States of matter and particle modelIB MYP Sciences: Revision notes
Section 1
Solids, liquids and gases
All matter is made of tiny particles (atoms, molecules or ions). The particle model explains the properties of the three states.
- Solid: fixed shape and fixed volume. It cannot be compressed or poured.
- Liquid: no fixed shape (it takes the shape of its container) but a fixed volume. It can flow and is almost impossible to compress.
- Gas: no fixed shape and no fixed volume, so it fills its container. It flows and is easily compressed.
Section 2
Arrangement, movement and forces
- Solid: particles are touching in a regular pattern. Strong forces hold them in place, so they only vibrate about fixed positions.
- Liquid: particles are close together but in an irregular arrangement. The forces are weaker, so they slide past each other.
- Gas: particles are far apart and move quickly and randomly in all directions. The forces between them are very weak.
Solids and liquids are hard to compress because their particles are already touching. Gases can be compressed because there are large spaces between the particles.
Particles do not change size when a substance changes state. Only their arrangement, spacing and movement change.
Section 3
Changes of state
Changes of state are physical changes: the particles stay the same, but their energy and arrangement change.
- Melting: solid to liquid
- Freezing: liquid to solid
- Boiling: liquid to gas, throughout the liquid, at the boiling point
- Evaporation: liquid to gas, at the surface only, at any temperature below the boiling point
- Condensation: gas to liquid
- Sublimation: solid straight to gas (for example dry ice)
In evaporation the fastest particles escape from the surface, so the remaining liquid cools. It is faster when it is warmer, windier, drier or when the surface area is larger.
Section 4
Heating and cooling curves
A heating curve shows temperature against time as a substance is heated.
- Sloping parts: the substance is in one state and the energy supplied raises the kinetic energy of the particles, so the temperature rises.
- Flat parts: a change of state is happening. The energy supplied is used to overcome the forces between particles, so the temperature stays constant.
The first flat part is at the melting point and the second is at the boiling point. A cooling curve is the reverse: flat parts show freezing and condensing, when particles release energy as forces form again.
On a flat part, never say 'no energy is being supplied'. Say the energy is used to overcome the forces between particles.
Section 5
Melting and boiling points
The melting and boiling points tell you the state at a given temperature. For example, a substance with melting point -7 °C and boiling point 59 °C is a liquid at 25 °C.
A pure substance melts at one sharp fixed temperature. An impure substance melts over a range and at a lower temperature. Stronger forces between particles mean higher melting and boiling points.
Section 6
Limits of the particle model
The model is useful, but it is simplified. In diagrams:
- particles are drawn as solid spheres, but real atoms and molecules are not hard balls
- the forces between particles are not shown
- the spaces and sizes are not drawn to scale
- a 2D picture cannot show the movement of particles.
A good scientist describes a model's strengths and its limits.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on States of matter and particle model
- At a school science fair in Dubai, a student shows a block of dry ice (solid carbon dioxide) in a bowl. The block gets smaller and a cold gas pours over the edge of the bowl, but no liquid collects in the bowl. The student explains the demonstration using the particle model, in which each particle is drawn as a small solid sphere.Explain, using the particle model, why a gas can be compressed into a smaller volume but a solid cannot.2 marks
- In Dubai in summer, wet laundry hung on a balcony dries within a couple of hours, even though the temperature is far below 100 °C and the water never boils. A student wants to explain why the water leaves the clothes and why the air near the laundry feels slightly cooler.Explain, in terms of particles, why evaporation makes the remaining liquid cooler.2 marks
- A student heats a solid sample of pure stearic acid in a water bath and records its temperature every minute. The temperature rises steadily from 20 °C, stays at 69 °C for several minutes while the solid melts, and then rises again.State the melting point of stearic acid and explain, in terms of particles and energy, why the temperature stays constant while it melts.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).