Properties of materials

1. Every material has a set of properties that let you identify it and decide what it can be used for.
2. Properties include the density, the melting point, the boiling point, the colour, the hardness and whether the material conducts electricity.
3. Pure water, for example, is colourless, freezes at $0{}^{\circ }C$, boils at $100{}^{\circ }C$ and has a density of $1000{kg/m}^3$.
4. These properties come from the particles the material is made of, so a material keeps its properties for as long as its particles stay the same.
5. Some properties look different in different states, since ice is hard while water flows, but those differences come from how the particles are arranged rather than from a change in the particles themselves.

Physical changes

1. In a physical change, no new substance is made, and the particles of the material stay the same particles.
2. Only the arrangement, spacing or movement of the particles changes.
3. Because the particles are unchanged, reversing the change brings back the original material with all of its original properties.
4. Every change of state is a physical change.
5. Ice that melts into water can be frozen again, and the new ice has the same melting point, density and appearance as the ice you started with.
6. Water that evaporates from a saucepan can be condensed on a cold lid, and the droplets that form are pure water again.
7. Candle wax that melts near the flame runs down the side of the candle and sets again as solid wax with the same melting point.
8. Dissolving is also a physical change, because salt stirred into water can be recovered as solid salt by evaporating the water away.
9. Changing the shape of a material, such as bending a paper clip or squashing a drinks can, is a physical change too, because the metal is still the same metal.

Chemical changes

1. In a chemical change, the atoms of the starting materials are rearranged to make one or more new substances.
2. The new substances have different properties from the materials you started with.
3. Reversing the conditions does not undo a chemical change, so cooling, freezing or drying the products does not bring back the original material.
4. Bread that has been toasted stays brown and crisp after it cools, because the toasting has formed new substances on its surface.
5. Frying an egg turns the runny, see-through egg white into a firm white solid, and letting the egg cool does not make it runny again.
6. Burning a match turns the wood and the chemicals in the match head into ash, smoke and gases, none of which become a match again when they are cooled.
7. Iron that rusts forms a flaky orange-brown solid that is weaker than iron and has quite different properties.
8. Signs that a new substance has formed include a new colour, a gas being given off, a new smell, or a change that cannot be undone by reversing the conditions.
9. Some chemical changes can be undone by carrying out a different chemical reaction, but that is not the same as reversing the conditions as you can for melting or freezing.

Telling the two changes apart

1. The test is to imagine reversing the change and then ask whether the material gets back its original properties.
2. If reversing the change gives back the original material with the same properties, the change is physical.
3. If reversing the conditions leaves a different material with different properties, the change is chemical.
4. How easy or quick the reversal is does not matter, because condensing all the steam from a boiling kettle back into water is awkward, yet boiling is still a physical change.
5. A big change in appearance is not proof of a chemical change either, because ice, liquid water and steam look very different but are all the same substance.