Matter and the kinetic theory of matter

This page covers a single Form 4 content standard: matter and the kinetic theory of matter. It opens the Matter and the Atomic Structure chapter and gives you the particle model you will use to explain states of matter, changes of state, dissolving, diffusion and gas behaviour throughout SPM Chemistry. Once you can describe particles confidently, a whole family of exam answers becomes routine.

What matter is

Matter is anything that has mass and occupies space (has volume). Everything around you, air, water, copper, your own body, is matter. All matter is made up of extremely small, discrete particles: atoms, molecules or ions. These particles are far too small to see, but the way they are arranged and how they move explains everything you observe about a substance.

The kinetic theory of matter

The kinetic theory of matter states that:

• Matter is made of tiny particles.
• The particles are in constant, random motion and therefore possess kinetic energy.
• The higher the temperature, the greater the kinetic energy and the faster the particles move.
• There are forces of attraction between the particles; the strength of these forces depends on how far apart the particles are.

These few ideas are powerful because they let you explain the three states of matter and the changes between them without memorising separate facts for each one.

The three states of matter

Solids, liquids and gases differ in three things you should always mention together: the arrangement of the particles, the forces of attraction between them, and the movement (kinetic energy) of the particles.

• Solid, particles are closely packed in an orderly, regular arrangement, held by strong forces of attraction. They can only vibrate and rotate about fixed positions, so a solid has a fixed shape and fixed volume.
• Liquid, particles are still close together but arranged randomly (not orderly), with weaker forces of attraction than in a solid. They can move and slide past one another, so a liquid has a fixed volume but no fixed shape (it takes the shape of its container).
• Gas, particles are very far apart in a completely random arrangement, with very weak, almost negligible forces of attraction. They move freely, quickly and randomly in all directions, so a gas has no fixed shape and no fixed volume and fills its whole container.

Key idea. When you compare states, answer in the same three terms every time, arrangement, forces of attraction, and movement of particles. A comparison that names all three earns full marks; one that mentions only “close” or “far apart” does not.

Evidence for moving particles

Two everyday observations support the kinetic theory. Diffusion is the spreading of particles from a region where they are more crowded to a region where they are less crowded, because of their random motion, for example, a coloured crystal spreading through still water, or a scent filling a room. Brownian motion is the small, random, jerky movement of visible specks (such as smoke or pollen grains) caused by invisible, fast-moving particles colliding with them; it is direct evidence that particles are moving constantly.

Worked example

Question. A purple crystal of potassium manganate(VII) is placed at the bottom of a beaker of cold, still water. After some time the whole solution turns purple, and it happens faster if the water is warm. Use the kinetic theory to explain both observations.

Step 1, Identify the process. The colour spreads because the potassium manganate(VII) particles move away from where they are crowded (near the crystal) to where they are less crowded. This spreading is diffusion.

Step 2, Explain the movement. The particles of both the solute and the water are in constant, random motion. As they move and collide, the coloured particles gradually mix evenly throughout the water, so the whole solution turns purple.

Step 3, Explain the effect of temperature. When the water is warmer, the particles have greater kinetic energy and move faster. Faster movement means the coloured particles spread out more quickly, so the solution turns purple in a shorter time.

Answer. The solution turns purple because the constantly moving particles diffuse and mix; it happens faster in warm water because the higher temperature gives the particles greater kinetic energy, so they move and diffuse more rapidly.

Practice question

Question. Using the kinetic theory of matter, explain why a solid has a fixed shape and a fixed volume, while a gas has neither.

Answer. In a solid, the particles are closely packed in a regular arrangement and held by strong forces of attraction, so they can only vibrate about fixed positions and cannot move from place to place, this gives the solid a fixed shape and fixed volume. In a gas, the particles are very far apart with very weak forces of attraction and move freely and randomly in all directions, so the gas has no fixed shape and no fixed volume and spreads to fill any container.

Exam tip

For any “explain in terms of particles” question, structure your answer around the same three points, arrangement, forces of attraction, and movement/kinetic energy, and always link fast movement to higher temperature. A common trap is to describe a gas as simply “particles that are far apart” while forgetting to mention that the forces of attraction are very weak and the movement is fast and random; the marking scheme usually wants all three ideas. Use the correct term diffusion for spreading and Brownian motion for the jerky movement of visible specks, and never say particles “disappear”, they only spread out.

Where this fits

This is the first standard of the Matter and the Atomic Structure chapter, and it feeds directly into the next standard on changes of state. To revise the whole chapter in order, use the revision notes, and to see more particle-model answers solved step by step, work through the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers check that your state comparisons always cover arrangement, forces and movement, because that is where full marks are won or lost. Secure the particle model here and later ideas such as changes of state and gas behaviour follow easily.

Quick recap

• Matter has mass and occupies space, and is made of tiny moving particles.
• The kinetic theory: particles move constantly; higher temperature means greater kinetic energy and faster movement.
• Compare solids, liquids and gases by arrangement, forces of attraction, and movement.
• Diffusion and Brownian motion are evidence that particles are always moving.