# What is the independent variable in Boyle's Law?

## Main Question or Discussion Point

I asked my teacher, and I was told that volume is the independent variable, and that pressure is dependent. The textbook I'm using said the same thing. After looking up some graphs of the law, I found that both pressure and volume were used as independent variables.

It seems counter-intuitive to me, that volume would be the independent variable. Does that mean that if I have two gasses, Gas A and Gas B (← lesser volume), that Gas B should be under greater pressure, because of it's lower volume? If pressure is measured as force in Newtons / unit of area, then why would volume matter, if we only care about the the 2D area in contact with the atmosphere? Unless, it is the surface area in contact with the environment that we care about for pressure, which would make sense.

To me, having pressure be the independent variable makes much more intuitive sense, as a greater pressure should decrease the volume.

Thanks for any help.

Bystander
Homework Helper
Gold Member
It seems counter-intuitive to me, that volume would be the independent variable.
"Intuition" is the last method to use in the sciences. Two variables: pressure and volume. Which one could be controlled? Volume. Therefore volume is the "independent" variable for the series of measurements used as the basis for Boyle's Law. Some other experimental situation, pressure might be more easily controlled and be the "independent" variable. "Independence" is NOT an absolute property of any particular variable, it is a matter of what variable is selected to be varied independently to observe the behavior of a dependent variable.

• AAAA
it is the surface area in contact with the environment that we care about for pressure, which would make sense.
Is this assumption correct?

Bystander
Homework Helper
Gold Member
"... the surface area in contact with the environment that we care about for pressure ...."
Is this assumption correct?
As stated, the "assumption" makes no sense.
Try rewording it, and we'll see if we can't straighten out the confusion.

Try rewording it, and we'll see if we can't straighten out the confusion.
When measuring pressure, it is measured as the force in Newtons/surface area.

Bystander
Homework Helper
Gold Member
More strictly, the normal force per unit area; "normal" meaning the component of force perpendicular to the surface. Close enough.
Does that mean that if I have two gasses, Gas A and Gas B (← lesser volume), that Gas B should be under greater pressure, because of it's lower volume?
Let's complete this question with the detail you need to understand it: if you have the same number of moles of two gases, A and B, or two same size samples, A and B, of a single gas, at the same temperature (that much was built into the discussion that introduced you to the concept), THEN, B is at greater pressure because of the lower volume. Make sense?

Yes, thank you. That makes a lot more sense.

DrDu