Which of the following processes are examples of chemical changes rather than purely physical changes: (A) cooking of food, (B) digestion of food, (C) freezing of water, (D) heating water?

Introduction / Context:
In everyday life, materials undergo many changes. Some of these changes are physical, where the substance retains its chemical identity, while others are chemical, where new substances are formed. Being able to distinguish between physical and chemical changes is a fundamental skill in introductory chemistry. This question presents four familiar processes and asks you to identify which ones are chemical changes: cooking food, digestion of food, freezing water, and heating water.

Given Data / Assumptions:

• Cooking of food involves heating and often complex reactions in ingredients.
• Digestion of food occurs in the body with the help of enzymes and acids.
• Freezing of water changes liquid water to solid ice without altering its chemical formula.
• Heating water raises its temperature or converts it to steam but does not change H2O into a different substance.
• Chemical changes produce new substances; physical changes alter state or appearance without changing composition.

Concept / Approach:
A chemical change is characterised by the formation of one or more new substances with different chemical properties and composition from the starting materials. Common signs include colour change, formation of gas, formation of a precipitate, and changes that are not easily reversible. Physical changes affect state, shape, or size but do not change the basic chemical identity. Cooking and digestion involve breaking and forming chemical bonds in food molecules, leading to new substances. Freezing and heating water change its state and temperature but keep the molecule H2O unchanged, so they are physical changes.

Step-by-Step Solution:
Step 1: Analyse cooking of food. When food is cooked, proteins denature, sugars caramelise, and many chemical reactions occur, creating new flavours and textures. This is a chemical change. Step 2: Examine digestion of food. In the digestive system, enzymes and acids break large biomolecules like proteins, fats, and carbohydrates into smaller ones, such as amino acids and simple sugars. New substances are formed, so digestion is a chemical change. Step 3: Consider freezing of water. When water freezes, it changes from liquid to solid, but each molecule remains H2O. This is a change of state only, so it is a physical change. Step 4: Consider heating water. Heating simply raises the temperature or may convert liquid to vapour, but H2O remains chemically the same. This is also a physical change. Step 5: Conclude that only processes A and B, cooking and digestion of food, are chemical changes, while C and D are physical changes.

Verification / Alternative check:
Think about reversibility and chemical identity. Ice can be melted back to water and water vapour can condense back to liquid without forming new substances, which is typical of physical changes. In contrast, once food is cooked, you cannot fully restore the raw ingredients because chemical changes have occurred. Similarly, once food is digested, the original food molecules no longer exist; they have been converted into smaller molecules absorbed by the body. These observations support the classification of A and B as chemical changes and C and D as physical changes.

Why Other Options Are Wrong:
Option B incorrectly includes freezing of water as a chemical change, but freezing does not produce a new substance. Option C lists only C and D, both of which are physical changes, so it fails to identify the actual chemical changes. Option D claims that all four processes are chemical changes, which is incorrect for freezing and heating water. Option E says that only cooking is a chemical change and ignores digestion, which is clearly a chemical process as well. Therefore, these options do not correctly separate chemical and physical changes.

Common Pitfalls:
A frequent confusion arises because both chemical and physical changes can involve heat, so some learners assume that any change involving heating must be chemical. Another mistake is to think that changes of state, such as freezing, must be chemical because the appearance changes. To avoid these errors, always ask whether the basic chemical substance before and after the process is the same. If H2O remains H2O, the change is physical. If new substances with different compositions are formed, the change is chemical.

Final Answer:
Among the listed processes, the chemical changes are A and B only (cooking of food and digestion of food).