Identification Of Ions
Resumen
Somewhere in every school laboratory there is a bottle whose label has peeled off. Inside is a white powder. It could be sodium chloride, it could be barium sulfate, it could be one of two dozen other things, and no amount of staring at it will settle the question. Yet a chemist with three reagents and a clean wire can name both halves of that compound inside ten minutes, without a single instrument more expensive than a Bunsen burner. That is the trick this lesson teaches you.
The method works because ions announce themselves. Heat certain metal ions and they glow in a colour that belongs to them alone. Offer others a hydroxide ion and they drop out of solution as a solid whose shade gives them away. Negative ions are quieter but just as findable, if you know which reagent to reach for and, crucially, which acid to add first. By the end you will be able to take a page of observations from an unknown solution and write down the formula of the salt that produced them.
Objetivos
- Flame tests can be used to identify metal ions. Lithium, sodium, potassium, calcium and barium compounds produce distinctive colours in flame tests: lithium compounds result in a crimson flame; sodium compounds result in a yellow flame; potassium compounds result in a lilac flame; calcium compounds result in a red flame; barium compounds result in a green flame. Required practical: Identify the metal ion in an unknown compound using flame testing techniques.
- Aluminium, calcium and magnesium ions form white precipitates with sodium hydroxide solution but only the aluminium hydroxide precipitate dissolves in excess sodium hydroxide solution.
- Copper(II), iron(II) and iron(III) ions form coloured precipitates with sodium hydroxide solution. Copper(II) forms a blue precipitate, iron(II) a green precipitate and iron(III) a brown precipitate.
- Carbonates react with dilute acids to form carbon dioxide. Carbon dioxide produces a white precipitate with limewater, which turns limewater cloudy white.
- Halide ions in solution produce precipitates with silver nitrate solution in the presence of dilute nitric acid. Silver chloride is white, silver bromide is cream and silver iodide is yellow.
- Sulfate ions in solution produce a white precipitate with barium chloride solution in the presence of dilute hydrochloric acid.
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- A compound produces a lilac flame in a flame test. Which metal ion does the compound contain? A. barium B. calcium C. lithium D. potassium Answer: D
- Sodium hydroxide solution is added to a solution of an unknown salt. A white precipitate forms and then dissolves when excess sodium hydroxide solution is added. Which metal ion is present? A. aluminium B. calcium C. iron(II) D. magnesium Answer: A
- Which colour of precipitate is formed when sodium hydroxide solution is added to a solution containing iron(III) ions? A. blue B. brown C. green D. white Answer: B
- A solution is acidified with dilute nitric acid and silver nitrate solution is added. A cream precipitate forms. Which ion is present in the solution? A. bromide B. chloride C. iodide D. sulfate Answer: A
- Why is dilute hydrochloric acid, rather than dilute sulfuric acid, added before barium chloride solution when testing for sulfate ions? A. Hydrochloric acid dissolves barium sulfate. B. Sulfuric acid contains sulfate ions and would give a precipitate anyway. C. Sulfuric acid is too concentrated to use in this test. D. Hydrochloric acid makes the precipitate change colour. Answer: B
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