1.1 Substances, states of matter and the particle model

What matter is, and how we model it

Matter is the 'stuff' that everything is made of. We define it as anything that has mass and volume. All matter is ultimately built from atoms. To explain how it behaves, scientists use the particle model: matter is made of tiny particles in constant motion, with forces of attraction between them. How much energy those particles have decides the state — solid, liquid or gas.

Why these categories actually differ

The dividing line between a compound and a mixture is the words chemically combined. In a compound, the atoms are joined by chemical bonds, so the only way to separate them is another chemical reaction. In a mixture nothing is bonded, so the parts keep their own properties and can be separated physically — by filtering, evaporating or letting them settle.

The same reasoning sits underneath the states of matter: heating gives particles more energy, so they break free of their fixed positions (solid → liquid) and then of each other (liquid → gas). The particles do not change — only their arrangement and movement do.

Is salt water a compound or a mixture? It contains water molecules and sodium chloride, but they are simply mixed — the salt is dissolved, not bonded to the water, and the ratio can vary. You can get the salt back by evaporating the water (a physical change, no reaction). So it is a mixture — and a homogeneous one, because it looks uniform throughout.

For 2000 years people believed everything was made from four "elements" — earth, air, fire and water. We now recognise about 118 real elements, and the difference between a mixture and a compound was only settled once chemists could weigh reactants and products carefully. The habit to carry forward is the one this whole unit is built on: a model is kept only as long as the evidence supports it. Next year, the bonding that holds a compound together (ionic and covalent) is exactly what you will study to explain why those fixed ratios appear.