Identify physical and chemical changes, and describe the differences between them
Chemical Reactions – Physical & Chemical Changes
What is a Physical Change?
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.
- Melting ice ➜ water (🌡️)
- Boiling water ➜ steam (💨)
- Crushing a can ➜ smaller can (🔩)
- Mixing sugar in tea ➜ sweet tea (🍵)
Notice: No new substances are formed.
What is a Chemical Change?
A chemical change is like mixing baking soda and vinegar to make a volcano – the reactants turn into new substances with different properties.
- Rusting iron ➜ iron(III) oxide (🛡️)
- Combustion of wood ➜ ash + CO₂ (🔥)
- Cooking an egg ➜ denatured proteins (🍳)
- Neutralising acid with base ➜ salt + water (⚗️)
Key sign: New substances appear, often with a new smell, colour, or gas.
Key Differences
| 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) |
Exam Tip Box
Quick Check
🔍 Question: When you boil water, is it a physical or chemical change?
?? Answer: Physical change – water turns to steam but remains H₂O.
Chemical Equation Example
Combustion of methane:
$$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{heat}$$
Here, methane (CH₄) and oxygen (O₂) combine to form new substances: carbon dioxide and water, releasing heat – a classic chemical change.
Revision
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