Look around you. The air you breathe, the water in your bottle, the graphite in your pencil and the phone in your pocket are all matter: anything that has mass and takes up space. There are millions of different kinds of matter, so scientists sort them into groups, much like a library sorts books. Once you know which group a sample belongs to, you can predict a lot about it: whether it can be separated, whether it will look the same everywhere, and whether it is made of one kind of particle or many.
Matter is made of particles
- All matter is made of tiny particles that are far too small to see.
- There are spaces between the particles.
- The particles are always moving. The hotter the matter, the faster they move.
- The particles attract each other. The attraction is strongest in solids and weakest in gases.
This is the particle theory of matter. It explains the three common states. In a solid, particles are packed tightly and vibrate in place, so a solid keeps its shape. In a liquid, particles are close but slide past each other, so a liquid flows and takes the shape of its container. In a gas, particles are far apart and move freely, so a gas spreads out to fill any space.
The classification of matter
A pure substance contains only one kind of particle, so every sample of it has the same properties. Pure water always freezes at 0 °C and boils at 100 °C (at normal air pressure at sea level). There are two kinds of pure substances. An element is made of only one kind of atom and cannot be broken down into simpler substances by chemical means: oxygen, iron and gold are elements. A compound is made of two or more different elements chemically joined in a fixed ratio: water is always 2 hydrogen atoms for every 1 oxygen atom, written .
A mixture contains two or more substances that are physically mixed but not chemically joined. Each substance keeps its own properties, and the amounts can change: you can make lemonade sweeter or weaker. A mechanical mixture (also called a heterogeneous mixture) has parts you can see, like granola or pizza. A solution (a homogeneous mixture) looks the same all the way through, because one substance has dissolved completely into another, like sugar in water or the gases that make up air.
| Type | What it is | Examples |
|---|---|---|
| Element | One kind of atom only | Oxygen gas, copper wire, helium in a balloon, a diamond (carbon) |
| Compound | Two or more elements chemically joined in a fixed ratio | Pure water, table salt (sodium chloride), carbon dioxide, sugar |
| Mechanical mixture | Two or more substances you can see as separate parts | Granola, a salad, sand and gravel, granite |
| Solution | Substances mixed so evenly it looks like one substance | Tap water, air, vinegar, brass (copper and zinc) |
| Suspension (a type of mechanical mixture) | Cloudy mixture whose particles settle out if left standing | Muddy water, orange juice with pulp, oil-and-vinegar dressing |
| Colloid (a type of mechanical mixture) | Tiny particles spread through another substance that do not settle | Milk, fog, whipped cream, gelatin |
Separating mixtures
Because the parts of a mixture are not chemically joined, they can be separated by physical methods. Filtration (pouring through filter paper) removes sand from water. Evaporation leaves salt behind when salt water dries. Distillation boils a solution and cools the vapour to collect pure water. Magnets pull iron filings out of sand. A water treatment plant uses several of these steps to turn lake water into safe tap water. Separating a compound into its elements, by contrast, needs a chemical reaction, such as passing electricity through water to split it into hydrogen and oxygen.