States of Matter | Middle school chemistry | Khan Academy

Matter is anything that occupies mass and takes up space. Matter is made of particles, which have kinetic energy—they are in constant movement. There are three states of matter: solids, liquids, and gases. In solids, particles are packed tightly together. Solids have a fixed shape and volume. In liquids, the particles flow around each other. Liquids have a fixed volume but take the shape of their containers. In a gas, particles move freely. Gases have no fixed volume or shape. When the particles of a system gain or lose kinetic energy, a phase change occurs as one state of matter changes into another.