States of Matter and Mixtures

What you'll learn

This topic forms the foundation of GCSE Chemistry and appears in both Foundation and Higher tier papers. You'll study how particles behave in solids, liquids and gases, explain changes of state using energy transfers, and master the separation techniques that are frequently tested in Paper 1. Understanding these concepts is essential because they underpin bonding, quantitative chemistry and many practical questions.

Key terms and definitions

State of matter — the physical form a substance takes (solid, liquid or gas) determined by the arrangement and movement of its particles.

Melting point — the temperature at which a solid changes to a liquid; a pure substance has a sharp, fixed melting point.

Boiling point — the temperature at which a liquid changes to a gas throughout the bulk of the liquid, not just at the surface.

Mixture — two or more substances (elements or compounds) that are not chemically bonded together and can be separated by physical methods.

Pure substance — a single element or compound that is not mixed with any other substance; has fixed melting and boiling points.

Filtration — a separation technique using filter paper to separate an insoluble solid from a liquid.

Distillation — a separation method that uses differences in boiling points to separate liquids from solids or to separate miscible liquids.

Chromatography — an analytical technique that separates mixtures based on different affinities of substances for a mobile phase and a stationary phase.

Core concepts

Particle model and the three states of matter

The particle model explains the properties of solids, liquids and gases in terms of particle arrangement, movement and energy. All substances are made of particles (atoms, molecules or ions) that are constantly moving.

Solids:

• Particles are arranged in a regular, fixed pattern
• Particles are very close together with strong forces of attraction between them
• Particles vibrate about fixed positions but cannot move from place to place
• Solids have a fixed shape and volume
• Solids cannot be compressed
• Solids have the highest density of the three states

Liquids:

• Particles are close together but not in a fixed arrangement
• Particles have weaker forces of attraction than in solids
• Particles can move around each other, allowing liquids to flow
• Liquids have a fixed volume but take the shape of their container
• Liquids cannot be easily compressed
• Liquids typically have lower density than the solid state (water is an exception)

Gases:

• Particles are far apart with negligible forces of attraction between them
• Particles move rapidly and randomly in all directions
• Gases fill any container and have no fixed shape or volume
• Gases can be easily compressed
• Gases have very low density compared to solids and liquids

Changes of state

Changes of state are physical changes — no new substances are formed and the change is reversible. The particle model explains what happens during these processes:

Melting (solid → liquid):

• Energy is transferred to the substance, increasing particle vibration
• Particles gain enough energy to overcome some forces of attraction
• The regular arrangement breaks down and particles can move around each other

Freezing (liquid → solid):

• Energy is transferred away from the substance
• Particles slow down and forces of attraction pull them into fixed positions
• A regular arrangement forms

Boiling (liquid → gas):

• Energy is transferred to the liquid
• Particles gain enough energy to completely overcome forces of attraction
• Particles move apart rapidly and randomly
• Occurs throughout the liquid at the boiling point

Evaporation (liquid → gas):

• Occurs at temperatures below the boiling point
• Only happens at the surface of the liquid
• Higher energy particles escape from the surface

Condensation (gas → liquid):

• Energy is transferred away from the gas
• Particles slow down and forces of attraction bring them closer together

Sublimation (solid → gas):

• Some substances change directly from solid to gas without melting
• Examples include solid carbon dioxide (dry ice) and iodine

During a change of state, temperature remains constant even though energy is being transferred. This energy is used to overcome forces between particles rather than increase their kinetic energy.

Purity and formulations

In chemistry, pure has a specific meaning different from everyday use. A pure substance contains only one element or compound with no other substances mixed in.

Testing purity:

• Pure substances have sharp, fixed melting and boiling points
• Impurities lower the melting point and raise the boiling point
• Impurities cause melting to occur over a range of temperatures rather than at a single point
• Comparing experimental melting points with data book values identifies substances and assesses purity

Formulations:

• A formulation is a mixture designed to produce a useful product with specific properties
• Each component is present in a carefully measured quantity
• Examples regularly tested include paints, fuels, cleaning products, medicines, fertilisers and cosmetics
• Paint formulation typically contains: pigment (colour), solvent (spreads paint), binder (forms coating), additives (improve properties)

Separation techniques: filtration and crystallisation

Filtration separates an insoluble solid from a liquid:

1. Place filter paper in a funnel
2. Pour the mixture into the funnel
3. Liquid passes through the filter paper (filtrate)
4. Insoluble solid remains on the filter paper (residue)
5. Common applications include separating sand from water or excess reactant from a solution

Crystallisation separates a soluble solid from a solution:

1. Heat the solution in an evaporating basin to evaporate some solvent
2. Stop heating when crystals start to form at the edge
3. Leave the solution to cool slowly
4. Crystals form as the solution becomes saturated and cannot hold all the dissolved solid
5. Filter to collect crystals and dry them
6. Used to purify salt solutions, copper sulfate and other soluble salts

Evaporation can also separate a soluble solid from solution but risks decomposing heat-sensitive substances. Continue heating until all liquid evaporates.

Separation techniques: distillation

Simple distillation separates a liquid from a solution:

1. Heat the solution in a flask
2. The liquid with the lowest boiling point evaporates first
3. Vapour passes into a condenser where it cools
4. Condensed liquid (distillate) collects in a receiving vessel
5. Used to obtain pure water from salt water or to separate ethanol from fermented solutions

Key apparatus: flask, thermometer (positioned at side arm level), condenser with cold water flowing in at the bottom and out at the top, heat source.

Fractional distillation separates miscible liquids with different boiling points:

1. Heat the mixture in a fractionating flask containing a fractionating column
2. The column contains glass beads or has a large surface area
3. Vapours condense and evaporate repeatedly as they rise through the column
4. The liquid with the lowest boiling point reaches the top first and distils over
5. Temperature on the thermometer shows which fraction is distilling
6. Used to separate crude oil into fractions and to separate liquid air into oxygen, nitrogen and argon

Fractional distillation is more effective than simple distillation for separating liquids with similar boiling points.

Chromatography

Paper chromatography separates mixtures of soluble substances (often dyes or inks):

Method:

1. Draw a pencil baseline 1-2 cm from the bottom of chromatography paper (pencil is insoluble)
2. Place small spots of each substance on the baseline
3. Place paper in a beaker containing shallow solvent (mobile phase) below the baseline
4. Cover the beaker to prevent evaporation
5. Solvent rises up the paper (stationary phase) carrying the solutes
6. Remove paper when solvent nearly reaches the top and mark the solvent front with pencil
7. Allow to dry

Interpreting chromatograms:

• Each spot on the final chromatogram represents a different component
• The number of spots shows how many different substances were in the mixture
• Components with greater affinity for the mobile phase travel further
• Pure substances produce single spots; mixtures produce multiple spots
• Comparing Rf values identifies unknown substances

Rf value calculation:

Rf = distance travelled by substance ÷ distance travelled by solvent

Rf values are always between 0 and 1 and are constant for a given substance in a specific solvent system.

Locating colourless substances:

• Spray chromatogram with locating agent
• Use UV light if substances fluoresce
• Common in food testing and forensic science

Worked examples

Example 1: Calculating Rf values

A student investigated food colourings using paper chromatography. The solvent travelled 8.0 cm from the baseline. Colouring A produced two spots: one travelled 6.4 cm and another travelled 3.2 cm.

(a) Calculate the Rf value of the spot that travelled 6.4 cm. [2 marks]

(b) Explain what the chromatogram shows about colouring A. [1 mark]

Model answer:

(a) Rf = distance travelled by substance ÷ distance travelled by solvent [1 mark]

Rf = 6.4 ÷ 8.0 = 0.8 [1 mark]

(b) Colouring A is a mixture containing at least two different dyes/substances. [1 mark]

Example 2: Changes of state

A student heated a pure solid substance and recorded its temperature every minute.

(a) Describe what happens to the particles as the solid changes to a liquid. [3 marks]

(b) Explain why the temperature remains constant during melting even though the substance is being heated. [2 marks]

Model answer:

(a) Particles gain energy and vibrate more [1 mark]. Forces of attraction between particles are overcome [1 mark]. Particles break free from fixed positions and can move around each other [1 mark].

(b) Energy is used to overcome forces of attraction between particles [1 mark] rather than increasing the kinetic energy/temperature of particles [1 mark].

Example 3: Separation methods

A student has a mixture containing sand (insoluble), salt (soluble) and water.

(a) Describe how the student could obtain a sample of pure, dry sand from this mixture. [2 marks]

(b) After removing the sand, describe how the student could obtain pure salt crystals from the remaining solution. [3 marks]

Model answer:

(a) Filter the mixture [1 mark]. Dry the residue/sand left on the filter paper [1 mark].

(b) Heat the solution/filtrate in an evaporating basin [1 mark] until crystals start to form OR until volume reduces [1 mark]. Allow to cool and crystallise, then filter and dry the crystals [1 mark].

Common mistakes and how to avoid them

• Mistake: Stating that particles expand or get bigger when heated. Correction: Particles themselves do not change size. They move faster and further apart, causing substances to expand.

• Mistake: Confusing evaporation and boiling, or using them interchangeably. Correction: Boiling occurs throughout the liquid at a specific temperature (boiling point); evaporation occurs only at the surface at any temperature below the boiling point.

• Mistake: Drawing particle diagrams with different-sized particles for the same substance in different states. Correction: Particles remain the same size in all three states; only their arrangement and movement change.

• Mistake: Writing that pure substances are "100% pure" or contain "no chemicals". Correction: In chemistry, pure means only one substance with no other substances mixed in. All substances are chemicals.

• Mistake: Placing the thermometer in the liquid during distillation rather than at the side arm. Correction: Position the thermometer bulb level with the side arm to accurately measure the temperature of vapour entering the condenser.

• Mistake: Using pen to draw the baseline in chromatography or placing spots below the solvent level. Correction: Always use pencil for the baseline (ink would dissolve and travel up the paper). Place spots above the solvent level so they don't dissolve directly into the solvent.

Exam technique for States of Matter and Mixtures

• "Describe" questions require you to state what happens without explaining why. Give observations or steps in sequence. For separation methods, include apparatus names and what happens at each stage. Expect 1 mark per valid point.

• "Explain" questions ask for reasons using particle theory or scientific principles. Use linking words like "because", "so" or "therefore". For particle explanations, mention particle arrangement, movement AND forces of attraction for full marks.

• Calculation questions for Rf values require the formula, correct substitution of values and the final answer. Always show working even if the calculation seems simple — this can earn method marks if the final answer is incorrect.

• Practical questions test knowledge of apparatus, procedures and safety. Learn the names of all apparatus (condenser, fractionating column, evaporating basin). Edexcel frequently asks you to identify mistakes in practical methods or suggest improvements.

Quick revision summary

Particles in solids vibrate in fixed positions; in liquids they move around each other; in gases they move rapidly and randomly. Changes of state are physical changes involving energy transfers to overcome or form forces between particles. Pure substances have sharp melting points; impurities lower melting points and broaden the melting range. Separation techniques: filtration (insoluble solid from liquid), crystallisation (soluble solid from solution), simple distillation (liquid from solution), fractional distillation (miscible liquids), chromatography (soluble mixtures). Rf = distance moved by substance ÷ distance moved by solvent.