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Flashcards · 9202 · Topic 4

Chemical analysis flashcards

21 cards on Topic 4, subtopics 3.4.1 to 3.4.3. Pick a subtopic or study them all.

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3.4.1 Purity and chromatography

Pure substance (in chemistry)
A single element or compound, with no other substance mixed in. Everyday "pure" (nothing added) is different: a "pure" natural product can still be a mixture.
How do impurities affect melting and boiling points?
A pure substance melts and boils at sharp, fixed values. Impurities lower the melting point and raise the boiling point, and make them happen over a range.
Why does purity matter for drugs and foodstuffs?
Drugs: the correct dose of the active compound, with no harmful impurities. Food: no harmful contaminants, and the contents match the label.
Choose the method: insoluble solid from a liquid; soluble solid from a solution; a liquid from a solution
Filtration; crystallisation; distillation.
Stationary and mobile phase in paper chromatography
Stationary phase: the paper. Mobile phase: the solvent.
Rf value, and a spot that moves 3.3 cm when the solvent front moves 5.5 cm
Rf = distance moved by substance ÷ distance moved by solvent, both from the baseline (to the centre of the spot). 3.3 ÷ 5.5 = 0.60. No units; never above 1.
How does paper chromatography separate the components of a mixture?
Each component is shared between the paper and the solvent. The more soluble it is in the solvent, the further it moves, so components with different solubilities separate.
Does one spot in one solvent prove a substance is pure?
No. Run it in a second solvent alongside a pure reference. One spot each time, level with the reference, is strong evidence.

3.4.2 Identification of common gases

Test for hydrogen
A lighted splint near the gas: a pop (2H₂ + O₂ → 2H₂O).
Test for oxygen
A glowing splint in the gas relights.
Test for carbon dioxide, and why it works
Bubble the gas through limewater (calcium hydroxide solution): it turns cloudy white. Ca(OH)₂ + CO₂ → CaCO₃ + H₂O — insoluble calcium carbonate forms.
Two tests for ammonia
Damp red litmus paper turns blue. Hydrogen chloride from concentrated hydrochloric acid held near it gives white smoke of ammonium chloride (NH₃ + HCl → NH₄Cl).
Test for chlorine
Damp blue litmus paper turns red, then is bleached white.

3.4.3 Identification of ions

Flame test colours for Li⁺, Na⁺, K⁺, Ca²⁺ and Ba²⁺
Li⁺: crimson. Na⁺: yellow. K⁺: lilac. Ca²⁺: red. Ba²⁺: green.
Flame test practical: how is the wire cleaned, and what can spoil the result?
Dip a nichrome loop in dilute hydrochloric acid and heat, repeating until the flame shows no colour. Sodium contamination gives a strong yellow that can hide other colours (especially lilac); a mixture of ions may show one colour only.
Sodium hydroxide solution: precipitates with Cu²⁺, Fe²⁺ and Fe³⁺
Cu²⁺: blue. Fe²⁺: green (slowly turns brown in air). Fe³⁺: brown.
Al³⁺, Ca²⁺ and Mg²⁺ all give a white precipitate with NaOH. How do you tell them apart?
Add excess NaOH: only aluminium hydroxide dissolves. For Ca²⁺ vs Mg²⁺ use a flame test: calcium gives red; magnesium gives no characteristic colour.
Test for carbonate ions
Add dilute acid: fizzing. Pass the gas into limewater: it turns cloudy white (carbon dioxide).
Test for halide ions, and the precipitate colours
Acidify with dilute nitric acid, then add silver nitrate solution. Cl⁻: white. Br⁻: cream. I⁻: yellow.
Test for sulfate ions, and why sulfuric acid is not used
Add dilute hydrochloric acid, then barium chloride solution: a white precipitate of barium sulfate. Sulfuric acid contains sulfate ions, so it would always give a precipitate.
A solution gives a brown precipitate with NaOH and a white precipitate with acidified BaCl₂. Name and formula?
Iron(III) sulfate, Fe₂(SO₄)₃ — two Fe³⁺ balance three SO₄²⁻.

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