Ideal gas: definition in chemistry
Ideal gas: An imaginary gas whose particles take up no space and neither attract nor repel each other, so it obeys the ideal gas law, PV = nRT, exactly.
In the ideal gas law, P is pressure, V is volume, n is the number of moles, T is the temperature in kelvins and R is the gas constant, 8.314 J/(mol·K). Real gases behave almost ideally at low pressures and high temperatures, when their particles are far apart and moving fast. At high pressures or low temperatures they deviate, because particle size and attractions start to matter, which is also why real gases can condense into liquids.
Examples
- 1 mole of gas at 0 °C and 1 atm takes up about 22.4 L
- Doubling the kelvin temperature at constant pressure doubles the volume
- Halving the volume at constant temperature doubles the pressure
- Car tire pressure rises after driving, as the air inside warms up
On the periodic table
Helium, at the top of group 18, behaves most like an ideal gas, because its tiny atoms barely attract one another.