States of matter and kinetic theoryIB MYP Chemistry: Revision notes
Section 1
Solids, liquids and gases
Matter is anything that has mass and takes up space. It exists mainly in three states of matter: solid, liquid and gas.
The kinetic theory of matter says that all matter is made of tiny particles that are always moving. The state of a substance depends on how much energy its particles have and how strongly they attract each other. This idea is called the particle model.
Section 2
Arrangement, spacing and movement
Solid
- Particles are packed closely together in a regular pattern
- They are held in place by strong forces and only vibrate about fixed positions
Liquid
- Particles are close together but in an irregular arrangement
- They can slide past each other, so a liquid can flow
Gas
- Particles are far apart, with lots of empty space between them
- They move randomly at high speed in all directions, and the forces between them are very weak
The particles in a solid are not still. They are always vibrating. Particles themselves do not melt, expand or change colour when a substance is heated. Only their movement and spacing change.
Section 3
Energy of the particles
Particles in a solid have the least kinetic (movement) energy. Particles in a liquid have more, and particles in a gas have the most.
When a substance is heated, its particles gain kinetic energy. In a solid they vibrate more strongly. If they gain enough energy the substance changes state, first to a liquid and then to a gas.
This is why a hotter gas has faster-moving particles than a cooler one.
Section 4
Explaining properties with the particle model
- Shape: solids have a fixed shape because their particles are held in fixed positions. Liquids take the shape of their container because the particles slide past each other. Gases fill their container because the particles move freely.
- Volume: solids and liquids have a fixed volume because their particles are touching. Gases have no fixed volume because the particles spread out.
- Compressibility: gases can be compressed because there is a lot of space between the particles. Solids and liquids are almost impossible to compress because the particles are already touching.
- Density: solids and liquids are dense because many particles are packed into a small volume. Gases have a low density because the particles are far apart, so there are few particles in the same volume.
Worked example: why can air be squashed in a syringe but water cannot? Air particles are far apart, so the plunger pushes them closer together. Water particles are touching, so they cannot move closer.
For any property question, name the state, describe the particle spacing or movement, then link it to the property.
Section 5
Limitations of the particle model
The particle model is useful but it is a simplified model. In diagrams:
- Particles are drawn as hard, solid spheres, but real particles are not
- The forces between particles are not shown
- Particles are all drawn the same size and shape, but real particles can differ
- A diagram is two-dimensional and cannot show the constant movement
- The spacing in a gas is usually not to scale, because the particles are really much further apart
A good scientist uses a model to explain results and understands where it does not match reality.
Must Know
- Solid: particles close, regular pattern, vibrate in place, least energy
- Liquid: particles close, irregular, slide past each other
- Gas: particles far apart, random fast movement, most energy
- Gases are compressible because of the space between particles
- Solids and liquids have high density because particles are close together
- The particle model has limitations, such as drawing particles as hard spheres
That's the notes covered.
Carry on to the next subtopic.
Exam questions on States of matter and kinetic theory
- In a science lesson in Lagos, students seal the nozzles of two identical syringes. One syringe holds 20 cm³ of air and the other holds 20 cm³ of water. When the students push each plunger in firmly, the air syringe squashes to about 5 cm³ but the water syringe hardly changes volume. The teacher then draws the particles in each syringe as small hard spheres.Explain why the water in the syringe hardly changes volume when the plunger is pushed.2 marks
- A metal worker in Seoul heats a solid iron bar from 20 °C to 300 °C in a workshop. The bar becomes slightly longer but stays solid and keeps its shape. Later in the day the same bar is heated in a furnace until it melts.Explain, using the particle model, why the iron bar becomes slightly longer when heated but stays solid.2 marks
- Students in Santiago want to test the hypothesis that a gas is more compressible than a liquid. They seal the nozzles of two identical 50 cm³ syringes, one filled with 50 cm³ of air and one with 50 cm³ of water. They place the same 5 kg mass on each plunger, wait until the plunger stops moving and read the volume. Safety glasses are worn throughout.Identify the independent variable, the dependent variable and one control variable in this investigation.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).