|Themes > Science > Chemistry > General Chemistry > Matter > Theories of Matter > Solid, Liquid and Gaseous States of Matter|
By Ron Kurtus (revised 5 March 2001)
There are three states or phases in which matter exists: solid, liquid and gaseous. Most materials can go from one state to another, depending on the temperature. The change of state can be dramatic.
Questions you may have about this are:
Description of states of matter
The difference between a solid, liquid and gas is very apparent when you look and even touch the materials. In most cases, they don't even look related. Water and ice don't look like they are from the same molecules. And steam or water vapor is essentially invisible.
A solid has a specific shape. If you put a liquid in a container, it will normally stay in the container and take the shape of the container. A gas will also take the shape of its container, but it will not normally stay in the container.
Gas has high speed molecules
From the Molecular Theory of Matter and the Theory of Heat, you know that molecules are in constant motion. They only stop moving at Absolute Zero. It is also a fact that similar molecules and atoms have an attraction to each other.
With a gas, the molecules (or atoms) have a fair amount of energy 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.
Liquids have slower molecules
When the gas is cooled sufficiently that the molecules are have less energy and are moving slower. The molecule force takes over and attracts the molecules together. This mild binding force holds the material together in the form of a liquid.
Molecules in solids vibrate
If a liquid is cooled even more, the molecule slow down so much that they get stuck in the structure of a solid. They are not moving freely about. Instead, most of them are simply vibrating in place.
Examples with different materials
Look at various materials to see their different states.
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 degrees F (0° C), it will freeze and become the solid we call ice. If you heat water above 212° F (100° C), it will boil and turn into a gas we call steam.
You've seem pictures of molten iron in a foundry. They heat the iron to a very high temperature, and it turns to a hot, yellow liquid. If iron was placed in an extremely hot environment like on the Sun's surface, it will boil and turn into a gas.
Helium is a gas at room temperature, but if it is cooled to a few degrees above Absolute Zero (degrees Kelvin), it would turn into a liquid. It is the only liquid that cannot be solidified by 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).
What happens when states change
When a material changes from one state to another, there is a dramatic change in the look at shape of the material. Most materials contract when they are cooled. Also, the temperature stays constant while the material changes states.
Expansion and contraction
Most substances expand when heated and contract when cooled. (You can demonstrate this with a balloon filled with air. See Experiment with Balloons.) This continues even after they have changed their states.
For example, water will contract until it starts to freeze and turn into ice. It then starts to expand and get larger. Ice will expand until the temperature reaches about -3° C, after which is will start to contract again with lowering temperatures.
Ice is one of the few materials that expands when cooled. Unfortunately, this is the cause of much of the damage to road in the winter. Water seeps into a small crack in the road, and then when it freezes, the ice expands and makes the crack bigger. Cars drive over the crack and loosen the material, so the water and ice can work again to make a pothole in the road.
Temperature stays constant
When you try to boil water, the energy you apply to the water is spent rising its temperature. Once the temperature reaches 212° F (100° C), the temperature stays constant and all of the energy is used to change the water into steam. Once all the water is converted to steam, the temperature of the steam can start to rise again.
Likewise, when you heat a piece of ice, its temperature will rise until it reaches 32° F (0° C). Then all of the energy goes into changing the ice into water. Once all of the ice is melted, the temperature of the water can rise again.
Most materials will change their states from solid to liquid to gas, as the temperature increases. This is caused by the energy of the moving molecules overcoming the molecular attraction forces.