B.1.8 – Gas Laws

Defined Words: gas laws, pressure, volume, temperature

 

The gas laws are a set of relationships between the four main measurements of a gas. Those measurements are pressure, volume, temperature, and number of molecules. Despite being named after gases, these laws act upon all types of matter, though the effects are slightly different. Here, we’ll stick with gases as the laws describe.

Gaseous molecules have no attraction to each other and are constantly moving and bouncing off the walls of their container. The total amount of force exerted on a defined area of the container gives a measurement of pressure.

Gases bounce around their container with mostly empty space between them, the amount of total space the gas takes up is called the volume. Gases are also fluid like liquid, which means they take the shape of the container they’re in. The volume of a gas is usually decided by the container.

Gases are in constant motion meaning they have kinetic energy. The amount of kinetic energy corresponds to the temperature. Greater kinetic energy means faster molecules and higher temperature. The last variable is the simplest, the exact number of molecules present in the container.

In order to determine the relationships, we need to keep two of these variables constant while altering a third in order to see its effect on the fourth. Let’s go over the relationships of pressure first.

If you increase the temperature of a gas in a container, the molecules will increase in speed and therefore would run into walls of the container with greater momentum. The faster moving molecules will slam into the walls with greater force, increasing the pressure. Pressure and temperature are “directly related.”

If you decided to increase the volume of the gas container, the molecules would have farther to travel within the container before they hit the walls. Fewer molecules run into the container walls over time, causing a lower overall force. Less force on a larger area lowers the pressure of the gas. Pressure and volume are “inversely related.”

With an increase in the amount of molecules in a container, more of them will be running into the walls. That means greater overall force, which means greater pressure. Pressure and number of molecules are “directly related.”

Next, we’ll tackle volume. If you increase the temperature of a container, the molecules will start to move faster and bounce off the walls harder. If the pressure is to remain the same, then the volume must increase. Volume and temperature are “directly related.”

We have already discussed the relationship between pressure and volume and know that they are inversely related. If you were to add more molecules to a container, it will increase in size to equalize the pressure. Volume and number of molecules are “directly related.”

Finally, we’ll look at the relationship between temperature and number of molecules. If you add more molecules to a rigid container, more molecules are now bouncing around the same sized container. The original kinetic energy diffuses to the newly added molecules. The overall energy of the system stays the same but the original molecules are moving slower, resulting in lower temperature. Temperature and number of molecules are “inversely related.”

If we take all of the individual relationships and put them together, we can form a relationship between all four variables at once. This relationship can be mathematically written as follows:

PV=nRT

P stands for pressure, V stands for volume, n is the number of molecules, T is the temperature, and R is a constant that allows us to convert between the various units of each measurement.

Nature tends toward equilibrium, so a change in one or two variables will mean that another variable must change to reverse the effects. For example, heating up a container will cause the pressure to rise unless the volume grows or molecules are removed from the container to balance the pressure. This is how a hot air balloon works: hot air expands the balloon but lowers the pressure of the air inside to let it fly.

I’m sure we’ve all been yelled at for leaving a window open and “letting the heat out.” In order to balance the pressure, the higher temperature gas will move toward lower temperatures. However, “letting the cool out” is technically impossible since the warm air from outside rushes in. Diffusion also plays a factor here, but the point is that these phenomena can be predicted.

We have spent the last 15,000 years learning about the world we live in and we have finally (mostly) figured it all out! When you truly understand how your world works, you can predict it and successfully manipulate it. All of your tools are in front of you; exploit your adaptation to conquer your world with knowledge.

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