Kinetic theory and gas behaviourIB MYP Sciences: Revision notes
Section 1
Particles in constant random motion
The kinetic theory says that all matter is made of particles that are always moving. In a gas the particles move quickly in random straight lines, collide with each other and with the container walls, and change direction after each collision.
The higher the temperature, the more kinetic energy the particles have and the faster they move.
Section 2
Gas pressure
Gas pressure is caused by gas particles colliding with the walls of their container. Each collision pushes on the wall, and billions of collisions every second add up to a steady pressure. It is the same in every direction.
Pressure does not come from particles hitting each other, or from the weight of the gas.
Do not say particles 'expand' when heated. They move faster; their size does not change.
Section 3
What changes the pressure?
For a fixed amount of gas:
- Temperature up: particles move faster, so collisions with the walls are more frequent and more forceful. Pressure rises (at constant volume).
- Volume down: the same particles hit the walls more often because they have less space. Pressure rises (at constant temperature).
- More particles in the same container: more collisions each second, so pressure rises.
The opposite changes reduce the pressure. When you answer, always refer to the frequency and force of collisions.
Use the phrase 'more often and with more force' to explain why heating a gas in a sealed container raises its pressure.
Section 4
Absolute zero and the Kelvin scale
Absolute zero is the lowest possible temperature, -273 °C. Particles have the least possible kinetic energy there and a gas would exert almost no pressure.
The Kelvin scale starts at absolute zero (0 K). To convert, K = °C + 273. For example, 27 °C = 300 K, and 0 °C = 273 K. A change of 1 K is the same size as a change of 1 °C.
Section 5
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion. It happens in gases and liquids, but not in solids.
It is faster in gases than in liquids because the particles are further apart, weakly attracted and collide less often. It is faster at higher temperatures because the particles move faster. Examples are perfume spreading through a room, or food colouring spreading in water.
Section 6
Brownian motion
Brownian motion is the random, jerky movement of tiny visible particles, such as smoke or pollen, in a gas or liquid. It happens because the invisible, fast-moving particles of the gas or liquid collide with them. At any moment the collisions on one side are more than on the other, so the visible particle is pushed in a random direction.
It is evidence that matter is made of particles that are moving constantly and randomly.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Kinetic theory and gas behaviour
- A hospital stores oxygen gas in sealed steel cylinders in a room with an air-conditioning system. The gas particles inside each cylinder are in constant random motion.A cylinder is left in hot sunlight. Explain, in terms of particles, why the pressure of the gas inside it increases.2 marks
- In a kitchen in Mumbai, the smell of spices frying on the stove reaches the far side of the room within a minute. In the same kitchen, a drop of food colouring added to a glass of water slowly spreads until the whole glass is coloured, without anyone stirring it.The cook finds that the smell of the spices spreads more quickly when the kitchen is warm. Explain this using the particle model.2 marks
- A class investigates how temperature affects diffusion. They add one crystal of potassium manganate(VII) to a beaker containing 200 cm³ of water and time how long it takes for the purple colour to spread evenly through the water. They repeat the test with water at different temperatures.State a testable hypothesis for this investigation, including a scientific reason.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).