The 4 Basic Properties of Gas
I want to take a look at four of the basic properties of gasses, and I want to talk about the properties of gasses in the context of a balloon, so that it gives us something to visualize when we’re talking about these properties.
Think of a balloon, and as you blow air into this balloon, you’re adding molecules to the air. As more of this air gets added to the balloon, the volume is going to increase, which makes sense.
Everyone knows when you blow air into a balloon, the balloon expands, and if you let air out of the balloon, then the balloon gets smaller, so volume is proportional to the amount of gas.
Now, for this to be true, the temperature and the pressure has to be constant, and you may be wondering, “Why is that?” Well, you’ll see later that temperature affects the volume, and the pressure affects the volume, so if the temperature and pressure are changing, then they’re also affecting the volume, in addition to the amount of gas.
We can no longer really see what’s really affecting the volume, so volume is no longer proportional to the amount of gas, but if temperature and pressure are constant through this entire time, we can pretty much take them out of the equation because they’re not affecting anything anymore because they’re not changing.
When temperature and pressure are constant, we can then say volume is proportional to the amount of gas. Now, the second property of gas is also very interesting. If you think about a balloon that has a fixed amount of air in it. In other words, the end’s tied off; you can’t add air, you can’t remove air.
Then you heat this balloon, it will expand, the volume will increase. As you heat it, the balloon will get bigger. Now, if you cool the balloon, then the volume will decrease, the balloon will get smaller. We can say the volume changes directly with temperature.
As temperature increases, volume increases, and as temperature decreases, volume decreases. Now for this to be true, the pressure of the balloon has to remain constant throughout the entire process. Now if you squeeze the balloon, the volume will decrease, because gasses are compressible.
We can say the volume changes inversely with the pressure, and this make sense. Say there’s lots of pressure, you’re pushing really hard on this balloon, the volume is going to decrease, so at that point the pressure’s increasing because you’re pushing harder, but the volume’s decreasing.
If the pressure decreases and you’re not pushing as hard, then the volume will increase, so that’s why we say volume changes inversely with the pressure. For this to be true, the temperature has to remain constant. Now, the fourth and final property of a gas: if you fix the volume of the balloon and add more air, then the pressure inside will increase.
We can say pressure changes directly with the amount of gas, and this makes sense because you think about air molecules inside that balloon, they’re moving all over the place. Well, if the amount of gas is large, then the molecules are going to hit the sides very often.
Think about the volume being the same throughout this whole time, the balloon is always going to be the same size, but if you add more air to it, then there’s more air molecules that are going to hit the side of the balloon more often, so that increases pressure.
If there’s less air inside the balloon, then the air molecules are going to hit the sides of the balloon less often, which decreases the pressure. That’s why we say pressure changes directly with the amount of gas. Now, for this to be true, like I said, the volume has to be constant, and the temperature also has to be constant. Those are the four basic properties of gasses.