States of Matter

By Ron Kurtus (revised 20 February 2016)

There are three major states or phases of matter: solid, liquid, gas. These states describe distinct physical characteristics of the material and are a function of temperature and pressure.

At very high temperatures, matter becomes a plasma or ionized gas. Some scientists designate a plasma as a fourth state of matter. There are also other forms of matter at both very high and very low temperatures that may be considered states of matter.

Major physical characteristics define each state and primarily concern volume and shape. Kinetic energy of the molecules and attractive forces determine the physical characteristics of each state. Many common materials can be seen in the various states of matter.

Questions you may have include:

• What are the physical characteristics of the states of matter?
• Why do the characteristics change?
• What are some examples of materials in different states?

Physical characteristics

The primary physical characteristics of the various states of matter are the volume and shape of the material. They are what really define the state. Other characteristics—such as color—may be different in the various states of matter, but they do not define the state as do volume and shape. Gravity has an effect on the shape of liquids and gases.

Solid

The solid state of matter is when the material has a definite volume or size and distinct shape at a given temperature.

At room temperature, a piece of iron at has a shape and size that does not change. Ice is another solid, but its temperature must be below 0^o C (32^o F).

Most solid materials expand with increasing temperatures, but they retain their shape.

Liquid

A liquid has a definite volume, but it takes the shape of its container with the help of gravity.

For example, if you pour water into a cup or container, it will take the shape of that container. If you put water in a balloon, the water will take the shape of the balloon, no matter how you change the shape of the balloon.

On the other hand, in outer space, where there is no gravity such as in the Space Shuttle, water might float out of its container. Its shape will vary, but its volume will remain constant if the air pressure and temperature are constant.

Most liquids expand with an increase of temperature and constant air pressure.

Gas

The volume of a quantity of gas is dependent on its temperature and the surrounding pressure. If affected by gravity, it will take the shape of its container, but much of it will also spread out into the surrounding area.

Assuming little or no convection or circulation, a heavier gas will settle to the bottom. For example, carbon dioxide gas (CO₂) is heavier than air. If a quantity of CO₂ is put in a cup, it sink to the bottom and take the shape of the cup. After a while, some of the CO₂ will mix in with the air, due to convection—perhaps becoming part of a solution with the air.

This can be demonstrated using Dry Ice (frozen CO₂) and pouring the "smoky" gas into a cup. It will settle to the bottom but will soon dissolve into the air.

If you put a gas in a cylinder and apply pressure with a piston, such as you might do with a tire pump, the volume of the gas can change considerably. This is not the case with water or a solid. Their volumes may change only slightly with an increase of pressure.

Reasons states have characteristics

The reason the various states of matter have their particular physical characteristics and behave in the way they do is a result of the motion of the molecules and the attraction between them.

Theories and forces

The Molecular Theory of Matter, Theory of Heatand the Molecular Attraction Force affect the states of matter.

Molecules in motion

The Molecular Theory of Matter and the Theory of Heat state that molecules are in constant motion. The greater the temperature, the greater the kinetic energy and thus the faster the molecules are moving. This also applies to substances made up of only atoms, such as iron or pure aluminum.

Molecular attraction

But also, Molecules are often attracted to each other, due to the Molecular Attraction Force. This force is strongest among similar molecules or atoms. It is a form of electrostatic attraction that also is the factor in static electricity.

Fight between the two

The kinetic energy of the molecules or atoms tends to spread them apart, while the molecular attraction tends to pull them together. The temperature or energy will then determine which force wins out.

Molecules in solids vibrate

If the kinetic energy (and thus the velocity) of the molecules of a material is so low that the molecular attraction between the molecules is much stronger than the forces pulling the molecules apart, the material will be a in solid state of matter. That means the molecules become fixed in place and often line up in a crystalline arrangement. The molecules or atoms still have some energy, but their movement is confined to just vibrating in place or rotation.

At the very lowest temperature possible—Absolute Zero (0 degrees Kelvin or 0^o K)—all motion stops and the atoms and molecules do not vibrate or even spin.

Molecules in liquids loosen structure

If the the molecules of the material have enough kinetic energy and are moving fast enough, they can break out of the constraints of a defined structure. The energy overcomes the much of the molecular force. This force is now only strong enough to hold the material together in the form of a liquid.

Molecules in gases run free

With a gas, the molecules (or atoms) are more energetic and are moving rapidly. Their kinetic energy is greater than the attractive force between them. Thus, a gas will easily spread and not stay in an open container.

Examples with different materials

Look at various materials to see their different states.

Water

A good example of how matter can exist in different states is water. It is normally a liquid at room temperature, but if you cool it below 32^o F (0^o C), it will freeze and become the solid we call ice. If you heat water above 212^oF (100^o C), it will boil and turn into a gas we call steam.

Iron

You've seen pictures of molten iron in a foundry. They heat the iron to a very high temperature, such that it turns into a glowing, yellow liquid. If iron was placed in an extremely hot environment like on the Sun's surface, it would boil and turn into a gas.

Helium

Helium is a gas at room temperature, but if it is cooled to a few degrees above Absolute Zero (degrees Kelvin), it turns into a liquid. It is the only liquid that cannot be solidified at atmospheric pressure by simply lowering its temperature. Scientists have been only able to solidify liquid Helium at the extreme pressures of 25 atmospheres. (1 atmosphere = 14.7 pounds per square inch).

Liquid Helium has some very strange properties. For example, if it is poured in a flask, it will first settle at the bottom of the flask and then start to climb up the sides and soon flow out of the container.

Summary

There are three accepted states of matter: solid, liquid, gas. There may be a fourth state, called the plasma state. As the energy of the moving molecules overcomes the molecular attraction forces, the material will change its state from solid to liquid to gas. The solid iron, liquid water and helium gas are examples of different states of matter at room temperature.