Identification of Ions & Gases (Cambridge (CIE) IGCSE Combined Science: Chemistry): Flashcards

Gently warm the solution with sodium hydroxide solution. A positive result is that the gas produced turns damp red litmus paper blue, confirming the presence of ammonia gas.

A flame test involves cleaning a nichrome or platinum wire loop, dipping it into the sample, then holding it in the blue Bunsen flame. The colour of the flame identifies the metal cation present.

True.

Zn²⁺ forms a white precipitate with a few drops of NaOH, but the precipitate dissolves in excess NaOH to form a colourless solution. This distinguishes zinc ions from calcium ions, whose precipitate does not dissolve.

In a flame test, sodium ions give a yellow flame, potassium ions give a lilac flame, and copper ions give a blue-green flame.

Fe²⁺ gives a green precipitate (iron(II) hydroxide). Fe³⁺ gives a red-brown precipitate (iron(III) hydroxide). Both precipitates do not dissolve in excess sodium hydroxide.

With excess ammonia solution, the light blue precipitate of copper(II) hydroxide dissolves to give a dark blue solution. The white precipitate of calcium hydroxide does not dissolve in excess ammonia.

True.

Li⁺ ions give a red flame. This is distinct from sodium (yellow), potassium (lilac), and copper (blue-green) flame colours.

When a few drops of NaOH are added to a Cu²⁺ solution, a light blue precipitate forms. In excess ammonia, this dissolves to give a dark blue solution.

Adding NaOH slowly prevents the precipitate from dissolving in excess before it can be observed. Some precipitates (e.g. zinc and aluminium hydroxides) dissolve in excess NaOH, so adding it too quickly could cause you to miss the precipitate forming.

Both Ca²⁺ and Zn²⁺ form white precipitates with a few drops of NaOH. Adding excess NaOH distinguishes them: the zinc hydroxide precipitate dissolves to form a colourless solution, while the calcium hydroxide precipitate does not dissolve.

Hold damp red litmus paper near the mouth of the test tube containing the gas. A positive result is the litmus paper turning blue, confirming the presence of ammonia.

Carbon dioxide is identified by bubbling it through limewater solution. A positive result is the solution turning cloudy white.

Use a burning (lighted) splint for hydrogen — it gives a squeaky pop. Use a glowing splint for oxygen — it relights. The splint type (burning vs glowing) is the key difference.

False.

Extinguishing a burning splint is not a definitive test for carbon dioxide — other gases such as nitrogen will also do this. The correct test is to bubble the gas through limewater, which turns cloudy white only in the presence of CO₂.

Hold a burning (lighted) splint at the open end of the test tube containing the gas. Hydrogen burns with a loud squeaky pop, caused by the rapid combustion of hydrogen with oxygen to form water.

Insert a glowing splint into the test tube containing the gas. If the gas is oxygen, the glowing splint will relight, confirming the presence of oxygen.

False.

The test for chlorine uses damp blue litmus paper (not red). Chlorine first turns it red (due to acid formation), then bleaches it white. Ammonia, not chlorine, turns red litmus blue.

When chlorine dissolves in water, it forms acidic products (including hydrochloric acid and hypochlorous acid). These acids turn the litmus red. The chlorine then bleaches the litmus white by oxidising the dye.

Chlorine gas is identified using damp blue litmus paper, which is first turned red then bleached white.

Ammonium ions (NH₄⁺) are aqueous cations present in solution. Ammonia gas (NH₃) is the gas released when ammonium ions are warmed with sodium hydroxide. The gas test uses damp red litmus paper, which turns blue in the presence of NH₃.