Study of Gas Laws

The One Thing

The behavior of a fixed amount of gas is governed by relationships among pressure, volume and temperature. Pressure and volume vary inversely at constant temperature, whereas volume and absolute temperature vary directly at constant pressure.

Reactions, Processes and Experiments

Key Terms

• Gas: A state of matter whose particles are far apart, move freely, and have neither a fixed shape nor a fixed volume.
• Pressure: The force exerted by gas particles when they collide with the walls of their container.
• Volume: The amount of space occupied by a gas, usually measured in litres or cubic centimetres.
• Temperature: A measure of the average kinetic energy, or motion, of gas particles.
• Boyle's law: For a fixed mass of gas at constant temperature, pressure is inversely proportional to volume.
• Charles' law: For a fixed mass of gas at constant pressure, volume is directly proportional to its absolute temperature.
• Absolute temperature: Temperature measured on the Kelvin scale, where 0 K is the lowest possible temperature.
• Kelvin scale: The temperature scale used in gas-law calculations; Kelvin temperature equals Celsius temperature plus 273.
• Fixed mass of gas: The amount of gas remains unchanged during the experiment.
• Constant temperature: The temperature remains unchanged while studying the relationship between pressure and volume.
• Constant pressure: The pressure remains unchanged while studying the relationship between volume and temperature.

Easily Confused

• Boyle's law and Charles' law: Boyle's law relates pressure and volume at constant temperature; Charles' law relates volume and absolute temperature at constant pressure.
• Pressure and volume: Pressure is caused by particle collisions with container walls, whereas volume is the space occupied by the gas.
• Celsius temperature and absolute temperature: Charles' law requires absolute temperature in kelvins, calculated using $T(K)=t(°C)+273$.
• Constant temperature and constant pressure: Boyle's law requires constant temperature; Charles' law requires constant pressure.
• Inverse and direct proportionality: In Boyle's law, pressure and volume change in opposite directions; in Charles' law, volume and absolute temperature change in the same direction.
• Fixed mass and changing conditions: Gas-law experiments require the amount of gas to remain unchanged while the relevant pressure, volume or temperature condition is varied.

What Gets Asked

• State or apply Boyle's law using $P\propto 1/V$, $PV=constant$, or $P1V1=P2V2$. The main mark-losing error is failing to state or maintain constant temperature.
• Explain what happens when the volume of a gas decreases or increases. The specific slip is reversing the relationship: decreasing volume increases pressure, while increasing volume decreases pressure.
• State or apply Charles' law using $V\propto T$, $V/T=constant$, or $V1/T1=V2/T2$. The main mark-losing error is using Celsius temperatures instead of kelvins.
• Convert temperatures using $T(K)=t(°C)+273$ before carrying out a Charles' law calculation.
• Explain gas pressure, compressibility or diffusion using particle motion. The relevant points are that particles move continuously, collide with container walls, have large spaces between them and mix freely.
• Apply the gas laws to examples such as compression of air in a syringe, inflation of balloons when heated, or increased pressure in a bicycle tyre on a hot day. The main slip is ignoring the required constant condition or the requirement that the gas mass remains fixed.