A difference between chemical and physical change is that in a chemical change

1. Analyze the question

The question asks for the fundamental distinction between a physical change and a chemical change. You need to identify which of the provided options is a unique characteristic of chemical changes that does not apply to physical changes.

2. Explain the concept

• Physical Change: This is a change in the state or appearance of matter without changing its chemical composition. No new substance is created. Examples include melting ice (solid to liquid), dissolving sugar in water, or tearing paper. These processes are often easily reversible.
• Chemical Change: This involves a chemical reaction where the original substances (reactants) are transformed into entirely new substances (products) with different chemical properties. This process is usually permanent and difficult to reverse. Examples include rusting of iron, burning wood, or the souring of milk.

3. Show the solution

Let’s evaluate the options:

• A (Oxygen is consumed): While oxygen is consumed in combustion (a chemical change), it is not consumed in all chemical changes (e.g., the decomposition of calcium carbonate).
• B (Heat is supplied): Heat can be supplied in both physical changes (like boiling water) and chemical changes (like thermal decomposition). Therefore, this is not a defining difference.
• C (The reversible process occurs): Physical changes are typically reversible, but some chemical changes are also reversible (e.g., the Haber process). This is not the primary distinction.
• D (A new substance is formed): This is the hallmark of a chemical change. In a chemical reaction, atoms are rearranged to form new chemical bonds, resulting in a product that is chemically different from the starting material. This never happens in a physical change.

4. Identify the answer

A chemical change is defined by the formation of a new substance, whereas a physical change only alters the form or state of the existing substance.

5. Tip

Memory Trick: Think of the "New Substance" rule. If you can't easily "undo" the change to get your original material back (like trying to turn ash back into wood), it is almost certainly a chemical change because a new substance has been formed!