Solids, liquids and gases

MATTER IS ANY PHYSICAL SUBSTANCE OR object that exists in the three dimensions of space. It can be as huge as a planet or a star, or as Small as one atom – or even as tiny as the sub-atoms particles inside an atom. Whatever its size, matter also exists in one of three main forms. These are solid, liquid and gas. They are called the three states of matter. A housebrick, lump of wood of sheet of steel are solid. The petrol for a car's engine of the oil for cooking food are liquid. A cylinder of oxygen in a hospital or an "empty” room contain gases. Each form of matter has its own features and properties. But the atoms and molecules in matter do not change for each different state. What changes is the way that the atoms or molecules can move about, or the way they are forced to stay still.

The same matter or substance can change state, from solid to liquid,or from liquid to gas.These processes are called meIting and boiling,and are shown on the next page.Another change of state happens when substances burn, or combust. In a vehicle engine, liquid petrol sprays into the cylinders inside the engine, along with air containing oxygen. The petrol catches fire an burns rapidly, combining with the Oxygen in amini-explosion. The result is not anotherliquid, like petrol, but a variety of gases. These leave the engineas exhaust fumes. Gases flow and expand, spreading out in all directions to fill their container. So the exhaust fumes from every vehicle spread out evenly through their container – the Earth's atmosphere.This is why pollution from vehicles is a worldwide problem. The glaze on a shiny vase is solid. Most solids are opaque. You cannot see through them. But clear glass, glazes and varnishes are see-through, or transparent. The glaze protects the beautiful colours and patterns of the paints beneath and allows them to show through. Solid water is called ice. In a solid, the molecules can move very little.They are held in a rigid framework or pattern by bonds between them. So a solid stays in the same shape, unless subject to powerful forces such as twisting or crushing. This is called water vapour. It floats in the air. In a gas, the molecules can move about very easily.This is why gases flow and take up the shape of the container they are in. But the molecules in a gas can also be squashed nearer together or moved farther apart. So a gas can be compressed into a smaller volume, or expand to fill its container.