1.2.1 Element, Compound or Mixture — Edexcel International GCSE Science Double Award (Modular) - Chemistry Unit 1 Revision Notes & Topical Questions

Part 1 of 7Key ideas

1

A pure substance has a sharp melting point; impurities lower the melting point and make it melt over a range.

2

Pure water melts at 0 °C and boils at 100 °C at normal pressure.

3

R_f = distance moved by spot ÷ distance moved by solvent front.

4

The start line is drawn in pencil because pencil (graphite) is insoluble and will not run.

5

A pure substance gives a single spot on a chromatogram; a mixture gives two or more spots.

6

In fractional distillation the liquid with the lowest boiling point distils over first.

7

Crystallisation is preferred to evaporating to dryness for salts that decompose or lose water of crystallisation on strong heating.

8

Compounds have properties quite different from the elements they are made from (e.g. sodium + chlorine → sodium chloride).

Part 2 of 7Learning objectives

1

Classify substances as elements, compounds or mixtures.

2

Use melting point data to decide whether a substance is pure.

3

Choose and describe suitable separation techniques.

4

Carry out paper chromatography and calculate R_f values.

Part 3 of 7Essential knowledge

Essential knowledge

Element

All substances can be classified into one of three types.

An element contains only one kind of atom (e.g. iron, Fe, or nitrogen, N₂).
A compound contains two or more elements chemically combined in a fixed ratio (e.g. carbon dioxide, CO₂).
A mixture contains different substances that are not chemically joined (e.g. air).

Particle diagrams help

Particle diagrams help identical single atoms or identical molecules of one atom type = element; identical molecules containing different atoms = compound; two or more different kinds of particle = mixture.

Pure

In chemistry, pure means a single substance only, not 'natural' or 'clean'. Pure substances melt and boil at one fixed temperature. Mixtures melt or boil over a range of temperatures, so a melting-point test is a quick purity check.

Filtration

Filtration removes insoluble solids: the solid stays on the filter paper (residue), the liquid passes through (filtrate).

Crystallisation

Crystallisation heat the solution to evaporate some water until it is saturated (crystals form on a cold glass rod), then leave it to cool; filter off and dry the crystals.

Simple distillation

Simple distillation boils a solution; the vapour passes into a water-cooled condenser and condenses to pure solvent. Fractional distillation uses a fractionating column so that liquids with close boiling points separate; the thermometer at the top shows which fraction is being collected.

Paper chromatography

Paper chromatography the paper is the stationary phase and the solvent is the mobile phase. Substances that are more soluble in the solvent and less attracted to the paper travel further, giving larger R_f values.

Interpreting chromatograms

Interpreting chromatograms if an unknown spot has the same R_f (same height) as a reference in the same solvent, they are likely the same substance. A spot that stays on the start line is insoluble in that solvent. The number of spots shows the minimum number of components.

Practical tips

Practical tips keep the start line above the solvent so spots do not dissolve into the beaker; use a lid so the air stays saturated with solvent vapour; use small spots so they do not smear; repeat with a different solvent if spots do not separate.

Impurities and boiling point

Impurities lower a substance's melting point and raise its boiling point, and make it melt or boil over a range. The closer the measured value is to the data-book value, the purer the sample.

Condenser connections

In distillation, cold water enters the condenser at the lower end and leaves at the upper end, flowing the opposite way to the vapour. This keeps the condenser full of water and cools the vapour efficiently.

Rf depends on the solvent

A substance always has the same Rf in the same solvent, but a different solvent gives a different Rf. Measure both distances from the start line, to the centre of the spot and to the solvent front.

Element defined by protons

An element is made of atoms that all have the same number of protons, and it cannot be broken down into anything simpler by chemical means. Compounds can only be separated into their elements by chemical reactions.

Part 4 of 7Process & sequence

1

Draw a pencil start line about 1 cm from the bottom of the chromatography paper.

2

Put small, concentrated spots of the unknown and of known reference substances on the line.

3

Stand the paper in a beaker of solvent so the solvent level is below the start line; cover with a lid.

4

Let the solvent rise up the paper until it is near the top.

5

Remove the paper and immediately mark the solvent front in pencil; allow it to dry.

6

Measure distances and calculate R_f values for each spot.

7

Compare spots and R_f values with the references to identify components.

Paper chromatographywrite these steps in this order; each step is a marking point.

1

Pencil start line

2

Spot samples on line

3

Stand in solvent below line

4

Solvent rises, spots separate

5

Mark solvent front

6

Calculate R_f values

Part 5 of 7Worked examples

0.60

A spot moves 4.2 cm while the solvent front moves 7.0 cm. Calculate its R_f value.

R_f = 4.2 ÷ 7.0 = 0.60. No units, because it is a ratio of two distances.

Impure

A student's sample of aspirin melts between 128 °C and 133 °C. Pure aspirin melts at 136 °C. What can be concluded?

The sample is impure: it melts over a range, and that range is below the pure melting point.
- Impurities lower the melting point and widen the range.

Part 6 of 7Summary

1

Every substance is an element, a compound or a mixture.

2

Pure substances have fixed, sharp melting and boiling points.

3

Mixtures are separated by physical methods based on differing properties.

4

Chromatography identifies substances by comparing R_f values.

5

One spot means the sample is probably pure.

Part 7 of 7Compare: Compare

Element, compound or mixture?
Feature	Element	Compound	Mixture
Types of atom	One	Two or more, bonded	Two or more substances, not bonded
Fixed composition?	Yes	Yes (fixed ratio)	No (any proportions)
Separated by	Cannot be split	Chemical reactions only	Physical methods
Melting point	Sharp	Sharp	Over a range
Example	Oxygen, O₂	Water, H₂O	Air, sea water

Choosing a separation method
Mixture	Method	What is collected
Sand and water	Filtration	Sand as residue, water as filtrate
Salt from salt solution	Crystallisation	Salt crystals
Water from sea water	Simple distillation	Pure water (distillate)
Ethanol and water	Fractional distillation	Ethanol first (lower b.p.)
Dyes in ink	Paper chromatography	Separate spots on paper

Pick the method from the physical property that differs: solubility, boiling point, or attraction to paper.

Key terms

Command terms

Give / State / Name

Recall a fact; one idea per mark.

Define

Give the exact meaning using the specification keywords.

Describe

Give an account of what happens / the pattern in data, quoting values; no reasons needed.

Explain

Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.

Suggest

Use knowledge in a new context; more than one answer may be accepted.

Compare

Similarities and differences; each point must refer to both items.

Calculate

Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.

Discuss

Explore the issue from different angles and link the points.

Evaluate / Assess

Weigh strengths and weaknesses and reach a supported judgement.

Deduce

Draw a conclusion from the information given.

Justify

Support a statement/choice with evidence.

Exam technique

Keywords the mark scheme looks for in this lesson

Element · Compound · Mixture · Pure substance · Filtration · Crystallisation · Distillation · Fractional distillation · Chromatography · R_f value

Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

Use specification keywords exactly (e.g. electronegativity, nucleophile, dynamic equilibrium, activation energy).

Link ideas

Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.

Use the information in the question

“discuss/evaluate” answers must use the data provided and match the number of marks.

Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.

Avoid vague words

“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.

Do not extrapolate a graph line beyond the data or to zero unless asked.

Six-mark extended responses are marked on content AND a logical, linked line of reasoning.