Gas Laws
Definition and meaning of Gas Laws in chemistry.
The gas laws are a set of simple rules describing how the pressure, volume, temperature, and amount of a gas depend on one another. Change any one of them and the others respond in a predictable way. Together they combine into a single equation called the ideal gas law.
In more detail
Gases behave far more predictably than solids or liquids. Their particles are spread far apart and barely interact, so a handful of relationships describe almost any gas. Boyle's law connects pressure and volume at a fixed temperature.
Squeeze a gas into half the space and its pressure doubles. Charles's law connects volume and temperature at fixed pressure, which is why a gas expands when you warm it. Gay-Lussac's law connects pressure and temperature at fixed volume, and it explains why a sealed can may burst if it gets hot.
Avogadro's law says that at the same temperature and pressure, equal volumes of any gas contain equal numbers of particles. Combining all four gives the ideal gas law, PV = nRT. P is pressure, V is volume, n is the number of moles, T is temperature, and R is the gas constant.
That one equation replaces the individual laws for most calculations. The most common mistake with any of these is using degrees Celsius. Every gas law needs absolute temperature, so you must convert to kelvin first.
Warming a gas from 10 to 20 degrees Celsius does not double its volume, because in kelvin that is only 283 to 293. Real gases follow these rules closely under everyday conditions. They drift away from them at very high pressure or very low temperature, where particles are crowded enough that their own size and their attraction to each other start to matter.
Key facts
| What They Relate | Pressure, volume, temperature, and amount of gas |
|---|---|
| Boyle's Law | Pressure and volume are inverse at constant temperature |
| Charles's Law | Volume and temperature rise together at constant pressure |
| Gay-Lussac's Law | Pressure and temperature rise together at constant volume |
| Avogadro's Law | Equal volumes of any gas hold equal numbers of particles |
| Combined Form | The ideal gas law, PV = nRT |
| Critical Rule | Temperature must always be in kelvin |
| Field | General Chemistry |
A balloon holds 2.0 liters of gas at 300 kelvin. Warm it to 600 kelvin while the pressure stays the same, and Charles's law predicts the volume doubles to 4.0 liters, because volume and absolute temperature rise together. In Celsius the same change runs from about 27 degrees to about 327 degrees, which looks nothing like doubling. That is exactly why the conversion to kelvin matters.
Frequently asked questions
Why does temperature have to be in kelvin?
The gas laws compare temperatures as ratios, and that only works from a true zero. The kelvin scale starts at absolute zero, the point where particle motion stops. Zero degrees Celsius is just the freezing point of water, so ratios taken in Celsius give the wrong answer.
What is the difference between the combined gas law and the ideal gas law?
The combined gas law compares one state of a fixed amount of gas to another, so the amount cancels out. The ideal gas law includes the number of moles and the gas constant, so it can handle a changing amount of gas and give absolute values.
Do real gases obey the gas laws exactly?
Not exactly, but close enough for most work at ordinary temperatures and pressures. The laws assume particles take up no space and do not attract each other. At high pressure or low temperature both assumptions fail, and real gases deviate noticeably.
Related terms
Sources & references
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