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Chemistry

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States of matter

Learning objectives · Understand the three states of matter in terms of the arrangement, movement and energy of the particles; Understand the interconversions between the three states of matter in terms of: the names of the interconversions; how they are achieved; the changes in arrangement, movement and energy of the particles; Know what is meant by the terms: solvent; solute; solution; saturated solution; CHEMISTRY ONLY — Know what is meant by the term solubility in the units g per 100 g of solvent; Understand how the results of experiments involving the dilution of coloured solutions and diffusion of gases can be explained

States of matter — the particle idea

SB p.3

Everything around you — metals, concrete, water, the air you breathe, even clouds — is made of particles that are far too small to see. That single idea is called the particle-model, and it is the key that unlocks the whole of this chapter. In it we look at how the particles are arranged in solids, liquids and gases, and at the ways the particles can move around. What the particles themselves actually are (atoms, ions or molecules) is a story for Chapter 3 — this chapter deliberately stays with the model, not the particle types.

By the end of this lesson you should be able to:

describe the three states of matter in terms of the arrangement, movement and energy of their particles;
understand the interconversions between the three states: their names, how they are achieved, and the changes in arrangement, movement and energy of the particles;
define the terms solvent, solute, solution and saturated solution;
(Chemistry only) define solubility in g per 100 g of solvent, plot and interpret solubility curves, and describe the solubility practical;
explain how experiments on the dilution of coloured solutions and the diffusion of gases are evidence for the particle-model.

Key point

The three states of matter are solid, liquid and gas. The particle-model explains their properties in terms of the arrangement, separation and motion of particles — and the kinetic-energy of those particles.

Prior knowledge

Open this chapter in the Rail All chapters

Particle theory in one pass: the three states differ in how close the particles are, how they move, and how much energy they carry - never in what the particles are. Melting, boiling, condensing and freezing are the interconversions the examiner names precisely; learn the direction and the energy change of each.

Page-true figure region in the book pane — Particle arrangement in solids, liquids and gases.

Simulationblock 5

States of Matter: Basics

Heat and cool solid, liquid and gas particles and watch the arrangement and energy change.

Interactive simulation — loads from PhET (University of Colorado Boulder) when you start it.

PhET Interactive Simulations · CC-BY-4.0Open directly

Fill the tableblock 6

Complete the particle-model table (one word or short phrase per cell).

State	Spacing of particles	Movement of particles
Solid
Gas

States of matter — facts around you

SB p.3

Try these three facts on for size — three everyday observations that the particle model has to explain:

You can't walk through a brick wall, but you can move — with some resistance, it pushes back against you — through water in a swimming pool. Moving through air is easy.
When you melt most solids their volume increases slightly. Most liquids are less dense than the solid they come from.
If you boil about 5 cm³ of water, the steam that comes off will fill an average bucket.

The particle-model explains all three at once. The particles in a solid are held by strong forces of attraction and cannot move out of your way — so the brick wall stops you. In a liquid the particles can be pushed aside — so you can swim through water. In a gas there are (almost) no forces between the particles, and they are far apart — so walking through air is effortless. And when a solid melts, small gaps open up between the particles, which is why most liquids are slightly less dense than their solids; when a liquid boils, the particles separate hugely, which is why 5 cm³ of water makes a bucketful of steam.

Key point

You can't walk through a brick wall because of the strong forces of attraction between the particles — they can't move out of your way.
You can swim through water because you can push the particles out of the way.
It is easy to move through a gas because there are (almost) no forces between the particles.

Did you know?

5 cm³ of water becomes enough steam to fill an average bucket. That is how much further apart gas particles are compared with particles in a liquid.

Common misconception

"You can walk through air because air has no particles." No — air is full of gas particles; you pass through easily because they are far apart and there are (almost) no forces between them, not because they are absent.

Exam hint

When you draw particle diagrams: in a solid the particles are close together and usually regularly packed; in a liquid make sure the particles are mostly touching their neighbours (the packing need not be regular — what matters is close together with some gaps).

Quick check

Question-bank join: spec 1.1 × 2 items — one on explaining "why can you walk through air but not a wall" in particle terms, one on particle diagrams for the three states.

Key terms in this section

particle model: — A model treating all matter as made of tiny moving particles; it explains the states of matter and their changes via particle arrangement, separation and motion.

Keep goingThe Three States of Matter 1-1-states-of-matter 1-6-ionic-bonding 2-3-gases-in-the-atmosphere

Particles in solids, liquids and gases

SB p.4

Here are the particle pictures for all three states, with the exact descriptions to memorise. In a solid, the particles are usually arranged regularly and packed closely together. They can only vibrate about fixed positions — they cannot move around — because strong forces of attraction hold them together.

In a liquid, the particles are still mostly touching, but some gaps have appeared between them. This is why liquids are usually less dense than the solids they come from. The forces between the particles are less effective, so the particles can move around each other; the arrangement is random rather than regular.

In a gas, the particles are much further apart and there are (almost) no forces of attraction between them. They move randomly at high speed in all directions. The energy story runs in the same order: the particles in a solid have less kinetic-energy than the particles in a liquid, which have less kinetic-energy than the particles in a gas.

Key point

Learn the three-part description for each state — arrangement (regular/random), separation (touching / touching with gaps / far apart) and motion (vibrate about fixed positions / move around each other / move randomly at high speed) — plus the energy ordering solid < liquid < gas.

Common misconception

"There are no forces at all between gas particles." Almost — almost no forces. Very weak attractions do exist; they are what let a gas condense to a liquid when it is cooled enough.

Exam hint

A 6-mark "compare the particles in solids and liquids" answer scores by covering arrangement, separation, motion and energy. Mark schemes accept phrases like "vibrate about fixed positions", "move around each other" and "move randomly and fast". "Particles in a liquid are freer" alone is too vague to score.

Quick check

Question-bank join: spec 1.1 × 2 items — draw/describe the three particle arrangements; compare kinetic energies. spec 1.2 × 1 item linking gaps to density on melting.

Key terms in this section

kinetic energy: — Movement energy. Gas particles have more kinetic energy than liquid particles, which have more than solid particles.

Sort & matchblock 3

Match each change of state to its direction of travel.

choose a match

Examiner note - state changes

Boiling happens THROUGHOUT a liquid at a fixed temperature; evaporation happens only at the SURFACE, at any temperature. Saying 'the water evaporates at 100 degrees' loses the marking point: it boils at 100 degrees. Write the state-change name the question asks for - 'state' or 'name' wants one word, not a sentence.

Check yourselfblock 7

Keep goingThe Three States of Matter 1-1-states-of-matter 1-6-ionic-bonding 1-7-covalent-bonding 2-3-gases-in-the-atmosphere

Melting, freezing, boiling and condensing

SB p.4

Heat a solid and the energy from the heat source makes its particles vibrate faster and faster. Eventually they vibrate so fast that the forces of attraction are no longer strong enough to hold them together — the particles can move around each other, and the solid meltings into a liquid. The temperature at which this happens is the melting-point. Energy has to be supplied to get there, because the liquid particles end up with more kinetic-energy than the solid particles had.

Cool the liquid again and its particles move more and more slowly, until the forces of attraction can hold them in fixed positions — packing more closely together into a solid. The liquid freezings. The temperature at which this happens is the freezing point — and although the two directions have different names, the melting point and the freezing point of a substance are exactly the same temperature.

Boiling is the liquid-to-gas change that happens when a liquid is heated so strongly that particles move fast enough to overcome all the forces of attraction between them. The stronger those forces are, the higher the boiling-point — more energy is needed to beat them. Cool a gas and its particles eventually slow enough for attractions to form and hold them together as a liquid: the gas condensations.

One warning before you leave this section: evaporation is not boiling. Evaporation occurs at any temperature; boiling occurs only at the boiling-point. UK puddles disappear in winter at temperatures well below 5 °C — the water certainly is not boiling; it is evaporating. Water evaporates at, say, 5 °C but boils only at 100 °C.

Key point

Melting point = freezing point (same temperature, different direction). Boiling happens only at the boiling point; evaporation happens at any temperature.

Common misconception

"A liquid must reach its boiling point before any of it can turn into a gas." No — evaporation happens at any temperature from the surface; boiling is the special, faster change that occurs throughout the liquid at one temperature.

Exam hint

"Name the change of state" questions (1 mark each): solid→liquid = melting; liquid→solid = freezing; liquid→gas = boiling or evaporation; gas→liquid = condensing. Mark schemes accept both spellings/gerunds ("condensation"/"condensing").

Quick check

Question-bank join: spec 1.2 × 3 items — name the four interconversions; explain why melting point and freezing point coincide; contrast evaporation and boiling in particle terms.

Key terms in this section

boiling point: — The temperature at which a liquid boils. The stronger the forces of attraction between particles, the higher the boiling point.
condensation: — The change of state from gas to liquid on cooling; particles slow down until forces of attraction hold them together.
freezing: — The change of state from liquid to solid on cooling; particles slow until attractions hold them in fixed, closely packed positions.
kinetic energy: — Movement energy. Gas particles have more kinetic energy than liquid particles, which have more than solid particles.
melting: — The change of state from solid to liquid when a solid is heated; particles vibrate faster until attractions can no longer hold them in fixed positions.
melting point: — The temperature at which a solid melts. It is exactly the same temperature as the freezing point of the substance.

Keep goingThe Three States of Matter 1-1-states-of-matter 1-7-covalent-bonding

Evaporation — and sublimation

SB p.5

evaporation is different from boiling, and the difference is all in the particles. In any liquid, the average speed of the particles varies with temperature — but at each temperature some particles are moving faster than the average and others more slowly. Some very fast particles at the surface of the liquid have enough kinetic-energy to overcome the forces of attraction between particles: they break away and form a gas. That is evaporation. You see no bubbling — an open liquid just slowly disappears.

In a closed container the story gains a second half. Gas particles collide with particles at the liquid surface; those moving slowly enough are held by the attractive forces and rejoin the liquid. So in a closed container evaporation and condensation are both occurring at the same time.

Some substances skip the liquid stage entirely. A small number can change directly from solid to gas — or gas to solid — at normal pressure. Solid to gas is sublimation; gas to solid is usually called deposition. Dry ice — solid carbon dioxide — subliming in a beaker makes the point vividly: the white cloud is not carbon dioxide (which is an invisible gas); it forms because the CO₂ gas is so cold that it makes water vapour in the air condense.

Finally, a definitions note you will use in §s08: in science, room-temperature usually means between 20 and 25 °C. There is not one fixed value, so when a change of state happens near room temperature, always make clear which value you are using.

Key point

Sublimation = solid → gas directly. Deposition = gas → solid directly. Both happen without a liquid forming, at normal pressure.

Did you know?

The white cloud over dry ice is condensed water vapour from the air, not the carbon dioxide itself — CO₂ gas is invisible.

Common misconception

"The white cloud around dry ice is carbon dioxide gas." No — carbon dioxide gas is invisible. The cloud is water vapour from the air condensing because the CO₂ gas is so cold.

Exam hint

"Explain why a puddle can disappear even when the temperature is below 5 °C." Mark schemes accept: some particles (at the surface) move faster than average / have enough energy to overcome the forces of attraction and escape from the liquid. Avoid answers that claim the puddle boils.

Quick check

Question-bank join: spec 1.2 × 3 items — evaporation vs boiling; closed-container evaporation + condensation; naming sublimation/deposition.

Key terms in this section

condensation: — The change of state from gas to liquid on cooling; particles slow down until forces of attraction hold them together.
deposition: — The direct change of state from gas to solid without forming a liquid; the reverse of sublimation. Some sources also call this de-sublimation.
evaporation: — Slow change from liquid to gas at any temperature; the fastest particles at the liquid surface have enough energy to overcome the attractions and break away.
kinetic energy: — Movement energy. Gas particles have more kinetic energy than liquid particles, which have more than solid particles.
room temperature: — In science, usually taken to mean a temperature between 20 and 25 °C. Because there is not one fixed value, always make clear which value you are using.
sublimation: — The direct change of state from solid to gas without forming a liquid, at normal pressure. Dry ice (solid carbon dioxide) sublimes at -78.5 °C.

Keep goingThe Three States of Matter 1-1-states-of-matter 1-7-covalent-bonding

Predicting state from melting and boiling points

SB p.6

Here is the rule that turns melting and boiling points into a prediction machine. A substance is:

a solid at temperatures **below its melting-point;
a liquid** between its melting point and its boiling-point;
a gas above its boiling point.

To decide the state at room temperature, use room-temperature (20–25 °C) and place the two fixed points on a temperature line. Oxygen's boiling point sits below room temperature, so oxygen is a gas at room temperature. Lithium's melting point sits above room temperature, so lithium is a solid. Bromine, as the worked example below shows, is a liquid.

Worked example

Bromine from −100 °C to 100 °C (melting point −7 °C, boiling point 59 °C):

At −100 °C — below bromine's melting point — bromine is a solid.
Heated to −7 °C it melts; from −7 °C to 59 °C it remains a liquid.
Room temperature (20–25 °C) falls inside that liquid range, so bromine is a liquid at room temperature.
Above 59 °C bromine is a gas.

Key point

Below the melting point → solid. Between melting point and boiling point → liquid. Above the boiling point → gas. The state at room temperature is decided entirely by where room temperature sits relative to the two fixed points.

Common misconception

"Anything liquid at room temperature must have a low boiling point." No — bromine's boiling point is 59 °C, which is not low, yet bromine is liquid at 20–25 °C because room temperature is above its melting point and below its boiling point.

Maths skills

Draw a horizontal temperature line, mark room temperature on it, then plot the melting and boiling points of the substance. Where the points land relative to the room-temperature mark gives the state — this is the fastest reliable exam technique for these questions.

Exam hint

"Explain why bromine is a liquid at room temperature" (2 marks): state that room temperature is above bromine's melting point and below its boiling point. Mark schemes accept "melting point below room temperature / boiling point above room temperature" — always quote the values you are using for room temperature.

Quick check

Question-bank join: spec 1.2 × 2 items — predict state at a stated temperature; explain state at room temperature from mp/bp.

Key terms in this section

boiling point: — The temperature at which a liquid boils. The stronger the forces of attraction between particles, the higher the boiling point.
melting point: — The temperature at which a solid melts. It is exactly the same temperature as the freezing point of the substance.
room temperature: — In science, usually taken to mean a temperature between 20 and 25 °C. Because there is not one fixed value, always make clear which value you are using.

Keep goingThe Three States of Matter 1-1-states-of-matter 1-7-covalent-bonding

Diffusion

SB p.6

Someone releases a smelly gas — ammonia, say — at the front of the lab. Within a minute or so everybody can smell it. That is not surprising: particles in a gas are free to move around. What needs explaining is why it takes so long. At room temperature ammonia particles travel at about 600 metres per second, so travelling in straight lines they would cross the lab in under 0.01 s. Instead, each particle is bouncing off air particles on the way — by the time the smell reaches every corner, an ammonia particle may have travelled 30 or more kilometres.

That spreading out is diffusion: the movement of particles from where they are at a high concentration (lots of them in a certain volume) to where they are at a low concentration (fewer in that volume), ending with particles spread out evenly.

Two classic experiments demonstrate diffusion:

Bromine and air: a lower gas jar holds brown bromine gas; the upper jar holds air. Remove the lids and the brown colour diffuses upwards until both jars are uniformly brown — air particles diffuse downwards at the same time. Random movement gives an even mixture of both gases everywhere.
Hydrogen and air: same experiment, but you must lower a lighted splint in at the end to find where the hydrogen went. People expect the much less dense hydrogen to end up only in the top jar — in fact you get identical explosions from both jars.

Different gases diffuse at different speeds. The classic evidence: a long glass tube with cotton wool soaked in concentrated ammonia solution at one end and concentrated hydrochloric acid at the other. Ammonia and hydrogen chloride gases diffuse along the tube and react where they meet: NH₃(g) + HCl(g) → NH₄Cl(s) — a white ring of solid ammonium chloride. The ring takes minutes to form (diffusion takes time) and forms closer to the hydrochloric acid end, because ammonia particles (relative molecular mass 17) are lighter than hydrogen chloride particles (36.5) and therefore move faster, travelling further in the same time.

Safety

These are teacher demonstrations only. The bromine and ammonia/hydrogen chloride demos must be prepared in a working fume cupboard with eye protection and chemical-resistant gloves; bromine inhalation can cause delayed reactions needing urgent medical attention. Skin contact and fume inhalation must be avoided; clean-up happens in the fume cupboard. (You will learn about relative molecular mass properly in Chapter 5.)

Key point

Diffusion = spreading out of particles from high concentration to low concentration. Lighter particles (lower relative molecular mass) diffuse faster.

Common misconception

"With hydrogen in the bottom jar, only the top jar would explode — hydrogen rises because it is light." No — after the lids are removed both gases diffuse into both jars, so the experiment gives identical explosions in both jars.

Exam hint

"Explain why the white ring forms nearer the hydrochloric acid end" (2–3 marks): ammonia particles are lighter (smaller relative molecular mass) so they move faster and travel further in the same time. Mark schemes accept "lighter/smaller mass particles move faster"; avoid teleology like "ammonia wants to travel further".

Quick check

Question-bank join: spec 1.3 × 4 items — define diffusion; explain the bromine jar result; the NH₃/HCl ring position; predict the ring position with HBr.

Key terms in this section

concentration: — How many particles there are in a certain volume: lots of particles in a volume is a high concentration; fewer in the same volume is a low concentration.
diffusion: — The spreading out of particles from where they are at a high concentration to where they are at a low concentration, ending in an even mixture.

Keep goingDiffusion & Dilution 1-1-states-of-matter

Diffusion in liquids — and how tiny particles are

SB p.8

diffusion through a liquid is very slow if the liquid is completely still. Place a small jar of strongly coloured potassium manganate(VII) solution in a gas jar of water and it can take days for the colour to spread through all the water. Two reasons, straight from the particle-model: particles in a liquid move more slowly than particles in a gas, and they are much closer together — so there is less space to move into without colliding with another particle.

This coloured solution then powers one of the most beautiful estimates in the whole course — estimating an upper limit on the mass of a single particle:

Dissolve 0.01 g of potassium manganate(VII) in 1 cm³ of water — a deep purple solution.
One drop is about 0.05 cm³, so there are 20 drops in 1 cm³, and each drop contains 0.0005 g.
Dilute the solution until the total volume is 10 000 cm³ — the colour is still just visible. There are now 200 000 drops, and for each drop to look purple, each drop must contain at least one 'particle'.
So at least 200 000 'particles' must fit in 0.01 g — each 'particle' can weigh no more than 5 × 10⁻⁸ g (0.00000005 g).

That estimate is not even close to the real answer — a potassium manganate(VII) 'particle' actually weighs about 2.6 × 10⁻²² g, and there are about 38 000 000 000 000 000 000 particles in 0.01 g. In reality you need enormous numbers of particles in each drop for the colour to register. Dilution does not remove particles — it only spreads them further apart.

Why the inverted commas around 'particle'? Potassium manganate(VII) is an ionic compound, so it contains more than one sort of particle — you will find out what that means in Chapter 7.

Key point

Diffusion in a still liquid is slow because liquid particles move slowly and are close together. The visibility of a dilute colour proves that a tiny mass contains an enormous number of particles.

Common misconception

"Diluting a solution removes some of the particles." No — dilution only spreads the same particles through a larger volume; the total number of particles is unchanged, which is exactly why the colour survives such huge dilution.

Maths skills

Standard form is the honest language for this section: 5 × 10⁻⁸ g upper limit, 2.6 × 10⁻²² g actual, 3.8 × 10¹⁹ particles. Chain the estimate one step at a time (mass per drop → number of drops → particles per drop) and each step is easy.

Did you know?

In 0.01 g of potassium manganate(VII) there are about 38 000 000 000 000 000 000 particles — roughly 38 quintillion. You need very large numbers of particles in each drop for your eye to see the colour at all.

Exam hint

"Suggest what this experiment tells us about the size of particles" (2 marks): the particles must be extremely small — a very large number of them fit into a tiny mass. Mark schemes accept "particles are very small"; avoid "particles are invisible" (no evidence) or "particles disappear".

Cross-curricular links

Maths: powers of ten and estimation — this section is a worked lesson in Fermi-estimate reasoning (upper bounds from visible evidence).

Quick check

Question-bank join: spec 1.3 × 3 items — why liquid diffusion is slow; the dilution-estimate logic; what the estimate proves about particle size.

Key terms in this section

diffusion: — The spreading out of particles from where they are at a high concentration to where they are at a low concentration, ending in an even mixture.
particle model: — A model treating all matter as made of tiny moving particles; it explains the states of matter and their changes via particle arrangement, separation and motion.
solution: — The liquid formed when a solute dissolves in a solvent.

Keep goingDiffusion & Dilution 1-1-states-of-matter

Solute, solvent, solution — and measuring solubility

SB p.8

When a solid dissolves in a liquid, the vocabulary is fixed: the substance that dissolves is the solute; the liquid it dissolves in is the solvent; the liquid formed is a solution. At the particle level, dissolving means the attractive forces between the particles in the solute are broken, while new attractive forces form between solvent particles and solute particles. Whether a particular solid is soluble in a particular solvent depends on whether the new forces are strong enough to overcome the old ones.

Only a certain amount of solute will dissolve in a fixed amount of solvent at a particular temperature. When that maximum is dissolved — with some undissolved solid still present — you have a saturated-solution. Keep some undissolved solid there: it prevents supersaturated solutions forming (an extension idea you do not need at International GCSE).

CHEMISTRY ONLY — everything from here to the end of §s13 is Paper 2 content (spec points 1.5C, 1.6C, 1.7C).

The solubility of a solid in a solvent at a particular temperature is defined as the mass of solute which must dissolve in 100 g of solvent at that temperature to form a saturated solution. For example, the solubility of sodium chloride in water at 25 °C is about 36 g per 100 g of water.

Practical: measuring the solubility of potassium nitrate at 40 °C. Weigh an evaporating basin; heat water in a boiling tube to just above 40 °C; add potassium nitrate stirring rapidly until no more dissolves (some solid left over); allow the solution to cool to exactly 40 °C; pour off some solution into the basin — solution only, no solid; weigh basin + contents; heat gently to evaporate off the water; re-weigh, then heat and re-weigh again — heating to constant mass — to be sure all the water has gone. With a recorded set of results (basin 25.72 g; basin + solution 58.00 g; basin + dry crystals 38.00 g): mass of solid = 12.28 g, mass of water = 20.00 g. Twelve point two eight grams dissolves in 20.00 g of water, so five times as much dissolves in 100 g — the solubility of potassium nitrate at 40 °C is 61.4 g per 100 g of water. In general: solubility (g per 100 g) = (mass of solute ÷ mass of solvent) × 100.

Key point

A saturated solution contains the maximum mass of dissolved solid at that temperature, with undissolved solid present. Solubility is always quoted with a temperature: "the solubility of sodium chloride at 30 °C is…".

Maths skills

From the balance readings: solid = 38.00 − 25.72 = 12.28 g; water = 58.00 − 38.00 = 20.00 g; scale to 100 g: 12.28 × (100 ÷ 20.00) = 61.4 g per 100 g. Learn the scaling equation — it appears in exam data questions constantly.

Common misconception

"Solubility is a fixed property — the same value at any temperature." No — solubility varies with temperature, which is why a temperature must always be quoted alongside the value.

Exam hint

"Why is the solution heated gently?" — so none of it spits out; spitting would lose solid and give a solubility reading lower than the true value. "Why heat to constant mass?" — to confirm all the water has evaporated. Mark schemes accept "mass no longer changes / all water removed".

Safety

Wear eye protection and heat gently — hot solid can spit out of the evaporating basin and cause burns.

Quick check

Question-bank join: spec 1.4 × 2 (solute/solvent/ solution/saturated definitions); spec 1.5C × 1 (define solubility, quote temperature); spec 1.7C × 2 (practical method + constant-mass reasoning).

Key terms in this section

saturated solution: — A solution containing as much dissolved solid as possible at a particular temperature; some undissolved solute must be present.
solubility: — The mass of solute that must dissolve in 100 g of solvent at a stated temperature to form a saturated solution, in g per 100 g of solvent. Always quote the temperature.
solute: — The substance that dissolves in a liquid to make a solution, e.g. sodium chloride when salt dissolves in water.
solution: — The liquid formed when a solute dissolves in a solvent.
solvent: — The liquid that a solute dissolves in, e.g. water in a salt solution.

Keep goingSolutions Solubility Practical: Investigate the Solubility of a Solid in Water at a Specific Temperature 1-1-states-of-matter 1-2-elements-compounds-and-mixtures

Solubility curves

SB p.10

CHEMISTRY ONLY. Solubility changes with temperature, so plot solubility against temperature and you get a solubility curve. Most solids have curves like those for potassium nitrate and potassium chloride: solubility increases with temperature — either dramatically (KNO₃) or only a little (KCl).

Solubility curves earn their keep by predicting crystals. Take potassium nitrate: at 90 °C, 200 g dissolves in 100 g of water; at 30 °C only 50 g will dissolve. So a solution containing 200 g of potassium nitrate in 100 g of water, cooled from 90 °C to 30 °C, must release 200 − 50 = 150 g of solid. We say the potassium nitrate crystallisations out of the solution — or precipitates out — as crystals.

One graph-drawing note: the solubility curve here is virtually a straight line, but that will not always be the case — if you are asked to draw a line of best fit, it can be either a straight line or a curve.

Key point

Mass of crystals formed on cooling = difference between the solubility values at the starting and finishing temperatures (for the same mass of solvent).

Maths skills

Cooling a saturated solution: crystals = solubility(hot) − solubility(cool), scaled to however much solvent you actually have (see §s13 for the scaling method).

Common misconception

"Cooling a saturated solution makes more solute dissolve." No — cooling lowers the solubility of most solids, so the excess solute comes out of the solution as crystals.

Exam hint

Read the axes before anything else — temperature on the horizontal axis, solubility (g per 100 g of water) on the vertical. Use dashed construction lines to read values; mark schemes allow small read-off tolerances.

Quick check

Question-bank join: spec 1.6C × 2 items — read a value off a solubility curve; calculate mass of crystals on cooling.

Key terms in this section

crystallisation: — The formation of solid crystals from a saturated solution as it cools and the solubility falls; also called precipitating out.
solubility: — The mass of solute that must dissolve in 100 g of solvent at a stated temperature to form a saturated solution, in g per 100 g of solvent. Always quote the temperature.

Keep goingSolubility 1-1-states-of-matter 1-2-elements-compounds-and-mixtures

Worked example — reading a solubility curve

SB p.11

CHEMISTRY ONLY. This worked example uses potassium chloride, with solubilities measured at five temperatures:

Temperature/°C	10	30	40	70	90
Solubility/g per 100 g of water	31.2	37.2	40.0	48.5	53.9

(a) Plot the curve. Temperature on the horizontal axis, solubility on the vertical one; plot the five points and join them smoothly (Figure 1.19).

(b)(i) Solubility at 50 °C. Reading the curve at 50 °C gives 42.5 g per 100 g of water.

(b)(ii) Maximum mass dissolving in 50 g of water at 40 °C. The curve gives 40 g per 100 g at 40 °C — so half the mass of water dissolves half the mass: 20 g in 50 g of water. For any other mass of water, scale with the equation: mass dissolved = (mass of water ÷ 100) × solubility. For instance, in 34.6 g of water at 40 °C: (34.6 ÷ 100) × 40 = 13.84 g of potassium chloride dissolves.

(b)(iii) When do crystals first appear? A hot solution contains 51.0 g of potassium chloride in 100 g of water, and is left to cool. Crystals appear as soon as the solution becomes saturated — that is, when 51.0 g is the maximum that can stay dissolved. Drawing a line across at 51.0 g shows this happens at 80 °C: below that temperature the solution is saturated and crystals form. (In different circumstances you might need to read the 10 °C value from the graph as well — for example, cooling to 10 °C leaves 31.2 g dissolved, so 51.0 − 31.2 = 19.8 g of crystals form.)

This lesson's practice is wired to the question bank below.

Worked example

The three readings above — 42.5 g per 100 g at 50 °C; 20 g in 50 g of water at 40 °C; crystals first below 80 °C for a 51.0 g solution — are the full exam skill set: read the curve, scale to the solvent mass, find the saturation temperature.

Maths skills

The scaling equation mass = (mass of water ÷ 100) × solubility works in both directions: given the water mass it predicts the dissolved mass; given the dissolved mass (51.0 g) it pins the temperature at which crystals first appear.

Common misconception

"A solubility curve is always a straight line." No — the potassium chloride curve here is virtually straight, but most curves bend (potassium nitrate's rises steeply). A line of best fit can be a curve.

Exam hint

Graph questions: draw dashed construction lines from the axis to the curve and quote values to the precision the grid allows. Mark schemes accept read-off tolerance — but a missing temperature or unit ("per 100 g of water") loses marks.

Quick check

Question-bank join: spec 1.6C × 3 items — read solubility at a temperature; scale to a non-100 g solvent mass; find the crystallisation temperature.

Summary

Matter is solid, liquid or gas; the particle-model explains states by particle arrangement, separation, motion and energy.
Interconversions: melting/freezing (solid↔liquid), boiling/condensing (liquid↔gas), evaporation (any temperature), sublimation/deposition (solid↔gas directly).
State at a temperature = where that temperature sits relative to the melting and boiling points (room temperature = 20–25 °C).
Diffusion = spreading from high to low concentration; lighter particles diffuse faster; liquids diffuse slowly.
Solute + solvent = solution; a saturated solution holds the maximum dissolved solid at that temperature; solubility is quoted in g per 100 g of solvent with a temperature (Chemistry only, with curves and the practical).

Can you…?

describe each state in terms of arrangement, separation and motion?
name all the interconversions and say how each is achieved?
predict the state of a substance at any temperature from its melting and boiling points?
explain the ammonia/hydrogen chloride white-ring experiment, including why the ring forms nearer one end?
define solute, solvent, solution and saturated solution?
(C) calculate a solubility in g per 100 g from experiment data?
(C) read a solubility curve to find dissolved mass, crystal mass and saturation temperature?

Key terms in this section

particle model: — A model treating all matter as made of tiny moving particles; it explains the states of matter and their changes via particle arrangement, separation and motion.

Keep goingSolubility 1-1-states-of-matter 1-2-elements-compounds-and-mixtures

States of matter

States of matter — the particle idea

The three states of matter

States of matter — facts around you

Particles in solids, liquids and gases

Melting, freezing, boiling and condensing

Evaporation — and sublimation

Dry ice and carbon dioxide's states

Predicting state from melting and boiling points

Diffusion

Diffusion in liquids — and how tiny particles are

Solute, solvent, solution — and measuring solubility

Solubility curves

Worked example — reading a solubility curve