What’re the general properties of gases?
General properties of gases include the following. They
- Expand: gases expand to fill the container in which they are put in. Just imagine blowing air into a balloon. Notice! The gas molecules in themselves do not expand. It’s the distance between the gas molecules that increase.
- Take the shape or volume of their container: gases take the shape of the container in which they are put in. Imagine blowing air into different shapes of balloon.
- Can be compressed: gases can be compressed. Just imagine the amount of air squeezed into a car tire.
- Can mix easily with other gases: just imagine the air around us. It consists of a mixture of gases: nitrogen, oxygen, carbon dioxide, and small amounts of other gases.
- Can exert pressure: feel your bike or car tire to see how hard it is.
- Have low density: gases are lighter than solids or liquids. Compare the size of a ballon filled with air to an equal size of a solid object.
Why do gases show these properties?
To explain gas properties, we have to turn to another theory called kinetic theory. This theory simply states that all atoms are in constant motion. And because of this motion, all matter always produce heat to some degree. However, to apply this theory to gases,
we have to make the following assumptions:
- Gases consists of small particles called molecules. The distance between these molecules is larger than their sizes. The total volume of the molecules is smaller than the entire space they occupy. Because of the larger distances between them, we can compress gases. And because of their smaller volume, gases have low density. This assumption explains gas property 3 and 6.
- Gas molecules are in constant random motion, colliding with each other and with the walls of their container. Because of this random motion, we can easily combine two or more gases to form a uniform mixture. For instance, natural gas has no detectable smell. For this reason, we often add a rotten-egg like smelling gas, hydrogen sulfide, to it so that we can detect when we have a gas leak in our homes. This assumption explains gas property 4 and 6.
- No attractive forces exist between gas molecules. This assumption explains gas property 1.
- When gas molecules collide, no kinetic energy is lost. However, energy can be transferred from one to the other.
- The average kinetic energy of gas molecules is proportional to the temperature in Kelvin. As you can tell, average here means that some molecules will be hotter, while others will be colder. In addition, as the temperature increases, average kinetic energy of the gas molecules increases as well. And as temperature decreases, average kinetic energy of the gas molecules decreases.
Gases that agree with these assumptions 100% are called ideal gases. However, in real life we have only real gases like oxygen, nitrogen, hydrogen and many more. So,
When do real gases behave like ideal gases?
They do under moderate conditions of temperature and pressure. These moderate conditions can be room temperature and standard atmospheric pressure.
When do real gases not behave like ideal gases?
At low temperature and high pressure. At these conditions, the gas molecules slow down, and become attracted to each other.
In all, the kinetic theory is just a model to help us understand gas behavior. For this reason, we don’t expect real gases to conform 100% with the theory.
To read more about how gases exert pressure, click here.