Which is the main difference between physical changes and chemical changes?

• A. Physical changes do not alter the chemical composition of a substance, while chemical changes result in the formation of new substances.
• B. Physical changes only occur in solids, while chemical changes occur in liquids and gases.
• C. Physical changes always require heat, while chemical changes do not.
• D. Physical changes are irreversible, while chemical changes are always reversible.

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the definition of a physical change. A physical change affects the form or appearance of a substance but does not change its chemical composition. Examples include changes in state (like melting or freezing) and shape.
2. Step 2: Understand the definition of a chemical change. A chemical change results in the formation of one or more new substances with different chemical compositions and properties. Examples include combustion, rusting, and digestion.
3. Step 3: Compare the two types of changes based on whether the chemical composition is altered. Physical changes do not alter the chemical identity, while chemical changes do.
4. Step 4: Evaluate the other options given in the problem. For example, physical changes can occur in any state of matter, not just solids; heat may or may not be involved in either type of change; and reversibility depends on the specific process, not the type of change alone.
5. Step 5: Conclude that the main difference is that physical changes do not alter the chemical composition of a substance, whereas chemical changes result in the formation of new substances.