1aStates of matter

Everything you meet in chemistry is made of particles. What makes a solid a solid, and a gas a gas, is not what the particles are — it is how they are arranged, how they move, and how much energy they have.

Those three words matter. Almost every question on this topic asks about one of them, and an answer that covers only arrangement when the question asked about movement will lose the mark.

The three states side by side

Reading the table properly

Solids and liquids are both "close together". That is why neither can be squashed much, and why both have a fixed volume. The difference is whether the particles can change places: in a solid they cannot, in a liquid they can. That single difference is why a liquid flows and a solid holds its shape.

Only gases have particles far apart. This is the reason a gas can be compressed into a smaller volume — there is empty space to push the particles into. A syringe of air compresses; a syringe of water does not.

The mistake that costs marks

"The particles in a solid do not move."

They do. They vibrate about fixed positions. They cannot travel from place to place, but they are never still. Examiners penalise "do not move" repeatedly, and the fix is one word: vibrate.

Energy, and what it means here

For the same substance, particles hold least energy as a solid and most as a gas. That is why energy must be supplied to melt something and again to boil it.

Be careful with one detail: at the exact melting point, solid and liquid are at the same temperature, so their particles have the same average kinetic energy. The energy that goes in at that moment is used to break the particles apart, not to speed them up. In an exam you will not usually be pushed this far, but it is why "a liquid's particles are always moving faster" is not quite true.

A change of state is a physical change. No new substance is made, no chemical bonds inside the molecules are broken, and the change can be reversed by putting the energy back or taking it away.

The names, and which way each one goes

Learn them in pairs of opposites: melting and freezing, boiling and condensing. If you can recall one, the other is simply the reverse.

Boiling is not evaporation

These are different processes and examiners do not accept one for the other.

• Boiling happens throughout the liquid, at one fixed temperature — the boiling point.
• Evaporation happens only at the surface, and at any temperature below the boiling point.

A puddle drying on a warm day is evaporating, not boiling. If a question asks what happens when water boils, "evaporation" will not be credited.

What the particles actually do

When you heat a substance:

1. The particles gain energy.
2. They move faster — vibrating harder in a solid, moving more quickly in a liquid.
3. Eventually they have enough energy to overcome the forces of attraction holding them together.
4. The particles separate, and the state changes.

That four-step chain is the answer to almost every "explain in terms of particles" question in this sub-topic. Write it in that order and the marks follow.

Where students go wrong

"The particles get hotter."

Particles do not get hotter. Temperature is a property of the substance as a whole; what changes for an individual particle is its energy and therefore its speed. Say that instead.

Diffusion

Diffusion is the spreading of particles from where they are more concentrated to where they are less concentrated, caused by the particles' own random movement.

It needs no stirring and no current. Drop coloured solution into still water and it will spread on its own, because the particles never stop moving.

Two things change how fast it happens:

• Temperature. Hotter particles move faster, so they spread more quickly.
• Particle mass. At the same temperature, lighter particles move faster and therefore diffuse faster.

That second point is what the classic ammonia and hydrogen chloride experiment demonstrates. Both gases are released at opposite ends of a tube; a white ring forms where they meet, and it forms nearer the hydrogen chloride end because ammonia's lighter particles have travelled further.

The language of solutions

These four terms are worth getting exactly right, because questions often ask for them by name.

Those last four words are part of the definition, not an optional extra. A solution saturated at 20 °C is usually not saturated at 60 °C.

Solubility (Separate Chemistry)

Solubility is measured in grams per 100 g of water in this course.

(The general definition is per 100 g of solvent; 4CH1 questions use water.)

Read that again: per 100 g of solvent, not of solution. The two are different, because the dissolved solute adds mass to the solution. Getting this wrong is the single commonest error in solubility calculations.

A solubility curve shows how solubility changes with temperature. For most solids solubility rises as temperature rises, which is why crystals appear when a hot saturated solution cools — the solution can no longer hold everything it did when it was hot.

Finding solubility experimentally (Separate Chemistry)

The method has one step students routinely forget:

1. Make a saturated solution at a known, controlled temperature.
2. Separate off some of the saturated solution only, leaving any excess solid behind.
3. Weigh that sample of solution.
4. Evaporate it to dryness and weigh the solid left.
5. Find the mass of water by subtracting the solid from the mass of solution.
6. Scale to 100 g of water.

Step 5 is the one that gets missed. Scaling against the mass of solution answers a different question.

These are the specific errors examiners record on this material. Each one is cheap to avoid once you know it is coming.

1. "Particles in a solid do not move"

They vibrate about fixed positions. This is probably the most penalised sentence in the whole topic. The particles cannot travel, but they are not still.

2. Describing arrangement when the question asked about movement

Questions frequently ask for both the arrangement and the movement. Two marks, two ideas. An answer that says "the particles are close together in a regular pattern" and stops has answered half the question.

Check what the question asked for, and give one clear statement for each part.

3. Using "evaporation" for boiling

Boiling happens throughout a liquid at a fixed temperature. Evaporation happens at the surface, at any temperature. They are not interchangeable, and only one of them will be on the mark scheme.

4. Saying the particles "get hotter"

Write about the particles' energy and speed. Heat is transferred to the substance; what changes for a particle is how much energy it has and how fast it moves.

5. Explaining diffusion by stirring or currents

If a question describes a tube that is still and undisturbed, an answer involving mixing or convection cannot be credited. The whole point of these questions is that diffusion happens on its own, driven by random particle movement.

6. Solubility per 100 g of solution

Solubility is always per 100 g of solvent. When a calculation gives you the mass of a saturated solution, subtract the dissolved solid to find the mass of water before scaling.

7. Forgetting units, or inventing abbreviations

Examiners report losing marks for units surprisingly often — including candidates writing "m" for minutes, which is not accepted. Write units out where there is any doubt, and check that the unit you have given matches the quantity you calculated.

Separate Chemistry only.

Solubility is quoted as grams of solute per 100 g of water at a stated temperature. The experiment measures both of those masses.

Apparatus

Evaporating basin, balance reading to 0.01 g, filter funnel and paper, water bath, tongs, thermometer.

Method

1. Make a saturated solution at the temperature you are investigating — add solid until no more dissolves and some stays undissolved.

2. Hold the solution at that temperature and filter off the excess solid, working fast and with warmed apparatus.

   A cold funnel cools the solution on its way through, and solid crystallises in the filter. That solute should have been counted, so the measured solubility comes out too low. Warm the funnel and flask first, or decant the clear solution off the excess solid instead of filtering it.

3. Weigh a clean, dry evaporating basin. Record the mass.

4. Add a sample of the filtered solution to the basin and reweigh.

5. Evaporate the water using a water bath, then gently over a low flame.

6. Cool and reweigh. Reheat, cool and reweigh until two masses agree — heating to constant mass.

The calculation

solubility = (mass of solute ÷ mass of water) × 100

The answer is g per 100 g of water, and the temperature must be quoted with it. A solubility without a temperature is meaningless.

Why each step

Filter at the working temperature. Let a saturated solution cool on the way to the filter and solid crystallises out — you would remove solute that should have been counted, and the result comes out low.

Heat to constant mass. This is the only evidence that all the water has gone. One weighing cannot tell you whether the sample is dry or merely drier.

Water bath first. Direct strong heat makes the solution spit, and any solid thrown out of the basin is lost from the measurement.

Where marks are lost

• Forgetting to subtract the mass of the basin — it appears in both weighings and cancels only if you subtract it.
• Quoting the answer per 100 cm³ of solution rather than per 100 g of water.
• Stopping after a single evaporation, with no constant-mass check.