Relationship between pressure, density, temperature, and state of matter

1. Apr 22, 2009

wavingerwin

How do pressure, density, state of matter, and temperature relate?

these are my points, are they right?:
1. Higher temperature makes an object less dense
2. Density governs the state of matter for the object, hence as water becomes less and less dense, it becomes gas (water vapour)
3. Higher temperature makes an object has more pressure, hotter gas has higher pressure.
4. for pint 3, does it works backwards instead? i.e if a gas is pressured, it will be hotter.

Thank you!

2. Apr 22, 2009

Bob S

1) For a gas, PV = nRT (the ideal gas equation) so if V is fixed, then the density remains constant as it is heated. Water at 4 degrees C is more dense than water at zero deg. C.
2) Not always true. Ice floats on water, for example, so your statement is false.
3) Generally true.
4) Above the critical point, the liqiud and gas phases combine into a single phase called a supercritical state. So matter then has only two states, not three.

3. Apr 23, 2009

Andy Resnick

Those thermodynamic variables you mention are all related via constitutive equations; equations that cannot be derived from elementary principles. The ideal gas law (and other refinements such as van der Waals, see http://en.wikipedia.org/wiki/Equation_of_state) are approximations on how real materials act.