States of Matter and Mixtures

What you'll learn

This topic covers the particle model of matter, how substances change state, and techniques for separating mixtures. You'll learn to explain physical processes at the particle level and apply practical separation methods. Understanding these fundamentals is essential for success across all GCSE Chemistry topics.

Key terms and definitions

State of matter — the physical form of a substance: solid, liquid or gas, determined by particle arrangement and movement

Pure substance — a single element or compound that is not mixed with any other substance, with a specific sharp melting and boiling point

Mixture — two or more elements or compounds physically combined but not chemically bonded, which can be separated by physical methods

Filtration — separation technique using a barrier (filter paper) to separate an insoluble solid from a liquid

Distillation — separation technique that uses differences in boiling points to separate liquids from a mixture

Chromatography — separation technique that separates substances based on their different solubilities and attractions to a mobile and stationary phase

Solvent — the liquid in which a solute dissolves to form a solution

Residue — the solid left behind on filter paper after filtration; the insoluble component

Core concepts

The particle model and states of matter

Matter exists in three main states: solid, liquid and gas. The particle model explains the properties of each state.

Solids:

• Particles arranged in a fixed, regular pattern
• Strong forces of attraction between particles hold them in position
• Particles vibrate about fixed positions but cannot move from place to place
• Fixed shape and volume
• Cannot be compressed
• Highest density of the three states

Liquids:

• Particles close together but randomly arranged
• Moderate forces of attraction between particles
• Particles can move past each other
• Fixed volume but take the shape of their container
• Cannot be compressed significantly
• Density lower than solids

Gases:

• Particles far apart and randomly arranged
• Very weak forces of attraction between particles
• Particles move rapidly and randomly in all directions
• No fixed shape or volume – fill their container
• Can be compressed easily
• Lowest density of the three states

The amount of energy particles possess determines the state of a substance. Temperature is a measure of the average kinetic energy of particles.

State changes and energy

State changes are physical changes that can be reversed. No new substances form during state changes.

State changes with increasing temperature:

• Melting — solid to liquid (at the melting point)
• Boiling/evaporation — liquid to gas (at the boiling point)
• Sublimation — solid directly to gas (e.g., solid carbon dioxide/dry ice)

State changes with decreasing temperature:

• Freezing — liquid to solid (at the freezing point, same temperature as melting point)
• Condensation — gas to liquid

During state changes, energy is transferred to or from the surroundings but the temperature remains constant until the change is complete. Energy is used to overcome forces of attraction between particles rather than increasing their kinetic energy.

Pure substances have sharp, specific melting and boiling points. Impure substances (mixtures) melt and boil over a range of temperatures. This difference helps identify pure substances and test for purity.

Physical vs chemical changes

Physical changes:

• No new substances formed
• Changes in state, shape or size
• Usually reversible
• Examples: dissolving, state changes, separating mixtures

Chemical changes:

• New substances formed
• Atoms rearranged; bonds broken and formed
• Usually irreversible
• Examples: combustion, oxidation, thermal decomposition

Separating mixtures

Different separation techniques exploit physical properties of substances.

Filtration:

• Separates insoluble solids from liquids
• Filter paper acts as a barrier with tiny pores
• Liquid passes through (filtrate), solid remains (residue)
• Example: separating sand from water

Crystallisation:

• Separates a soluble solid from a solution
• Solution heated gently to evaporate some solvent
• Solution becomes saturated as concentration increases
• When cooled, crystals form as solid can no longer stay dissolved
• Crystals filtered and dried
• Example: obtaining copper sulfate crystals from solution

Simple distillation:

• Separates a liquid from a solution
• Solution heated until liquid boils
• Vapour rises, passes through condenser
• Vapour cools and condenses back to liquid
• Pure liquid (distillate) collected
• Example: obtaining pure water from salt water
• Only effective when components have very different boiling points

Fractional distillation:

• Separates mixtures of liquids with similar boiling points
• Uses a fractionating column filled with glass beads or rods
• Provides a temperature gradient
• Vapours condense and evaporate repeatedly as they rise
• Liquid with lowest boiling point reaches top first and is collected
• Example: separating ethanol from water, crude oil separation

Chromatography (paper chromatography):

• Separates dissolved substances based on different solubilities
• Small spot of mixture placed on baseline (drawn in pencil)
• Paper placed in suitable solvent (mobile phase)
• Solvent rises up paper (stationary phase) by capillary action
• Different substances travel different distances
• More soluble substances/those with weaker attraction to paper travel further
• Produces a chromatogram
• Rf value = distance moved by substance ÷ distance moved by solvent
• Each substance has a characteristic Rf value in specific conditions
• Used to identify unknown substances by comparison

Purity and formulations

In everyday language, "pure" means natural or clean. In chemistry, pure means containing only one substance.

Testing purity:

• Pure substances have specific, sharp melting and boiling points
• Impurities lower melting points and raise boiling points
• Melting and boiling occur over a range rather than at a fixed temperature
• Compare measured melting/boiling point with data book values

Formulations:

• A formulation is a mixture designed for a specific purpose, with precise proportions of components
• Each component has a specific function
• Examples include:
  • Pharmaceuticals (medicines contain active ingredient, binder, coating)
  • Fuels (petrol contains octane, additives to improve performance)
  • Cleaning products (detergent, water, fragrance, colourants)
  • Paints (pigment for colour, solvent, binder, additives)
  • Alloys (metals mixed in specific proportions for desired properties)

Formulations are made by mixing components in carefully measured quantities. Quality control ensures each batch has the correct composition.

Practical techniques

You must be able to describe and explain practical methods used to separate mixtures:

Setting up filtration:

1. Fold filter paper into a cone
2. Place in funnel
3. Pour mixture slowly down glass rod to prevent splashing
4. Collect filtrate in beaker below

Setting up simple distillation:

1. Heat solution in flask
2. Thermometer positioned at neck of flask
3. Vapour passes into condenser
4. Cold water flows through outer jacket of condenser (in at bottom, out at top)
5. Collect distillate in receiving vessel

Carrying out paper chromatography:

1. Draw baseline in pencil (not pen – ink would dissolve)
2. Place spot of mixture on baseline
3. Baseline must be above solvent level
4. Allow solvent to rise until near top
5. Remove and mark solvent front immediately
6. Allow to dry (chromatogram)
7. Calculate Rf values

Worked examples

Example 1: State changes and particle behaviour

Question: A student investigated how the temperature of stearic acid changed as it cooled from liquid to solid. Explain, in terms of particles, what happens when stearic acid freezes. (3 marks)

Answer:

• Particles lose energy / kinetic energy decreases (1 mark)
• Particles move more slowly and come closer together (1 mark)
• Forces of attraction between particles hold them in fixed positions / regular arrangement forms (1 mark)

Examiner tip: Questions about state changes in terms of particles require you to mention energy, particle movement and forces between particles.

Example 2: Calculating Rf values

Question: A student used paper chromatography to analyse food colourings. The solvent travelled 8.0 cm from the baseline. One food colouring produced a spot that travelled 6.4 cm from the baseline.

(a) Calculate the Rf value for this food colouring. (2 marks) (b) The student's teacher said the food colouring was a pure substance. Explain how the chromatogram shows this. (1 mark)

Answer: (a) Rf = distance moved by substance ÷ distance moved by solvent (1 mark) Rf = 6.4 ÷ 8.0 = 0.8 (1 mark)

(b) The food colouring produced only one spot / did not separate into different colours (1 mark)

Examiner tip: Rf values have no units and are always between 0 and 1. Show your working clearly.

Example 3: Separation techniques

Question: A student has a mixture of salt (sodium chloride) dissolved in water with some sand mixed in.

(a) Describe how the student could obtain pure dry sand from the mixture. (2 marks) (b) Describe how the student could then obtain pure water from the salt solution. (3 marks)

Answer: (a) Filter the mixture (1 mark) The residue is sand, which can be dried in an oven / left to dry (1 mark)

(b) Set up simple distillation apparatus (1 mark) Heat the salt solution so water boils and evaporates (1 mark) Water vapour condenses in the condenser and pure water is collected (1 mark)

Examiner tip: Name the specific technique, then describe the key steps. Filtration separates insoluble solids; distillation separates liquids from solutions.

Common mistakes and how to avoid them

• Confusing physical and chemical changes — remember that state changes and dissolving are physical (reversible, no new substances). Burning and reacting are chemical (new substances formed).

• Saying particles expand when heated — particles themselves do not change size. They gain energy and move further apart, causing the substance to expand.

• Drawing the baseline in pen during chromatography — always use pencil. Pen ink would dissolve in the solvent and interfere with results.

• Placing the baseline below the solvent level — the spots would dissolve directly into the solvent rather than being carried up the paper.

• Confusing filtrate and residue — filtrate is the liquid that passes through; residue is the solid left on the filter paper.

• Thinking pure substances in chemistry mean natural — chemically pure means only one substance, whether natural or synthetic. Pure water from a lab is chemically pure even though it's not from a natural source.

Exam technique for "States of Matter and Mixtures"

• "Describe" questions require you to state facts about a process or observation. For separation techniques, include apparatus and key steps (typically 1 mark per valid point).

• "Explain in terms of particles" means you must refer to particle movement, arrangement, forces between particles and energy changes. Generic answers without particle-level detail score zero.

• Method questions should be answered in logical order with specific detail. Name apparatus, state temperatures/conditions where relevant, and explain what you would observe.

• Rf value calculations always show the formula first (Rf = distance by substance ÷ distance by solvent), then substitute values. Give answers to 2 significant figures and never include units.

Quick revision summary

Matter exists as solid, liquid or gas depending on particle arrangement and energy. State changes are physical and reversible. Pure substances have sharp melting/boiling points; mixtures melt/boil over a range. Separation techniques exploit physical differences: filtration (solubility), crystallisation (solubility), distillation (boiling point), chromatography (solubility and attraction to paper). Formulations are useful mixtures with precise compositions. Always explain particle behaviour in terms of energy, movement, arrangement and forces.