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

Hint: In a chemical change, one substance combines with another substance to form a whole new product. Chemical reactions are often irreversible except by further chemical reactions.

Complete step by step answer:
Chemical changes occur when a substance reacts/combines with another substance to form a new product, or one substance can chemically decompose into two or more substances. Chemical reactions are often irreversible except by further chemical reactions, i.e. once the reaction moves forward, it can’t be brought back by any means.
Absorption and evolution of energy takes place in chemical reactions. The reactions in which energy is evolved are called exothermic reactions while the reactions in which the energy is absorbed are called endothermic reactions.
Now, in the given question:
A. Growth of a plant
Growth of a plant is an irreversible process. Also, the process of photosynthesis is a chemical change as it involves the formation of a new product. Hence, the growth of a plant is a chemical change.

B. Rusting of iron
Rusting of iron is an irreversible process which involves the formation of a new product. Hence, it is a chemical process

C. Mixing of iron filings and sand
Mixing of iron filings and sand is a reversible process as they can easily be separated by using a magnet. Thus, it is a physical change.

D. Cooking of food
Cooking of food is an irreversible process with certain reactions taking place which form a different product. Hence, it is a chemical change.

E. Digestion of food
It is a chemical change as the process is irreversible and the digestion of food produces different products than reactants.

F. Freezing of water
It is a physical change as the process is reversible. Ice can be melted to obtain water.

G. Burning of candle
It is a chemical change. Wax is made up of hydrocarbons which melts and becomes a gas and the hydrocarbons are broken down.

Hence, the correct options are (A), (B), (D), (E), (G).

Note: Remember that the physical changes are the exact opposite of chemical changes. They are reversible changes with no involvement of energy.