... states that the product of the pressure and the volume of a gas is directly proportional to the temperature. The significance of the word 'ideal' is that this would be true of an ideal gas (or, if you like, in an ideal world). In practice, the law works quite well at normal temperatures and everyday pressures, but if the temperature or the pressure is very high, or very low, it's only an approximation.
The ideal gas law can be expressed as:
PV = nRT
where P, V and T represent the pressure, volume and temperature respectively. n is the number of moles of gas that are present – a mole being a defined 'amount of substance'; and R is something called the universal gas constant, which as its name implies is just a number. R can be expressed in lots of different ways, and depends on the units that you're using to define the temperature, volume and pressure; in the SI system, one way of expressing it is in joules per kelvin per mole. It's actually slightly under 8.315 joules per kelvin per mole.
© Haydn Thompson 2021