Physical and Chemical Changes
Matter undergoes changes that can be classified as physical or chemical changes based on whether the composition of the substance is altered.
Differences Between Physical and Chemical Changes
| Feature | Physical Change | Chemical Change |
|---|---|---|
| Definition | A change in which only the physical properties of a substance (such as shape, size, state) are altered, but its chemical composition remains the same. | A change in which the chemical composition of the substance is altered, forming a new substance with different properties. |
| Reversibility | Usually reversible | Irreversible |
| New Substance Formation | No new substance is formed | A new substance is formed |
| Energy Change | Usually involves little or no energy change | Involves absorption or release of energy (heat, light, gas, etc.) |
| Examples | Melting ice, breaking glass, dissolving salt in water | Rusting of iron, burning wood, digestion of food |
Examples of Physical and Chemical Changes
Physical Changes:
- Melting of ice → Water changes from solid to liquid but remains H₂O.
- Boiling of water → Water evaporates into steam but remains the same chemically.
- Cutting paper → Only the size and shape change, not the chemical composition.
- Dissolving sugar in water → Sugar dissolves but can be recovered by evaporation.
- Stretching a rubber band → The shape changes but not the material.
Chemical Changes:
- Rusting of iron → Iron reacts with oxygen and moisture to form iron oxide (rust).
- Burning of wood → Wood reacts with oxygen to form ash, carbon dioxide, and water vapor.
- Cooking food → Heat causes chemical reactions that change raw ingredients into new substances.
- Digestion of food → Enzymes break down food into new substances that can be absorbed by the body.
- Souring of milk → Bacteria break down lactose into lactic acid, forming a new substance.