Question

Which of the following are chemical changes?

1. Growth of a plant
2. Rusting of iron
3. Mixing of iron filings and sand
4. Cooking of food
5. Digestion of food
6. Freezing of water
7. Burning of a candle

Answer 1

Chemical Change

• A chemical change is a change in which the molecular composition of the substance is completely altered and a new product is formed.
• The change is irreversible and permanent.

Physical Change

• A physical change is a change in which the physical properties of a substance are changed.
• It does not create a new product.
• This type of change is reversible and not permanent.

(a) Growth of a plant

• The growth of a plant is a chemical change as new cells are formed.
• The plant cannot go back to its previous state after growth.
• Hence, the growth of a plant is a chemical change.

(b) Rusting of iron

• The rusting of iron is a chemical change.
• When iron is exposed to water and oxygen, it forms rust.
• The rust formed is a new product and iron cannot be formed from rust.
• Hence, rusting of iron is a chemical change.

(c) Mixing of iron fillings and sand

• Mixing of iron fillings and sand is a physical change.
• Iron fillings can be separated from the sand by using a magnet.
• The chemical composition of neither the iron nor the sand is changed in this process.
• Hence, the mixing of iron fillings and sand is a physical change.

(d) Cooking food

• Cooking food is a chemical change.
• During cooking the chemical composition of the reactants are changed.
• New products are formed after cooking.
• Hence, cooking food is a chemical change.

(e) Digestion of food

• Digestion of food is a chemical change.
• During the digestion of food, the food is broken down into simpler substances by the action of enzymes.
• New products are formed after digestion.
• Hence, the digestion of food is a chemical change.

(f) Freezing of water

• Freezing of water is a physical change.
• During the freezing of water, the physical state of water changes from liquid to solid.
• The chemical composition of water does not change during freezing.
• Hence, the freezing of water is a physical change.

(g) Burning of candle

• Burning of a candle is a chemical change.
• During the burning of a candle, the wax is burnt and it releases carbon dioxide.
• The molecular composition of the candle is changed in the process and a new product is formed.
• Hence, the burning of a candle is a chemical change.

Therefore, (a) Growth of a plant, (b) Rusting of iron, (d) Cooking food, (e) Digestion of food, and (g) Burning of a candle are chemical changes.