Physicist: Lord Kelvin (and others of his ilk) noticed that when you hold the volume of an ideal gas constant you get a nice, linear relationship between pressure and temperature.
Temperature vs. Pressure. PV = nRT works for all ideal gases, independent of the particular gas in question (helium, water vapor, sulfur hexafluoride, ...). Different gases become liquids or solids (and stop exerting pressure) at different temperatures, so they drop off of the "ideal gas graph" at different points.
By the by, an ideal gas is just a gas where you can assume that the particles are bouncing off of each other much harder than they’re trying to stick together (the gas is hot enough that it’s a long way from condensing). This assumption allows you to use the “billiard ball model” of gas dynamics, which in turn leads to the ideal gas law (
Although it’s impossible to cool anything off completely, and despite the fact that all of the gases that Lord Kelvin was working with became liquids when chilled enough, it was still easy to graph temperature vs. pressure (even around room temperature) and then extend the line to find the temperature where the pressure should be zero. Kelvin figured that this would be a much more natural place for “zero” to be, and he carefully measured it (by extending the line) to be around -273.15°C, which is now 0°K (zero degrees Kelvin).
Using Kelvins instead of Celsius means that you can bust out the ideal gas law without needing to adjust anything. If you wrote the ideal gas law using Celsius instead, it would be 
hmmm. I never thought about that question. Nice simple answer too.
What if pressure comes below zero?
Pressure is caused by molecules bouncing off of each other. You can make the pressure higher by bouncing them harder (raise the temperature), or you can make the pressure higher by adding more gas, so there are more molecules to do the bouncing.
When you remove the gas the pressure drops. If you remove all of the gas there’s nothing left to bounce, and the pressure is zero. Since there’s nothing left to remove, the pressure can’t drop any further.
Similarly, if you drop the temperature the molecules so down. If you (could) drop the temperature to zero, the molecules would stop moving altogether, and the pressure would be zero again. Since you can’t get any slower than stopped, the pressure can’t be dropped any lower.
Long story short: pressure has to be zero or higher.
>and he carefully measured it (by extending the line) to be around -273.15°C, which is now 0°K (zero degrees Kelvin).<
I do apologize for commenting this, but it always annoys me when units are not correctly used: The Kelvin scale has no degrees !
(For the rest: great site , beautiful questions and great answers. Keep it up!)
thanks for your valuable clarifications………….they are simply great.