Flashcards · YCH11 · AS Topic 3

Unit 3 – Practical Skills in Chemistry I (AS Level) flashcards

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Accuracy vs precision: Tight repeats can all be wrong if a systematic error is present. Accurate: close to the true value. Precise: repeat results close to each other.
Random error vs systematic error: Random: unpredictable scatter, reduced by repeating and averaging. Systematic: the same shift in every reading, not removed by repeats.
Percentage uncertainty of a titre or a temperature change: Double the single-reading uncertainty (two readings are subtracted), then divide by the value and × 100. Add percentage uncertainties for the overall result.
Reflux (set-up): Vertical condenser, open at the top, anti-bumping granules, water bath or electric heater for flammable liquids.
Distillation (set-up): Thermometer bulb at the side arm; water into the condenser at the bottom; collect over a narrow range.
Using a separating funnel: Stopper, invert, vent through the tap; let layers settle; drain the lower layer.
Drying an organic liquid: Add anhydrous salt until some moves freely and the liquid is clear; decant or filter.
Mass by difference vs taring: Weighing by difference accounts for solid left in the weighing bottle.
Test for the ammonium ion: Warm with NaOH(aq): ammonia is given off, which turns damp red litmus blue.
Flame test colours for Ca²⁺, Sr²⁺ and Ba²⁺: Ca²⁺ orange-red, Sr²⁺ red, Ba²⁺ pale green.