1.2 Elements, Compounds & MixturesEdexcel IGCSE Chemistry: Revision notes
Section 1
How do you classify a substance as an element, compound or mixture?
- Element: a substance made of only one type of atom, cannot be broken down chemically
- Compound: two or more different elements chemically combined in fixed proportions, formed via a chemical reaction
- Mixture: two or more substances (elements and/or compounds) physically combined, not chemically bonded, in no fixed proportion
Mixtures can be separated by physical methods (no new substances form); compounds can only be split into elements by chemical reactions.
Air, salt water and crude oil are all mixtures, not compounds — students often wrongly call air a compound because it has a fixed composition; it is a mixture of gases physically combined.
Section 2
How do melting and boiling points show purity?
A pure substance (a single element or compound) melts and boils at one exact, fixed temperature.
A mixture melts or boils over a range of temperatures, because the different substances present interfere with each other's particle arrangement and interactions.
This is used practically: measuring the melting or boiling point of a sample (or a small melting-point range) can indicate whether it is pure or contains impurities.
State clearly: 'melts/boils at a fixed, sharp temperature' for pure substances and 'melts/boils over a range of temperatures' for mixtures — examiners look for exactly this phrasing.
Section 3
What techniques separate mixtures?
| Technique | Separates | How it works |
|---|---|---|
| Filtration | Insoluble solid from a liquid | Solid is trapped by filter paper; liquid (filtrate) passes through |
| Crystallisation | Dissolved solid from a solution | Solvent is evaporated (often by gentle heating) until the solution is saturated, then cooled so crystals form |
| Simple distillation | A solvent from a solution (or a liquid from dissolved solids) | Liquid is heated to evaporate the solvent, which is then cooled and condensed back to a pure liquid |
| Fractional distillation | Miscible liquids with different boiling points | Mixture heated in a fractionating column; liquid with the lowest boiling point evaporates first and is collected, others follow at their own boiling points |
| Paper chromatography | Different soluble substances in a mixture (e.g. inks/dyes) | Substances travel different distances up paper depending on their solubility and attraction to the paper |
Section 4
How is chromatography interpreted?
In paper chromatography, a spot of the mixture is placed near the bottom of chromatography paper, which is then placed in a solvent (below the spot). As the solvent moves up the paper (mobile phase), it carries the components of the mixture at different rates depending on their solubility in the solvent and how strongly they stick to the paper (stationary phase).
- A single spot that separates into several spots shows the original substance was a mixture.
- Comparing spots to known reference substances run alongside can identify components.
- The Rf value (retention factor) is calculated as:
Rf = distance travelled by substance ÷ distance travelled by solvent
Each substance has a characteristic Rf value in a given solvent, allowing identification by comparing to known Rf values.
If a dye spot travels 6 cm and the solvent front travels 10 cm, Rf = 6/10 = 0.6.
Section 5
How would you investigate paper chromatography of inks or food colourings?
- Draw a pencil baseline near the bottom of chromatography paper (pencil, not pen, as it will not dissolve).
- Place small spots of each ink or food colouring on the baseline.
- Stand the paper in a shallow layer of solvent (below the baseline) in a covered container.
- Allow the solvent to rise up the paper, carrying the colours with it, until near the top; then remove and mark the solvent front.
- Measure distances travelled by each spot and the solvent to calculate Rf values, and compare the pattern of spots to identify components.
Must Know
- Elements = one type of atom; compounds = elements chemically combined in fixed ratio; mixtures = physically combined, no fixed ratio
- Pure substances have a single fixed melting/boiling point; mixtures melt/boil over a range
- Filtration separates insoluble solid from liquid; crystallisation separates dissolved solid from solution
- Simple distillation separates a solvent from a solution; fractional distillation separates liquids with different boiling points
- Chromatography separates mixtures based on differing solubility/attraction to paper
- Rf = distance moved by substance ÷ distance moved by solvent
That's the notes covered.
Carry on to the next subtopic.