

(d) Melting of wax

(e) Beating aluminium to make aluminium foil

(f ) Digestion of food

Answer:

(a) Photosynthesis is the process through which plants convert light energy into chemical energy. It involves a chemical reaction in which water combines with carbon dioxide to produce glucose and oxygen. Since in this process, new chemical species are produced so it is classified as a chemical change.

(b) Dissolving sugar in water is a physical change because sugar molecules are dispersed in the water but the individual sugar molecules remain unchanged. 

(c) Burning of coal is a chemical change because the chemical composition of the coal changes when it is burned to form carbon dioxide along with heat and light.

(d) Melting of wax is a physical change because the physical state of wax changes to liquid while the chemical composition of the wax remains the same.

(e) When we beat the aluminium metal to form aluminium foil, we change its physical appearance while its chemical composition remains the same. Hence, it is a physical change.

(f) Digestion of food is a chemical change because the larger molecules break down into smaller molecules with the help of the salivary gland to form a new substance. 


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