Pearson Edexcel International GCSE Chemistry 4CH1: States of matter; elements, compounds and mixtures – Revision Notes

These notes condense sub-topics 1(a) States of matter and 1(b) Elements, compounds and mixtures of the Pearson Edexcel International GCSE Chemistry (4CH1) specification, Issue 3 (September 2024): points 1.1 to 1.13. The qualification is untiered. Points 1.5C, 1.6C and 1.7C (solubility) are Paper 2 only; the rest can appear on either paper. For full explanations and worked examples, use the study guide for this unit.

Also useful: the 4CH1 Chemistry course hub, the printable checklist, the practice questions for this unit, the wider Principles of chemistry revision notes and the free 4CH1 all-topics diagnostic.

1.1 The three states

	Solid	Liquid	Gas
Arrangement	Close, regular	Close, irregular	Far apart, random
Movement	Vibrate about fixed positions	Move around each other	Fast, random, all directions
Energy	Lowest	Middle	Highest
Forces between particles	Strongest	Weaker	Weakest

Use all three words, arrangement, movement, energy, in any “describe” answer. Each is usually a separate mark.

1.2 Changes of state

From → to	Name	Achieved by	Particles
Solid → liquid	Melting	Heating	Gain energy, overcome some forces, start moving around each other
Liquid → gas	Boiling / evaporating	Heating	Gain energy, overcome remaining forces, move far apart
Gas → liquid	Condensing	Cooling	Lose energy, move closer, forces hold them together
Liquid → solid	Freezing	Cooling	Lose energy, settle into fixed positions in a regular pattern
Solid → gas	Sublimation	Heating	Go straight from fixed positions to far apart

Must-know distinctions

• Evaporation: at the surface only, below the boiling point. Boiling: throughout the liquid, at the boiling point.
• Temperature is constant during a change of state of a pure substance: the energy overcomes forces between particles instead of raising the temperature.
• Freezing point and melting point of a pure substance are the same temperature.

1.3 Evidence for particles

Diffusion: the random movement of particles from high concentration to low concentration until evenly spread.

• Faster in gases than in liquids.
• Faster at higher temperature (particles have more energy).
• Faster for lighter particles.
• Almost instant into a vacuum (no other particles to collide with).

Dilution of a coloured solution (e.g. potassium manganate(VII)): repeated dilution makes the colour paler but it remains visible for many dilutions. Meaning: a small amount of solid contains a huge number of tiny particles; dilution spreads them among more water particles, so fewer coloured particles per cm³.

Method in steps: dilution factor

1. Work out the factor for one step: final volume ÷ volume taken (1 cm³ made up to 10 cm³ = 10).
2. Raise it to the number of steps.
3. Concentration left = 1 ÷ (that number).

1.4 Solution words

Term	Meaning
Solvent	The liquid that does the dissolving
Solute	The substance that dissolves
Solution	Solute dissolved in solvent
Saturated solution	No more solute can dissolve at that temperature

1.5C–1.7C Solubility – Paper 2 only

Solubility = mass of solute that dissolves in 100 g of solvent at a stated temperature, making a saturated solution. Unit: g per 100 g of solvent.

Method in steps: solubility from a practical

1. Make a saturated solution at a fixed temperature: excess solid, stir, water bath, some solid left over.
2. Weigh an empty evaporating basin, then basin + some clear saturated solution.
3. Evaporate the water; heat to constant mass.
4. Mass of solute = (basin + solid) − basin.
5. Mass of water = (basin + solution) − (basin + solid).
6. Solubility = mass of solute ÷ mass of water × 100.

Method in steps: crystals from a solubility curve

1. Read the solubility at the start and end temperatures from the curve.
2. Subtract: that is the mass of crystals per 100 g of water.
3. Scale by (actual mass of water ÷ 100).

Reading a curve: solubility on the y-axis, temperature on the x-axis, smooth curve. On the curve = saturated; below = unsaturated. A steep curve means solubility changes a lot with temperature, so cooling gives a big crop of crystals.

Small reminder. 7.2 g of a salt saturates 40 g of water at 25 °C. Solubility = 7.2 ÷ 40 × 100 = 18 g per 100 g of water at 25 °C.

1.8 Element, compound or mixture

	Element	Compound	Mixture
Made of	One type of atom	Two or more elements chemically combined	Two or more substances not chemically combined
Proportions	–	Fixed	Can vary
Properties	Its own	Different from its elements	Each part keeps its own
Separated by	Cannot be broken down chemically	Chemical reaction only	Physical methods
Examples	Cu, Cl₂, S	CO₂, CaCO₃, H₂O	Air, sea water, crude oil, ink

Particle diagrams: count the kinds of “unit”. One kind, one type of atom = element. One kind, different atoms joined = compound. More than one kind = mixture. A diatomic molecule such as Cl₂ is still an element.

1.9 Pure or impure?

• Pure substance: sharp, fixed melting point and boiling point (pure water: 0 °C and 100 °C at normal atmospheric pressure).
• Mixture: may melt or boil over a range of temperatures.
• Test: measure the melting point and compare with the data-book value.

1.10 Choosing a separation method

You want to…	Use
Remove an insoluble solid from a liquid	Filtration
Get a soluble solid out of its solution	Crystallisation
Get the solvent (e.g. pure water) out of a solution	Simple distillation
Separate liquids that mix and have different boiling points	Fractional distillation
Separate or identify dissolved substances such as dyes	Paper chromatography

Method in steps: crystallisation

1. Heat the solution to evaporate some of the water.
2. Dip in a glass rod; when crystals form on it as it cools, the solution is saturated.
3. Leave to cool so crystals form.
4. Filter off the crystals; wash with a little cold water; dry between filter papers.

Distillation points to remember: condenser water enters at the bottom; the thermometer bulb sits level with the side arm; in fractional distillation the lower-boiling liquid distils first and the thermometer reading stays steady while each liquid comes over.

1.11–1.13 Chromatography

• Baseline in pencil (ink would dissolve and run).
• Solvent level below the baseline (spots would dissolve into the solvent).
• Lid on the beaker; mark the solvent front before the paper dries.
• One spot = pure; several spots = mixture; spot level with a reference = same substance; spot left on baseline = insoluble in that solvent.

Rf = distance moved by substance / distance moved by solvent

Both distances are measured from the baseline. Rf has no units and is less than 1. Compare Rf values only for the same solvent and paper.

Quick self-test

1. Name the change of state from gas to liquid.
2. Why does the temperature of a pure liquid stay the same while it boils?
3. State two differences between evaporation and boiling.
4. 1 cm³ of a dye solution is added to 99 cm³ of water, and this is repeated once more with the new solution. What fraction of the original concentration remains?
5. Define a saturated solution.
6. (Paper 2 only) 4.5 g of a solid dissolves in 15 g of water to make a saturated solution at 30 °C. Calculate its solubility.
7. (Paper 2 only) A salt has a solubility of 64 g per 100 g of water at 70 °C and 24 g per 100 g at 15 °C. A saturated solution in 200 g of water is cooled from 70 °C to 15 °C. What mass of crystals forms?
8. (Paper 2 only) A solid has a solubility of 26 g per 100 g of water at 20 °C. Will 15 g dissolve completely in 50 g of water at 20 °C?
9. Classify: (a) bromine, Br₂; (b) magnesium oxide; (c) a solution of sugar in water.
10. A sample melts between 71 °C and 76 °C. What does this tell you?
11. Which method separates hexane (boiling point 69 °C) from heptane (boiling point 98 °C), which mix completely?
12. A spot travels 2.1 cm and the solvent front travels 7.0 cm. Calculate the Rf value.

Answers

1. Condensing.
2. The energy supplied is used to overcome forces between particles, not to make them move faster.
3. Evaporation happens only at the surface and below the boiling point; boiling happens throughout the liquid and only at the boiling point.
4. Each step dilutes 100 times: 1/100 × 1/100 = 1/10 000.
5. A solution in which no more solute can dissolve at that temperature.
6. 4.5 ÷ 15 × 100 = 30 g per 100 g of water (at 30 °C).
7. (64 − 24) × 200/100 = 80 g.
8. No. 50 g of water holds at most 26 × 50/100 = 13 g, so 2 g stays undissolved.
9. (a) Element; (b) compound; (c) mixture.
10. It is impure (a mixture): a pure substance melts at one fixed temperature.
11. Fractional distillation.
12. 2.1 ÷ 7.0 = 0.30.

Where marks are usually lost

• Describing a state with only arrangement, and leaving out movement or energy.
• Writing “particles in a solid don’t move” instead of “vibrate about fixed positions”.
• Saying the temperature rises during melting or boiling of a pure substance.
• Explaining diffusion as particles “wanting” to spread out, instead of random movement and collisions.
• Defining a saturated solution without “at that temperature”.
• (Paper 2 only) Dividing by the mass of the solution instead of the mass of water, or forgetting to scale to the actual mass of water.
• (Paper 2 only) Giving solubility without units, or with “g” alone instead of g per 100 g of water.
• Calling a mixture of two elements a compound, or calling O₂ a compound because it has two atoms.
• Choosing filtration for a dissolved solid, or simple distillation for two liquids that mix.
• Measuring Rf distances from the bottom of the paper, or giving Rf greater than 1.

Official syllabus

Pearson Edexcel International GCSE in Chemistry (4CH1) specification, Issue 3, September 2024, Pearson Education Limited. Topic 1 Principles of chemistry, sub-topics (a) States of matter and (b) Elements, compounds and mixtures, points 1.1–1.13.