The Properties of a Gas: Pressure, Volume, Temperature, Amount

A Gas Is Described with 4 Quantities

Unlike a solid, a gas has neither a fixed shape nor a fixed volume: it fills all the available space. To fully describe the state of a given amount of gas, you only need to know 4 quantities:

• the pressure P, exerted by the collisions of gas molecules against the walls of the container (SI unit: the pascal, Pa);
• the volume V occupied by the gas (SI unit: the cubic meter, m³);
• the temperature T, which reflects the average agitation of the molecules (SI unit: the kelvin, K);
• the amount of substance n, the number of moles of gas present (unit: the mole, mol).

The Ideal Gas Model

An ideal gas is a simplified model in which gas molecules are assumed to be point-like (their own volume is negligible) and not to interact with each other except during collisions. This model describes most real gases very well under typical temperature and pressure conditions.

Why These Quantities Are Linked

These 4 quantities are not independent: if you compress a gas (you reduce its volume) without changing its temperature or its amount, its pressure necessarily increases, because the molecules hit the walls more often. Likewise, if you heat a gas at constant volume, the molecules move around more and the pressure increases. This mutual dependence is what the ideal gas law describes, presented in the next lesson.

Watch Out for Temperature Units

In chemistry, the temperature used in gas calculations is always in kelvin, never in degrees Celsius. The conversion is simple:

T(K) = theta(deg C) + 273.15

Common Mistake

Forgetting to convert the temperature to kelvin is the most frequent mistake: using a value in degrees Celsius directly in an ideal gas calculation gives a completely wrong result, because the relationship between pressure and temperature is only proportional in kelvin, not in Celsius.