# Q & A: It’s Shrinking

Q:
what will happen when you decrease the volume of a container? will something increase?
- brittany desoto (age 13)
christian life, baton rouge, la
A:
To answer this equation, I'll use my favorite gas equation, the ideal gas law. PV=nRT

That might look a little tricky, but I'll explain each part.
P is the pressure.
V is the volume.
n is a number related to the number of gas molecules in the container.
R is the ideal gas constant.
T is the temperature.
So if you multiply P times V, you get the same as n times R times T.

Now back to the question. If you decrease the volume of the container, something else must change to keep our equation correct. R can't change because it is always the same. P can change. If you squish the container, then the pressure would go up. Or you could change n. As you make the volume smaller, the gas just goes away into the air around your container. You can even make T smaller. So the gas gets colder if you keep the pressure constant while you decrease the volume.

In the end, what happens depends on what you prevent from changing. But PV=nRT will always be true.

(published on 10/22/2007)

## Follow-Up #1: measuring irregular volumes

Q:
How do you measure the volume of an irregular shape without using water displacement method?
- Anonymous
A:
The water displacement method might not work for some material that would be damaged by the water. In some cases, you could then use essentially the same technique but with oil, or even with some granular material, such as sand or mustard seeds, replacing the water.

If for some reason none of those displacement methods work, then it's hard to answer the question without knowing a little more about the practical situation. If you have a set of measurements of the surface coordinates of the material, you could numerically integrate to calculate the volume.

Mike W.

(published on 11/23/2011)