Flashcards · 5070 · Topic 12

Experimental techniques and chemical analysis flashcards

30 cards on Topic 12, subtopics 12.1 to 12.5. Pick a subtopic or study them all.

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Apparatus to measure time, temperature, mass and volume: Time: stopwatch. Temperature: thermometer. Mass: balance. Volume: burette, volumetric pipette, measuring cylinder, gas syringe.
Solvent: A substance that dissolves a solute.
Solute: A substance that is dissolved in a solvent.
Solution: A mixture of one or more solutes dissolved in a solvent.
Saturated solution: A solution containing the maximum concentration of a solute dissolved in the solvent at a specified temperature.
Residue: A substance that remains after evaporation, distillation, filtration or any similar process.
Filtrate: A liquid or solution that has passed through a filter.

Apparatus for an acid–base titration: Burette, volumetric pipette, conical flask and a suitable indicator.
When do you stop adding from the burette, and how many runs do you need?: Stop when the indicator just changes colour (the end-point). Repeat until you have concordant titres.
Why not use universal indicator in a titration?: It changes colour gradually across the pH range, so it gives no sharp end-point. Use methyl orange or thymolphthalein.

Why is the baseline on a chromatogram drawn in pencil, above the solvent?: Pencil does not dissolve and run. Above the solvent so the spots do not dissolve into the solvent in the beaker.
How does a chromatogram show that a substance is pure?: A pure substance gives a single spot.
Rf value: distance moved by the substance ÷ distance moved by the solvent front, both measured from the baseline.
A spot moves 4.5 cm and the solvent front 7.5 cm. What is the Rf?: 4.5 ÷ 7.5 = 0.6
How are colourless substances seen on a chromatogram?: By spraying the paper with a locating agent.

Filtration separates…: An insoluble solid from a liquid. The solid is the residue; the liquid is the filtrate.
Crystallisation: what it separates and the method: A dissolved solid from its solution. Heat to evaporate some solvent until saturated, then cool; filter off the crystals, wash with a little cold solvent and dry.
Simple distillation separates…: A solvent from a dissolved solid (for example, water from salt water). The solvent evaporates and is condensed in a condenser.
Fractional distillation separates…: Miscible liquids with different boiling points (for example, ethanol and water). Uses a fractionating column; the liquid with the lower boiling point distils first.
Separating one soluble solid from an insoluble one: Add a suitable solvent to dissolve the soluble one, then filter.
How do melting and boiling points show whether a substance is pure?: A pure substance has a sharp, fixed melting point and boiling point. Impurities lower the melting point and raise the boiling point, and make them happen over a range.

Tests for hydrogen and oxygen: Hydrogen: a lighted splint gives a squeaky pop. Oxygen: relights a glowing splint.
Tests for carbon dioxide and ammonia: Carbon dioxide: turns limewater milky. Ammonia: turns damp red litmus paper blue.
Tests for chlorine and sulfur dioxide: Chlorine: bleaches damp litmus paper. Sulfur dioxide: turns acidified potassium manganate(VII) from purple to colourless.
Aqueous sodium hydroxide: precipitate colours for Cu²⁺, Fe²⁺ and Fe³⁺: Cu²⁺: light blue. Fe²⁺: green. Fe³⁺: red-brown. All insoluble in excess.
Zn²⁺ and Al³⁺ both give a white precipitate that dissolves in excess NaOH. How do you tell them apart?: Use aqueous ammonia: the zinc precipitate dissolves in excess; the aluminium precipitate does not.
Test for halide ions, and the precipitate colours: Acidify with dilute nitric acid, add aqueous silver nitrate. Cl⁻: white. Br⁻: cream. I⁻: yellow.
Test for sulfate ions: Acidify with dilute nitric acid, add aqueous barium nitrate: white precipitate.
Test for carbonate ions: Add dilute acid: effervescence, and the gas turns limewater milky.
Flame test colours: Li⁺, Na⁺, K⁺, Ca²⁺, Ba²⁺, Cu²⁺: Li⁺: red. Na⁺: yellow. K⁺: lilac. Ca²⁺: orange-red. Ba²⁺: light green. Cu²⁺: blue-green.