States of matter and the particle modelIB MYP Physics: Revision notes
Section 1
The particle model
The particle model says that all matter is made of tiny particles that are always moving. The three states of matter are solid, liquid and gas. The state depends on how the particles are arranged, how strong the forces between them are and how they move.
The model helps us explain everyday observations, but it is a simplified picture: real particles are not hard coloured balls.
Section 2
Solids, liquids and gases
Solid: particles are packed closely in a regular pattern, with strong forces between them. They vibrate about fixed positions.
Liquid: particles are close together but in a random arrangement. The forces are weaker, so the particles slide past each other.
Gas: particles are far apart and move randomly at high speed. The forces between them are almost zero.
Section 3
Explaining the properties of matter
- Solids keep their shape because the strong forces hold the particles in fixed positions.
- Liquids flow and take the shape of their container because the particles slide past each other, but they are hard to compress because the particles are already touching.
- Gases are easy to compress because there is lots of empty space between particles, and they fill their container because the particles move freely.
- Solids and liquids have a fixed volume. Gases do not.
Section 4
Changes of state
Changing state is a physical change. The particles stay the same, but their energy and arrangement change.
- melting: solid to liquid (heating)
- freezing: liquid to solid (cooling)
- boiling: liquid to gas throughout the liquid, at the boiling point
- evaporation: liquid to gas at the surface, at any temperature below the boiling point
- condensation: gas to liquid (cooling)
- sublimation: solid directly to gas (for example dry ice)
While a substance is changing state, the temperature stays constant. The energy supplied is used to overcome the forces between the particles, not to make them move faster.
Evaporation and boiling are different. Evaporation happens only at the surface and at any temperature; boiling happens throughout the liquid at one temperature.
Section 5
Brownian motion
Brownian motion is the random zigzag movement of small visible specks (such as smoke or pollen) in air or water.
The specks are pushed about by collisions with fast-moving, invisible particles of the air or water. The collisions on different sides are not equal, so the speck moves in a random direction. This is evidence that matter is made of tiny moving particles.
Section 6
Diffusion
Diffusion is the spreading of particles from where there are many of them (a high concentration) to where there are fewer (a low concentration), until they are evenly mixed. It happens because the particles move randomly.
Examples: a smell of perfume spreading across a room, or ink spreading in water.
Diffusion is faster in gases than in liquids, and faster at a higher temperature, because the particles have more kinetic energy and move faster.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on States of matter and the particle model
- Ice cubes are taken from a freezer and left in a glass in a hot kitchen in Dubai. The ice slowly changes to water, and later some of the water disappears from the glass.Describe how the arrangement and movement of the particles in the water differ from those in the ice.2 marks
- A student in Seoul looks at specks of smoke in a sealed glass cell under a microscope. The specks move in random zigzag paths. In another lesson, a teacher opens a bottle of perfume at the front of a still classroom, and after a minute students at the back can smell it.Explain why the perfume smell would reach the back of the room more quickly on a warmer day.2 marks
- A class in Cape Town heats crushed ice gently in a beaker. A thermometer is held in the beaker and the temperature is read every minute. The temperature rises steadily from -8 °C to 0 °C in 4 minutes, stays at 0 °C for 6 minutes while the ice melts, rises steadily to 100 °C over 10 minutes, and then stays at 100 °C while the water boils.Explain, using the particle model, why the temperature stays at 0 °C for 6 minutes even though the beaker is still being heated.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).