Separation & chemical tests

Physical processes separate mixtures (no new substances); chemical processes are needed to break compounds apart. Choose the technique by what you are separating:

• filtration: insoluble solid from a liquid
• evaporation / crystallisation: dissolved solid from its solution
• simple / fractional distillation: liquids by boiling point
• separating funnel: immiscible liquids
• chromatography: dissolved substances, by how far they travel.

In paper chromatography, $R_f=\frac{\text{distance moved by substance}}{\text{distance moved by solvent}}$. A pure substance gives a single spot.

• Hydrogen: a lit splint gives a squeaky pop.
• Oxygen: relights a glowing splint.
• Carbon dioxide: turns limewater cloudy.
• Chlorine: bleaches damp blue litmus (turns red then white).

• Carbonates: fizz with dilute acid (CO₂ released).
• Halides: with silver nitrate: chloride → white, bromide → cream, iodide → yellow precipitate.
• Sulfates: with barium chloride → white precipitate.
• Metal cations with sodium hydroxide: ${\text{Al}}^{3+},{\text{Ca}}^{2+},{\text{Mg}}^{2+}$ → white; ${\text{Cu}}^{2+}$ → blue; ${\text{Fe}}^{2+}$ → green; ${\text{Fe}}^{3+}$ → brown.
• Flame tests: Li crimson, Na yellow-orange, K lilac, Ca red-orange, Cu green.
• Water: turns anhydrous copper(II) sulfate from white to blue.

• Swapping the halide precipitate colours (chloride white, bromide cream, iodide yellow).
• Mixing up the splint tests (pop = hydrogen, relight = oxygen).
• Reading $R_f$ upside down (it is substance ÷ solvent, always less than 1).