Question

Which of the following are chemical changes?
(a) Growth of a plant
(b) Rusting of iron
(c) Mixing of iron filings and sand
(d) Cooking of food
(e) Digestion of food
(f) Freezing of water
(g) Burning of a candle

Answer 1

Growth of a plant, rusting of iron, cooking of food, digestion of food, burning of a candle are chemical changes, because here the chemical composition of substance changes.
The following are chemical changes:

(b) Rusting of iron – This is a chemical change because iron reacts with oxygen in the air to form iron oxide (rust), a new substance with different properties.

(d) Cooking of food – This involves chemical reactions that change the molecular structure of the food, resulting in new substances.

(e) Digestion of food – In digestion, food is broken down by enzymes into simpler substances, which is a chemical process.

(g) Burning of a candle – When a candle burns, the wax (a hydrocarbon) reacts with oxygen in the air, producing carbon dioxide, water, and heat. This is a chemical change.

Not chemical changes:

(a) Growth of a plant – This involves a series of biochemical processes, but it's not considered a simple chemical change, as it involves the synthesis of new compounds through photosynthesis.

(c) Mixing of iron filings and sand – This is a physical change because no new substances are formed, and the iron filings and sand can be easily separated.

(f) Freezing of water – This is a physical change because water changes from liquid to solid, but its chemical composition remains the same.