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Science

Gas Laws

Overview

Students investigate how pressure, volume, and temperature of a gas are related, discovering the gas laws that describe the behaviour of all ideal gases.

Learning Objective
Students understand Boyle's law and Charles's law and can apply them to predict the behaviour of gases under changing conditions.

Resources needed

  • A syringe or inflatable bag
  • Ice and hot water for temperature investigation

Lesson stages

0 / 7 done
  1. 1 Ask: what happens to the air in a syringe when you push the plunger in? (It compresses — volume decreases, pressure increases).
  2. 2 Introduce Boyle's law: at constant temperature, pressure and volume of a gas are inversely proportional. P x V = constant.
  3. 3 Demonstrate: compress a sealed syringe and feel the increased pressure. Double the pressure, halve the volume.
  4. 4 Introduce Charles's law: at constant pressure, volume and temperature are directly proportional. V/T = constant (temperature in Kelvin).
  5. 5 Demonstrate: a balloon in hot water expands; in ice water, it contracts.
  6. 6 Introduce the combined gas law: PV/T = constant. Useful when both pressure and temperature change.
  7. 7 Apply: a weather balloon rises — as it goes higher, external pressure falls, so the balloon expands. What happens to the gas inside?

Tap a step to mark it as done.

Variations

  • Plot pressure vs volume data and verify the inverse relationship.
  • Calculate the volume of a gas at a different temperature or pressure using the combined gas law.
  • Discuss real vs ideal gases: real gases deviate from the laws at very high pressure or very low temperature.
More information

Teach: pressure, volume, temperature, Kelvin, Boyle's law, Charles's law, inversely proportional, directly proportional. Always use Kelvin for temperature in gas law calculations — Celsius gives wrong results because 0°C is not zero molecular motion.

Focus on Boyle's law (pressure-volume) with the syringe demonstration before introducing Charles's law (volume-temperature).

Can students state Boyle's law and Charles's law in words and as equations? Can they use the combined gas law to calculate an unknown variable given two states of a gas?

A syringe demonstrates Boyle's law. A balloon in warm and cold water demonstrates Charles's law. Both experiments require only very cheap materials.

Students often use Celsius in gas law calculations. This consistently gives wrong answers. Establish firmly that temperature must be in Kelvin for any gas law calculation. Zero Kelvin (absolute zero) is the true zero of temperature.

Gas laws underpin atmospheric science, engineering, refrigeration, and the behaviour of gases in biological systems. They are also a beautiful example of mathematically precise empirical laws discovered before molecular theory explained them.

Activity details
Secondary Challenging 25 minutes 10 minutes prep Small Group, Whole Class Indoor & Outdoor
Topics
Gas Laws Pressure Temperature