Which of the following sets comprises chemical changes ?

The correct answer option is: Rusting of iron, burning of paper, digestion of food.

To determine which set comprises chemical changes, let us understand the difference between physical and chemical changes:

1. Physical Change: A change in which no new substance is formed. The chemical composition remains the same, and the change is usually reversible (e.g., changes of state like boiling or physical modifications like shredding).

2. Chemical Change: A change in which one or more new substances with entirely different chemical properties are formed. These changes are usually irreversible.

Let us analyze the processes given in the options:

- Rusting of iron: Iron reacts with oxygen and moisture to form iron oxide (rust), which is a new chemical compound. Hence, it is a chemical change.
- Burning of paper: Paper reacts with oxygen to form ash, carbon dioxide, and water vapor. New chemical substances are formed, so it is a chemical change.
- Digestion of food: Complex food molecules are broken down into simpler chemical substances by enzymes and acids. New substances are formed, making it a chemical change.

Let us also look at why the other options are incorrect:

- Boiling of water, breaking of glass, dissolving salt/sugar in water, and shredding of paper are all physical changes because no new chemical substances are created.

Therefore, the set that comprises only chemical changes is Rusting of iron, burning of paper, digestion of food.