I didn't see this in any other question, although it seems to be a fundamental question of the gas laws. I understand that Charles' Law practically has to use Kelvin temperature, because you would end up with a negative number in some cases, but can someone explain clearly what Charles did with the three gases and why the volume can only be proportional to the Kelvin temp and not the Celsius temp? thanks.

  • $\begingroup$ Imagine the following scenario. You want to find a relationship between the number of people and the total number of hands. But to facilitate easier counting, your count of hands is the number of hands - 273. Are you going to find the simple relationship that you were supposed to find? $\endgroup$
    – Zhe
    Commented Nov 29, 2018 at 20:35

2 Answers 2


It is partially about definitions and partially an about making the formulae simpler

Consider how you work out the relationship between volume and temperature of a gas. You measure the volume of a fixed amount of gas at different temperatures and plot the results on a chart (volume on Y, temperature on X). The points show a linear relationship (for gases whose behaviour is close to ideal at the range of chosen temperatures).

Where does the line cross the Y-axis (temperature)? It isn't at 0 °C, is it? It is at about -273 °C. So the formula could look like this: V=x(T+273) with temperature in Celsius. Or, you could redefine the temperature scale and make it simpler: V=xT with T in Kelvin. This is far simpler in practice and keeps the maths neater.

But this is, essentially, how we know what absolute zero is. This is the very definition of the term (which works on other temperature scales as well but only Americans still care about Fahrenheit). We chose 0 as the original base for the Celsius scale because that is the freezing point of water not because of anything to do with gas laws. So that is an arbitrary point that happens to give a more complicated gas law. Hence the Kelvin scale where the zero relates to the gas law and makes it simpler.

So volume is proportional to temperature in Celsius only if you adjust the temperature because the scale has an arbitrary point chosen for zero. Kelvin keeps things simpler and neater.


First you must know how was Charles' discovered his law? See on wikipedia Jacques Charles

Here is a quote from Wikipedia

Around 1787 Charles did an experiment where he filled 5 sites to the same volume with different adds. He then raised the temperature of the sites to 80 °C and noticed that they all increased in volume by the same amount. This experiment was referenced by Gay-Lussac in 1802 when he published a paper on the precise relationship between the volume and temperature of a gas. Charles' law states that under constant pressure, an ideal gas' volume is proportional to its absolute temperature. The volume of a gas at constant pressure increases linearly with the absolute temperature of the gas. The formula he created was $\frac{V_1}{T_1} = \frac{V_2}{T_2}$

As all statements of experimental laws are completely based on the results of experiment performed and cannot be altered. Charles'law is also an experimental law, and it was discovered from the experiment performed by Jacques Charles and he found that the Volume of a ideal (close to Ideal) gas is directly proportional to absolute temperature.

You are saying that

Why volume can't be proportional to temperature in Celcius?

But how it can be if it is not the result of the experiment.

Suppose a cricketer just scored six and you are asking why can't we write his score as four instead of six. In the same way Charles discovered that Volume is directly proportional to absolute temperature and you are saying why it is not temperature in Celcius.

If you want to know why volume can only be proportional to absolute temperature - experiment is the proof.


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