Properties of Gases: quick revision, 15 bites

What Makes a Gas a Gas

No shape, no fixed volume — just fill.

A gas has **no fixed shape and no fixed volume**. Its particles are far apart and move freely, so a gas expands to fill whatever container it is placed in.

Takeaway. Weak forces + free, fast particles = a gas fills any container completely.

The Four Measurable Properties of a Gas

Four numbers describe any gas.

Takeaway. P, V, T, and n are the four variables that fully describe a gas sample.

Kinetic Molecular Theory: Why Gases Behave This Way

Tiny fast particles explain everything.

The **kinetic molecular theory** explains gas behaviour by treating a gas as a large number of tiny particles in constant random motion.

Takeaway. Gas pressure comes from countless particle collisions with the container walls.

The Ideal Gas Equation

One equation links all four variables.

This single relation ties pressure, volume, temperature and amount together for an **ideal gas**.

Takeaway. PV = nRT is the master equation connecting all gas properties.

Boyle's vs Charles's Law

Two laws, two variables each.

Takeaway. Boyle: P and V move oppositely. Charles: V and T move together.

Combined Gas Law

Boyle + Charles in one shot.

Use this when **amount of gas (n) stays constant** but P, V, and T all change.

Takeaway. Combined gas law handles any change in P, V, or T when moles stay fixed.

Temperature Must Be in Kelvin

Kelvin, not Celsius — always.

All gas law calculations require temperature in **Kelvin (K)**, because gas laws are based on absolute temperature.

Takeaway. Convert Celsius to Kelvin before plugging into any gas law.

Why Tyres Look Flat in Winter

Cold air shrinks — your tyres know.

Takeaway. Temperature changes in daily life visibly change gas pressure and volume.

Don't Mix Up Pressure and Force

Pressure isn't the same as force.

Takeaway. Pressure is force per unit area — not force itself.

Diffusion: How Gases Mix on Their Own

Gases spread without any help.

**Diffusion** is the spontaneous spreading of gas particles from a region of higher concentration to lower concentration, due to their random motion.

Takeaway. Diffusion is direct evidence that gas particles are in constant random motion.

Gases Are Highly Compressible

Squeeze a gas — it shrinks easily.

Because gas particles are **far apart with lots of empty space**, gases can be compressed easily — unlike solids and liquids.

Takeaway. Big gaps between particles make gases the most compressible state of matter.

Remember the Gas Laws with 'BCC'

Three laws, three letters.

Takeaway. BCC = Boyle, Charles, Combined — the three core gas laws.

Exam Checklist for Gas Law Problems

Three checks before you solve.

Takeaway. Unit consistency and Kelvin conversion prevent most gas law errors.

Air Is a Mixture of Gases

You breathe a gas mixture.

Takeaway. The air around you is a mixture of gases, each contributing to total pressure.

Real Gases Aren't Perfectly Ideal

Ideal is a model, not reality.

Takeaway. Ideal gas law works well under normal conditions but breaks down at extremes.