Solids, Liquids, and Gases: States of Matter

The states of matter are the distinct physical forms in which a substance can exist; the three most familiar are solid, liquid and gas. A solid has a fixed shape and volume, a liquid has a fixed volume but takes the shape of its container, and a gas has neither a fixed shape nor a fixed volume. In brief: ice, liquid water and steam are the same substance, H₂O, in three different states.

The state of a substance depends mainly on temperature and pressure. Heating or cooling can move a substance from one state to another without changing what it is made of. This article compares the three states, explains how particles are arranged in each, shows a summary table, works through examples and mentions the other states of matter.

What are the properties of a solid?

A solid keeps its own shape and volume. Its particles are packed closely together in fixed positions and only vibrate in place. A solid cannot be poured, and it is very hard to compress. Examples include a brick, a coin, ice and a wooden table.

Some solids are crystalline, with particles arranged in a regular repeating pattern, as in salt and quartz. Others are amorphous, with no long-range pattern, as in glass and many plastics.

What are the properties of a liquid?

A liquid has a fixed volume but no fixed shape. It flows and takes the shape of the bottom part of its container. Liquid particles are still close together, but they can slide past each other. Liquids are almost impossible to compress.

A liquid forms a flat surface under gravity, and a small amount of liquid forms rounded droplets because of surface tension. Water, milk, oil and mercury are examples.

What are the properties of a gas?

A gas has no fixed shape or volume and expands to fill the entire container. Its particles are far apart and move quickly in all directions. Because of the large spaces between the particles, gases are easy to compress. This is why air can be pumped into a tire.

Gases are usually much less dense than solids or liquids. Examples include air, oxygen, carbon dioxide and the natural gas used for cooking.

Solids, liquids and gases compared

Which state is a substance in at room temperature?

The state at room temperature (about 20 °C) depends on the melting and boiling points. A substance is a solid if its melting point is above room temperature, a liquid if it melts below and boils above room temperature, and a gas if its boiling point is below room temperature.

Worked example: classifying substances

Use the table above to decide what happens to each substance at the given temperature.

1. Water at 50 °C. This lies between 0 °C and 100 °C, so water is a liquid.
2. Water at −5 °C. This is below the melting point of 0 °C, so water is a solid (ice).
3. Ethanol at 90 °C. This is above the boiling point of 78 °C, so ethanol is a gas (vapor).
4. Mercury at −50 °C. This is below the melting point of −39 °C, so mercury is a solid.
5. Oxygen at −200 °C. This lies between −218 °C and −183 °C, so oxygen is a liquid. Liquid oxygen at this temperature is used in rocket engines and hospitals.

Are there more than three states of matter?

Yes. Plasma is a gas-like state in which atoms have lost electrons and the material contains free charged particles. It makes up the Sun and other stars and is found in lightning and neon signs. Other states include Bose-Einstein condensates, which form near absolute zero, and liquid crystals, which show properties between those of liquids and solids. For most school chemistry, solid, liquid and gas are enough.

How did scientists study states of matter?

In the 1600s, Robert Boyle investigated how the volume of a gas changes with pressure, one of the first quantitative studies of a state of matter. In the 1800s, scientists developed the kinetic theory, which explains the behavior of gases by the motion of particles. In 1908 the Dutch physicist Heike Kamerlingh Onnes liquefied helium, and in 1995 the first Bose-Einstein condensate in a dilute gas was produced in a laboratory.

Which mistakes are common?

Mistake 1: Thinking that gases are not matter

Gases have mass and take up space. Air pushes on the walls of a balloon.

Mistake 2: Saying that solids do not move at the particle level

Particles in a solid vibrate. They do not change position, but they are not still.

Mistake 3: Thinking that powders are liquids

A pile of sand can be poured, but each grain is a small solid. The material is still a solid.

Mistake 4: Believing that steam is the visible white cloud

Steam, meaning gaseous water, is invisible. The white cloud above a kettle is made of tiny droplets of liquid water that have already condensed.