What is a physical change?
A physical change changes how a substance looks, its size, shape or state (solid, liquid, gas). It does not make a new substance.
The particles stay the same. They only move closer or farther apart, or get mixed with other particles.
- Melting ice, boiling water, freezing.
- Dissolving salt or sugar in water.
- Cutting, tearing, crushing, bending.
- Stretching a rubber band, magnetising a needle.
Many physical changes are easy to reverse: freeze the water again, or boil off the water to get the salt back.
What is a chemical change?
A chemical change (also called a chemical reaction) makes one or more new substances. The new substances have new properties.
Inside, bonds between atoms break and the atoms join up in a new way. The atoms themselves are not created or destroyed, so the total mass stays the same.
- Rusting of iron, burning of wood or a candle.
- Cooking food, ripening and rotting of fruit.
- Milk turning sour, digestion of food.
- Baking soda fizzing in vinegar.
Most chemical changes are hard to reverse.
Signs that a chemical change happened
- Gas is given off (bubbles, fizzing).
- Colour changes.
- Heat or light is given out (or heat is taken in).
- A solid (precipitate) forms in a liquid.
- A new smell or taste.
A sign is a clue, not proof. Boiling water also makes bubbles, but it is physical. Always ask: is there a new substance?
Physical and chemical properties
A property is something that describes a substance.
- Physical properties can be seen or measured without making a new substance: colour, smell, state, hardness, density, melting point, boiling point, solubility, electrical conductivity.
- Chemical properties show up only when the substance changes into a new one: whether it burns (flammability), whether it rusts, how it reacts with acid or with oxygen.
Example: copper is shiny, red-brown and conducts electricity (physical properties). Copper slowly forms a green layer in moist air (a chemical property).
Changes in material systems: a quick comparison
| Physical change | Chemical change | |
|---|---|---|
| New substance? | No | Yes |
| Particles | Same molecules, only moved | Atoms regrouped into new molecules |
| Reversing | Usually easy | Usually hard |
| Energy change | Usually small | Often large (heat, light) |
| Total mass | Same | Same |
Some events have both. A burning candle: the wax melts (physical) and the wax vapour burns (chemical).
Try it: a kitchen test
- Put a spoon of sugar in warm water and stir. Is the sugar gone? Taste a drop (only kitchen items!). Leave a saucer of it in the sun for two days. What is left?
- Put half a spoon of baking soda in a glass and add vinegar. What do you see and hear? Can you get the baking soda back?
- Decide for each: physical or chemical? Then open the last 3D step and check.
Key formulas and definitions
- Physical change: same substance, new form (state, size, shape)
- Chemical change: reactants → new products
- Signs: gas, colour change, heat/light, precipitate, new smell
- Mass before = mass after (in both kinds of change, in a closed container)
Worked examples
1. Is dissolving salt in water physical or chemical?
Physical. The salt particles spread through the water, but they are still salt. Evaporate the water and you get the salt back.
2. Why is rusting a chemical change?
Iron joins with oxygen and water to make rust (hydrated iron oxide), a new brown, flaky substance with different properties. It cannot be turned back into iron by a simple physical method.
3. A candle burns. List one physical and one chemical change in it.
Physical: wax melts near the flame. Chemical: wax vapour burns to make carbon dioxide and water, giving heat and light.
4. Classify the properties of magnesium: (a) silvery and shiny, (b) burns with a bright white flame, (c) melts at 650 °C.
(a) physical, (b) chemical (it makes new magnesium oxide), (c) physical.
5. 10 g of baking soda reacts with 50 g of vinegar in an open glass. The final mass is 55.6 g. Was mass lost?
No mass is destroyed. 4.4 g of carbon dioxide gas escaped into the air. In a closed bottle the mass would stay 60 g.
Common mistakes
- Thinking every change with bubbles is chemical. Boiling makes bubbles of steam, but it is a physical change.
- Thinking dissolving is chemical because the solid "disappears". The particles are still there, just spread out.
- Saying all physical changes are reversible and all chemical ones are not. It is a good rule of thumb, not a definition (tearing paper cannot be undone, but it is physical).
- Thinking mass is lost when something burns. The products (gases) escape; atoms are never destroyed.