http://www.ck12.org Chapter 2. Matter
It’s Crowded in Here
Imagine a container of gas molecules like the one in theFigure2.48. The container in the sketch has a lid that
can be pushed down to shrink the volume of the gas inside. Notice what happens as the lid is lowered. The gas
molecules crowd closer together because there is less space for them to occupy and they have nowhere else to go.
Gas molecules have a lot of energy. They are always moving and bouncing off each other and anything else in their
path. When gas molecules bump into things, it creates pressure. Pressure is greater when gas molecules occupy a
smaller space, because the greater crowding results in more collisions. In other words, decreasing the volume of a
gas increases its pressure. If you go to the URL below, you can watch a simulation of this.
http://www.grc.nasa.gov/WWW/K-12/airplane/aboyle.html
FIGURE 2.48
You Go Up and I’ll Go Down
As the volume of gas in the container pictured in theFigure2.48 gets smaller, the pressure of the gas molecules
becomes greater. When two variables change in opposite directions like this, the variables have an inverse, or
“upside-down,” relationship.
Q:How could you show an inverse relationship with a graph? Sketch a graph to show what the relationship between
gas volume and pressure might look like. Let the x-axis represent volume (V) and the y-axis represent pressure (P).
A:Did you sketch a graph like the one in theFigure2.49? Let’s see why this graph is correct. Find the point on the
line where volume is smallest. That’s were pressure is highest. Then find the point where volume is largest. That’s