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There are four phases: solid, liquid, gas, and plasma. Solids are composed of atoms that are tightly bound together, so they hold their shapes and have fixed volumes. Liquids are not held together as tightly as solids. A liquid conforms to the shape of its container, but the volume is still fixed. Gases have weak molecular forces, so they will fill whatever container they are in and adjust their shape and volume. Gases are easily compressible unlike the previous two states, since their volumes are not fixed.

You've probably heard of solid, liquid, and gas, but what is plasma?

Plasma is a relatively new state we have begun studying. A plasma occurs in high temperatures or pressures, such as on the Sun. In such extreme conditions, electrons are stripped from their shells around the nucleus and they leave behind a positively charged ion. The mixture of gases is known as a plasma.

Matter is generally categorized as either a solid, liquid, or a gas. A phase change is the term used to describe any time a sample changes from one form of matter to another.

When a sample's molecules are at their slowest, it is said to be in its solid state. Take an ice cube for example. As the H$_{2}$O molecules in the ice cube are heated, they become excited and begin to move more rapidly.

Eventually the molecules will begin to move so rapidly that the ice will melt, changing from a solid into a liquid through a process known as fusion. The temperature at which this phase change occurs is is known as the sample's melting/freezing point and is a unique property to each compound.

If more heat is added, the liquid water molecules will undergo evaporation as it gets closer to the sample's boiling point, eventually becoming a gas.

Cooling a sample can cause phase changes as well.

When a gas is cooled to a temperature below its boiling point, it returns to its liquid phase in a process called condensation. Cooling a liquid sample back down below its melting point causes it to return to its solid phase in a process known as crystallization or freezing.

Under certain unique conditions, some compounds can bypass the liquid state in their phase change and jump from solid to gas, or gas to solid.

For example, once compounds like CO$_{2}$ (dry ice) that have extremely low melting/freezing points are heated, they undergo sublimation and turn directly into gas from their solid state.

Another example is the process through which snow is formed in clouds. In sub-freezing air temperatures, the gaseous water vapor that makes up clouds undergoes deposition and transforms from a gaseous form into ice crystals within the cloud. Once these crystals grow to be too heavy, they fall down in the form of a snowflake.

There are 4 phases, or states, of matter: solid, liquid, gas, and plasma. The difference between them is determined by the strength of interactions of the composite molecules. From the left to the right in the list above, the interactions become weaker.

Solids

Solids have very strong interactions between their molecules. This results in the molecules being close together and moving very little. In solids, molecules tend to just vibrate in their positions. Because of this lack of motion, solids have a well defined shape and volume. Most things in our day to day lives are solids.

Liquids

The component molecules of a liquid still strongly interact with each other, but it is not as strong as in solids. Since it is weaker, molecules are able to move past one another. What this means is that unlike solids, liquids do not have a definite shape. They do, however, have definite volume. If you're having trouble understanding this, think of water. Water is probably the liquid that we deal with the most in our lives. When you transfer water between containers, the amount of water (volume) doesn't change, but it takes on the shape of whatever container it is in.

Gases

In the gas phase there is almost no interaction between molecules. In fact, the "ideal gas" does not interact at all. Since there is so little interaction between gas molecules, they can go pretty much anywhere. Gases have neither a defined volume or shape. Let's think of a balloon. When we blow up a balloon, there is a whole bunch of gas stuck in that volume and shape. If we release the gas from the balloon, it fills up the entire room. So the gas has changed both shape and volume. The gas we deal with the most is air, which is actually a mixture of gases. Air is composed primarily of nitrogen gas, oxygen gas, and carbon dioxide.

Plasma

Plasma has very similar properties to gas. The big difference between the two is that plasma is composed of ions. A neon sign is a common example of a plasma.