4.8 Chemical analysis

"What does 'pure' mean in chemistry?

A single element or compound with nothing else mixed in. (NOT the everyday meaning of 'natural' or 'unadulterated'.)"

"What does 'pure' mean in everyday language (vs chemistry)?

Everyday: natural / unadulterated (e.g. 'pure milk' — but milk is still a mixture of fats, proteins, water, etc). Chemistry: a single element or compound only."

"How can melting and boiling points show if a substance is pure?

A pure substance melts/boils at a SHARP specific temperature. A mixture melts/boils over a RANGE of temperatures."

"What is a formulation?

A mixture that has been deliberately designed as a useful product, where each component has a specific purpose and the proportions are carefully measured."

"Give 4 examples of formulations.

Any 4 of: fuels, cleaning agents, paints, medicines, alloys, fertilisers, foods."

"What is chromatography used for?

Separating mixtures and helping to identify the substances in them."

"What are the two 'phases' in chromatography?

The STATIONARY phase (e.g. the paper, which doesn't move) and the MOBILE phase (the solvent, which moves through the paper)."

"What is the formula for Rf value?

Rf = distance moved by substance ÷ distance moved by solvent."

"What range of values can Rf have?

Between 0 and 1. (The substance can never move further than the solvent that carries it.)"

"What does the Rf value depend on?

The substance itself, the solvent used, and the stationary phase (paper). The SAME substance has DIFFERENT Rf values in different solvents."

"What does a pure substance look like on a chromatogram?

A SINGLE spot — in every solvent tested."

"What does a mixture look like on a chromatogram?

Multiple spots — one for each component of the mixture."

"Why is the start line drawn in PENCIL, not pen?

Pen ink contains dyes that would dissolve in the solvent and run with it, ruining the experiment. Pencil (graphite) doesn't dissolve."

"Why must the solvent level start BELOW the start line?

If the solvent touched the spots, it would dissolve them into the reservoir at the bottom instead of carrying them up the paper."

"Required practical 6: what does it investigate?

How paper chromatography can be used to separate and tell the difference between coloured substances. Students calculate Rf values."

"What is the test for hydrogen gas?

Hold a LIT splint at the open end of a test tube of the gas — it burns rapidly with a squeaky 'pop' sound."

"What is the test for oxygen gas?

Insert a GLOWING (not lit) splint into a test tube of the gas — the splint RELIGHTS."

"What is the test for carbon dioxide?

Bubble the gas through limewater (calcium hydroxide solution) — it turns CLOUDY/milky."

"What is the test for chlorine gas?

Hold DAMP blue litmus paper in the gas — it is BLEACHED white. (It doesn't just go red — it goes white. Damp is essential — dry litmus won't react.)"

"What flame test colour does lithium give?

Crimson red."

"What flame test colour does sodium give?

Yellow."

"What flame test colour does potassium give?

Lilac."

"What flame test colour does calcium give?

Orange-red."

"What flame test colour does copper give?

Green."

"Why might a flame colour be masked in a flame test?

If the sample contains a mixture of ions, a strong colour (especially sodium's bright yellow) can hide weaker colours from other ions."

"What forms in general when sodium hydroxide is added to a metal ion solution?

A precipitate (often coloured) of the metal hydroxide."

"Which 3 metal ions form a WHITE precipitate with sodium hydroxide?

Aluminium (Al³⁺), calcium (Ca²⁺), and magnesium (Mg²⁺) — all three give white precipitates initially."

"How do you tell aluminium ions apart from calcium and magnesium?

Add EXCESS sodium hydroxide — the aluminium hydroxide precipitate dissolves to a colourless solution, but calcium and magnesium hydroxide stay as solid precipitates."

"What colour precipitate do copper(II) ions form with sodium hydroxide?

Blue."

"What colour precipitate do iron(II) ions form with sodium hydroxide?

Green."

"What colour precipitate do iron(III) ions form with sodium hydroxide?

Brown."

"What is the test for carbonate ions?

Add a dilute acid — carbon dioxide gas is produced (you'll see fizzing). The CO₂ can be confirmed by passing through limewater, which turns cloudy."

"What is the test for halide ions?

Add dilute nitric acid, then silver nitrate solution. A coloured precipitate of the silver halide forms."

"What colour precipitate forms with chloride ions (+ silver nitrate)?

White."

"What colour precipitate forms with bromide ions (+ silver nitrate)?

Cream."

"What colour precipitate forms with iodide ions (+ silver nitrate)?

Yellow."

"What is the test for sulfate ions?

Add dilute hydrochloric acid, then barium chloride solution. A WHITE precipitate (barium sulfate) forms."

"Required practical 7: what does it cover?

Using chemical tests (flame tests, hydroxide tests, carbonate test, halide test, sulfate test) to identify the ions in an unknown ionic compound."

"What are 3 advantages of instrumental methods over chemical tests?

1) Accurate; 2) Sensitive — can detect very small amounts; 3) Rapid (fast)."

"What is flame emission spectroscopy used for?

Identifying metal ions in solutions AND measuring their concentrations, by analysing the line spectrum produced when the sample is heated in a flame."

"How does flame emission spectroscopy work?

The sample is put into a flame; the light given out is passed through a spectroscope. The output is a line spectrum unique to each metal ion — which can be analysed to identify the ions and their concentrations."