A physical change is like shifting the shape of a piece of clay – the clay is still clay. It involves a change in state, size, or appearance but the molecular structure stays the same.
Notice: No new substances are formed.
A chemical change is like mixing baking soda and vinegar to make a volcano – the reactants turn into new substances with different properties.
Key sign: New substances appear, often with a new smell, colour, or gas.
| Physical Change | Chemical Change |
|---|---|
| Reversible (often) e.g., melting/solidifying | Usually irreversible e.g., combustion |
| No new substances (same molecules) | New substances formed (different molecules) |
| No energy change visible (except heat/colour) | Energy released or absorbed (heat, light, sound) |
| Often a change of state (solid ↔ liquid ↔ gas) | Change in chemical composition (new bonds form) |
Tip: When you see a new substance (different colour, gas, or smell) or a reversible process, think chemical change. If only the state changes (ice ↔ water), it’s a physical change.
🔍 Question: When you boil water, is it a physical or chemical change?
??
Answer: Physical change – water turns to steam but remains H₂O.
Combustion of methane:
\$CH4 + 2O2 \rightarrow CO2 + 2H2O + \text{heat}\$
Here, methane (CH₄) and oxygen (O₂) combine to form new substances: carbon dioxide and water, releasing heat – a classic chemical change.