Chemistry tests

Hydrogen gas test

Bring a lit splint to the mouth of the test tube. If hydrogen is present, a squeaky pop sound is heard as it burns.

Oxygen gas test

Insert a glowing splint into the test tube. If oxygen is present, the glowing splint relights.

Carbon dioxide gas test

Bubble the gas through limewater (calcium hydroxide solution). The limewater turns cloudy or milky if carbon dioxide is present.

Ammonia gas test

Hold damp red litmus paper at the mouth of the test tube. It turns blue in the presence of ammonia gas because ammonia is alkaline.

Chlorine gas test

Hold damp blue litmus paper in the gas. It turns red and then is bleached white in the presence of chlorine gas.

Type of splints for hydrogen and oxygen tests

A wooden splint is used for both hydrogen and oxygen tests. For hydrogen, use a lit splint; for oxygen, use a glowing splint.

Result for carbon dioxide bubbled through limewater

The limewater turns cloudy or milky because calcium carbonate forms.

Effect of ammonia gas on litmus paper

Damp red litmus paper turns blue in ammonia gas.

Effect of chlorine gas on litmus paper

Damp blue litmus paper is bleached white in chlorine gas.

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Flame test procedure

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Why use platinum or nichrome wire in flame tests?

Because they are unreactive metals that don’t produce their own flame colours.

What do flame tests identify?

They identify metal ions (cations) based on the colour produced when heated.

Lithium ion (Li⁺) flame colour

Red flame.

Sodium ion (Na⁺) flame colour

Yellow flame.

Potassium ion (K⁺) flame colour

Lilac flame.

Calcium ion (Ca²⁺) flame colour

Orange-red flame.

Copper(II) ion (Cu²⁺) flame colour

Blue-green flame.

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Cations and Hydroxides test principle

Some metal ions form coloured precipitates when reacted with sodium hydroxide (NaOH). These are called metal hydroxides.

Copper(II) ion with NaOH

Forms a blue precipitate of copper(II) hydroxide, Cu(OH)₂.

Iron(II) ion with NaOH

Forms a sludgy green precipitate of iron(II) hydroxide, Fe(OH)₂.

Iron(III) ion with NaOH

Forms a red-brown precipitate of iron(III) hydroxide, Fe(OH)₃.

Ammonium ion (NH₄⁺) with NaOH

Produces ammonia gas (NH₃). The gas turns damp red litmus paper blue.

Definition of a precipitation reaction

A reaction in which an insoluble product forms and falls out of solution (a precipitate).

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Anion definition

A negatively charged ion.

Cation definition

A positively charged ion.

Examples of halide ions

Chloride (Cl⁻), bromide (Br⁻), and iodide (I⁻).

Test for halide ions

Add nitric acid (HNO₃), then silver nitrate solution (AgNO₃).

Result for chloride ions (Cl⁻)

Forms a white precipitate of silver chloride, AgCl.

Result for bromide ions (Br⁻)

Forms a cream precipitate of silver bromide, AgBr.

Result for iodide ions (I⁻)

Forms a yellow precipitate of silver iodide, AgI.

Reason for adding nitric acid before silver nitrate

It removes carbonate or hydroxide ions that could form their own precipitates and cause false results.