Elements, Compounds & Mixtures

All substances are classed as element, compound, or mixture.
Element: substance made of one type of atom; cannot be split into simpler substance; 118 elements in the Periodic Table.
Compound: pure substance formed from two or more elements chemically combined; cannot be separated into elements by physical methods; e.g. CuSO₄, CaCO₃, CO₂.
Mixture: combination of two or more substances (elements and/or compounds) not chemically bonded; can be separated by physical methods (filtration, evaporation, etc.).

Pure substance: consists of a single element or a single compound with no other substances.
Distinguish purity by melting and boiling points: pure substances melt/boil at sharp, specific temperatures; impure substances show a range.
Impure substances generally have lower melting points and higher boiling points than the pure substance.
Examples: pure water has MP = 0 \text{ °C} and BP = 100 \text{ °C} ; tap water is a mixture due to dissolved impurities.
Purity assessment often uses melting point analysis with a melting point apparatus.

Used to separate components of mixtures based on physical properties.
Key methods: distillation (boiling point differences), filtration (solubility/particle size), crystallisation (solubility changes), chromatography (solubility/partitioning).

Simple distillation: separates a liquid and soluble solid from a solution or separates a pure liquid from a mixture of liquids with different boiling points.
- Process: heat until the component with the lowest boiling point vaporises; vapour condenses in the condenser and is collected as distillate; solid remains.
- Example: water-salt solution yields water distillate with salt left behind.
- Note: In ethanol–water separation, ethanol bp \approx 78^{\circ}\text{C} and water bp \approx 100^{\circ}\text{C}; stop heating when temperature rises past 78 \text{ °C} to avoid collecting water.
Fractional distillation: separates two or more miscible liquids with closer boiling points using a fractionating column to improve separation.

Filtration separates an undissolved solid from a liquid/solution.
Process: pass mixture through filter paper; liquid (filtrate) passes through; solid remains as residue.
Also applicable with centrifugation.

Used to separate a dissolved solid from a solution when the solid is much more soluble in hot solvent than in cold solvent.
Process: dissolve solid in hot solvent to create a saturated solution; allow to cool slowly; crystals form; collect crystals by filtration; wash with cold distilled water and dry.
Useful for obtaining pure crystalline solids.

Used to separate substances with different solubilities in a given solvent.
Procedure (summary):
- Draw a pencil baseline on chromatography paper and apply spots of the sample along the line.
- Place paper in solvent so baseline is above solvent level; solvent travels up by capillary action, carrying components at different rates.
- Substances with higher solubility move further; different components separate as distinct spots.
- Pencil (not ink) must be used to mark baseline; keep baseline above solvent.

Use chromatogram to compare mixture components to known substances.
Pure substances produce a single spot; impure substances produce multiple spots.
If two substances are the same, they produce identical spots (same Rf).
A reference known compound is often included as a reference spot to assist identification.
Example interpretation: brown ink (mixture) shows multiple spots; red, yellow, blue (pure) each show a single spot.

Rf value identifies components of mixtures; depends on solvent, but is constant for a given substance under fixed conditions.
Calculation:
R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}}
Properties:
- 0 < R_f < 1
- No units (unitless)
- If solvent changes, R_f can change.
Exam tip: chromatograms in exams are often black-and-white; compare dot positions and alignments rather than colours.

Question: A chromatogram shows multiple spots; determine unknown components by comparing R_f values with known substances under the same solvent.

Aim: Separate and identify a mixture of food colourings using paper chromatography.
Apparatus: beaker, chromatography paper, lines/spots, capillary tubes, ruler, paper clip, etc.
Method (summary):
- Draw a horizontal pencil line ~2 cm from paper end.
- Spot known colourings A–D and unknown U on the line using separate capillary tubes.
- Ensure spots are small (2–3 mm).
- Place paper in beaker with a small amount of water; solvent front should rise but not reach top.
- Clip the paper so it stands upright; allow solvent to run at least ¾ of the way up.
- Remove, dry, and rejoin with a fresh pencil line near wet edge to mark solvent front.
- Measure distance moved by solvent and distance moved by each spot; calculate R_f values for A–D and unknown U.
Analysis: substances with identical R_f values are the same substance; compare with known substances and visually compare spots.

Question: Sodium chloride is a soluble solid. Which technique separates a soluble solid from a solution? A filtration, B simple distillation, C crystallisation, D chromatography
Answer: C crystallisation (soluble solid crystallises out of solution when solvent is evaporated or solution becomes supersaturated).