We will be looking closely at matter, and the elements that make up matter and their various combinations.
Matter comes in three forms. Each different form is called a phase of matter. Matter comes in liquid, solid, and gas phases. Matter in the solid phase has shape and takes up a definite space. Solids can be hard (like a table) or can be soft (like a stuffed animal). In the liquid phase, matter can change shape, but it takes up a definite amount of space and has a definite volume. Matter in the gas phase can change shape and size. A gas will expand to fill the container it is in (shown below), so it does not take up a consistent amount of space. Its shape is determined by the container it is in.

Matter can change forms, moving from one phase to another.

Gases can become liquids and liquids can become solids. For example, when you boil a liquid, it becomes a gas. When you freeze a liquid, it becomes a solid. When you melt something, it becomes a liquid. Liquid becomes gas through evaporation, such as when you heat a kettle of water on the stove, turning it into steam (gas molecules shown at the right). Gas, when it is cooled, becomes liquid through a process called condensation.