Qualitative Analysis: Tests for IonsEdexcel GCSE Chemistry: Flashcards

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Why must the test for an ion be unique?

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Why must the test for an ion be unique?: So a positive result can only be caused by that specific ion, allowing certain identification.
What flame colour does lithium (Li+) give?: Red.
What flame colour does potassium (K+) give?: Lilac.
What flame colour does copper (Cu2+) give?: Blue-green.
How do you distinguish aluminium ions from calcium ions using sodium hydroxide?: Both give a white precipitate, but aluminium hydroxide dissolves in excess NaOH while calcium hydroxide does not.
What colour precipitate does iron(II) give with sodium hydroxide?: Green.
What colour precipitate does iron(III) give with sodium hydroxide?: Orange-brown.
How do you test for ammonium ions?: Warm with sodium hydroxide; ammonia gas is released, which turns damp red litmus paper blue.
How do you test for carbonate ions?: Add dilute acid; carbon dioxide gas is released, which turns limewater cloudy.
How do you test for sulfate ions?: Add dilute hydrochloric acid, then barium chloride solution; a white precipitate (barium sulfate) forms if sulfate is present.
How do you test for chloride, bromide and iodide ions?: Add dilute nitric acid, then silver nitrate solution: chloride gives white precipitate, bromide gives cream, iodide gives yellow.
Why is dilute acid added before barium chloride or silver nitrate in the sulfate/halide tests?: To remove carbonate ions first, which would otherwise also give a false-positive precipitate.
What advantage do instrumental methods of analysis have over traditional wet-chemistry tests?: They may improve sensitivity, accuracy and speed.
How does a flame photometer identify the concentration of ions in a solution?: By comparing the reading against a calibration curve built from known standard concentrations.