I want to take a look at four of the basic properties of gases and I want to talk about the properties of gases 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, as you blow air into this balloon your adding molecules to the air. As more of this air gets added to the balloon the volume is going to increase, which makes sense.

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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.

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 will see that temperature affects the volume and the pressure affects the volume. If the temperature and pressure changes and they’re also affecting the volume in the addition to the amount of gas and we can no longer really see what’s really affecting the volume, so volume is still no longer proportional to the amount of gas.

But, if temperature and pressure are constant through this entire time then we can pretty much take them out of the equation because they’re 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 ends tied off, you can’t add air and you can’t remove air and 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, then the balloon will get smaller, so 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 volume has to remain constant throughout the entire process. Now if you squeeze the balloon the volume will decrease because gases are compressible, so we can say the volume changes inversely with the pressure and this makes sense.

Say there’s lots of pressure pushing really hard on this balloon, the volume is going to decrease, so at that point, the pressure is increasing, because you’re pushing harder, but the volume is 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 and 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, so we can say pressure changes directly with the amount of gas. This makes sense, because you think of air molecules inside of that balloon, they’re moving all over the place, well if volume or if the amount of gas is large, then the molecules are going to hit the sides very often.

You 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 hit the side of the balloon more often, so that increases pressure. But there’s less air inside that balloon and the air molecules are going to hit the sides of the air balloon less often, which decrease 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 gases.