Which of the following are chemical changes?  I. cooking of food II. digestion of food III. freezing of water IV. water is heated up

Analyze the question: You need to identify which processes are chemical changes, meaning a new substance is formed, and which are only physical changes.

Concept:

• Chemical change: A new substance with different properties is formed. It is usually irreversible, and often involves a change in color, a gas, heat or light, or a new smell.
• Physical change: Only the state, shape, or size changes. No new substance is formed, and it is usually easy to reverse.

Step-by-step solution:

| Process | Type | Reason | |---|---|---| | I. Cooking of food | Chemical | Heat triggers reactions, such as proteins denaturing and starch breaking down. The food gets a new taste, texture, and color, and you cannot "uncook" it. | | II. Digestion of food | Chemical | Enzymes break large molecules into smaller ones, for example starch into glucose and proteins into amino acids. New substances are formed. | | III. Freezing of water | Physical | Liquid water becomes ice (H₂O → H₂O). It is the same substance in a different state, and melting reverses it. | | IV. Heating water | Physical | Warming or boiling water only raises the temperature or changes its state to steam. It is still H₂O. |

So only I and II are chemical changes.

Answer: A. I and II

Exam tip: Ask yourself, "Is a new substance formed?" Changes of state (melting, freezing, boiling, condensation, evaporation) are always physical. Cooking, digestion, rusting, burning, and rotting are chemical.