Edexcel GCSE Chemistry (1CH0) · Topic 2 — States of matter and mixtures
Mini-Lesson
States of Matter & Mixtures
This mini-lesson covers the key ideas of Edexcel Topic 2: the particle model and the three states, the changes of state & heating curves, pure substances vs mixtures, and how to separate mixtures — distillation, filtration, crystallisation and chromatography (with Rf) — plus making water potable.
Make a guess: what happened to the water in the pan?
Mission 1Did the water particles swell up?
Mission 2How can paper pull an ink apart?
Work through each screen, answer the questions as you go (some wordy, some calculations) and collect ⭐ stars. Press Start when you're ready.
The three states
Particles in solids, liquids & gases
Particle theory models matter as small solid spheres. The arrangement, movement and energy of the particles decides the state:
Same particles, three arrangements. Heating gives the particles more energy to overcome the forces between them.
State symbols in equations: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water).
Quick check
Read the particle diagram
?The particles are close together but arranged randomly and can slide past one another. Which state is this?
Changes of state
Melting, boiling, freezing, condensing
Changing state is a physical change — it is reversible and conserves mass; no new substance is made. Each change has a name:
Melting (solid → liquid) and freezing (liquid → solid) both happen at the melting point.
Boiling (liquid → gas) happens at the boiling point; evaporating (from the surface) and condensing (gas → liquid) also happen below it.
Sublimation is solid → gas directly (and deposition is gas → solid).
Sort it
Name the change of state
Tap the change of state happening in each case.
Mission 1 check
Did the water particles swell up?
?Water boils in an open flask standing on a balance. The balance reading slowly goes down. What has happened to the water?
Heating curves
Reading a heating curve
Heat a solid steadily and plot temperature against time. The graph has two flat plateaus — at each one, energy is going into overcoming the forces between particles (changing state), not raising the temperature:
The lower plateau is melting (solid→liquid); the higher, longer plateau is boiling (liquid→gas). Temperature is constant during a change of state.
Key idea: on a flat section both states are present and the temperature holds steady because the added energy overcomes the forces between particles instead of speeding them up.
Quick check
Interpret the curve
?On a heating curve, what is happening during the second (higher, longer) flat section?
Extension · not on the Edexcel spec
Limits of the simple particle model
The simple particle model is useful, but it is a simplification. It helps to know its limitations:
It shows particles as solid spheres — real particles are not solid balls.
It shows all particles as the same size, when in reality they differ.
It treats particles as having no forces between them, yet these forces of attraction are exactly what must be overcome to change state.
Extension · Quick check
What the model misses
?Which is a genuine limitation of the simple particle model?
Mission 2 of 2
How can paper pull an ink apart?
Make a guess: how can paper pull an ink apart?
Pure substances & mixtures
Pure vs mixture
In chemistry a pure substance is a single element or compound with nothing else in it. A mixture contains two or more substances not chemically bonded together.
A pure substance has a sharp, fixed melting point and boiling point. A mixture melts and boils over a range of temperatures — that's how melting-point data tests purity.
Common slip: "pure" in chemistry does not mean "clean" or "natural". Pure orange juice is really a mixture; only a single element or compound is chemically pure — and it has a sharp melting point.
Quick check
Is it pure?
?A solid melts sharply at exactly 80 °C. A second solid softens and melts gradually between 60 °C and 75 °C. What can you conclude?
Separating mixtures
Choosing a technique
Because a mixture isn't chemically bonded, it can be separated by physical methods. Match the method to the mixture:
Filtration — separates an insoluble solid from a liquid (e.g. sand from water).
Crystallisation — gets a soluble solid back from its solution by heating to evaporate solvent, then cooling so crystals form.
Simple distillation — separates a liquid from a solution (e.g. water from salt water) by boiling then condensing.
Fractional distillation — separates a mixture of liquids with different boiling points, using a fractionating column.
Paper chromatography — separates a mixture of soluble, coloured substances (e.g. dyes in an ink).
Distillation apparatus
Fractional distillation
The mixture is heated; the liquid with the lowest boiling point vaporises first, rises up the fractionating column, then condenses in the water-cooled condenser and is collected. Higher-boiling liquids stay behind or condense lower down.
Simple distillation uses the same flask + condenser to separate a liquid from a solution; the fractionating column is the extra part that separates liquids with close boiling points.Match it
Which separation technique?
Tap a mixture on the left, then the technique on the right that separates it.
Chromatography · core practical
Chromatography & the Rf value
Paper chromatography separates soluble substances. The paper is the stationary phase; the solvent that rises up it is the mobile phase. More-soluble substances travel further. Each spot's position is described by its Rf value:
Rf = distance moved by spot ÷ distance moved by solventmeasure both distances from the baseline (start line) to the centre of the spot, and to the solvent front
Rf = 4.0 ÷ 5.0 = 0.8. Measure from the baseline to the centre of the spot, and from the baseline to the solvent front.
Watch out: Rf has no units and is always less than 1 (the spot can't pass the solvent front). Draw the baseline in pencil so it doesn't dissolve, and measure to the spot's centre.
Calculate
Your turn — calculate Rf
1On a chromatogram a spot moves 3.0 cm from the baseline while the solvent front moves 12.0 cm. Calculate the Rf value.
(no units)
Hint: Rf = 3.0 ÷ 12.0. The answer is less than 1 and has no units.
Calculate
One more Rf
2A dye spot travels 6.4 cm and the solvent front travels 8.0 cm. What is the Rf value of the dye?
(no units)
Hint: Rf = 6.4 ÷ 8.0.
Mission 2 check
How can paper pull an ink apart?
?A green food colouring is spotted on a pencil line and run in water. It splits into a yellow spot near the top and a blue spot near the bottom. Why did the two dyes separate?
Treating water
Making water potable
Potable water is water that is safe to drink — it is not the same as pure water, because it still contains dissolved substances. To make ground water or waste water potable:
Sedimentation — suspended solids settle to the bottom and are removed.
Filtration — passing the water through filter beds removes finer insoluble solids.
Sterilisation — microbes are killed, e.g. by chlorination (chlorine gas), ozone or UV light.
Potable ≠ pure: potable water is safe to drink but is a mixture (it has dissolved salts). Distillation would make it pure but needs a lot of energy.
Quick check
Safe to drink
?Which stage of water treatment is used to kill microbes so the water is safe to drink?
Recap
The key points to know
Three states: solid (regular, vibrating, least energy), liquid (random, close, slide), gas (spread out, most energy).
Changes of state: melting/freezing at the melting point; boiling at the boiling point (evaporating and condensing also happen below it); sublimation = solid→gas. Physical & reversible; mass conserved.
Heating curve: flat plateaus = changing state at constant temperature.
Pure: single element/compound with a sharp melting point; mixtures melt over a range.
Separation: filtration, crystallisation, simple & fractional distillation, paper chromatography.
Rf: distance spot ÷ distance solvent (no units, < 1).