methods of identifying common gases

How do you test for hydrogen gas?

Place a lit splint at the mouth of a test tube containing the gas.

What is the positive result for hydrogen?

A squeaky pop sound is heard.

Why does hydrogen make a squeaky pop?

Hydrogen reacts explosively with oxygen in the air to form water (H₂O).

How do you test for oxygen gas?

Insert a glowing splint into a test tube containing the gas.

What is the positive result for oxygen?

The glowing splint relights.

Why does oxygen relight a glowing splint?

Oxygen supports combustion, allowing the splint to reignite.

How do you test for carbon dioxide gas?

Bubble the gas through limewater (aqueous calcium hydroxide).

What is the positive result for carbon dioxide?

The limewater turns cloudy (milky white).

Why does limewater turn cloudy in the presence of carbon dioxide?

Carbon dioxide reacts with calcium hydroxide to form calcium carbonate (CaCO₃), which is insoluble and appears as a white precipitate.

What happens if excess CO₂ is bubbled through limewater?

The cloudy solution clears again because calcium carbonate reacts with more CO₂ to form soluble calcium hydrogen carbonate.

How do you test for chlorine gas?

Hold a piece of damp blue litmus paper in the gas.

What is the positive result for chlorine?

The blue litmus paper turns red and then white.

Why does chlorine turn blue litmus paper red first?

Chlorine is acidic in water, so it temporarily makes the litmus paper red.

Why does chlorine then turn the litmus paper white?

Chlorine is a bleaching agent and removes the colour from the paper.

How do you test for halide ions (chloride, bromide, iodide)?

Add dilute nitric acid (HNO₃) followed by silver nitrate (AgNO₃) solution.

What are the results for different halide ions?

• Chloride (Cl⁻) → White precipitate (AgCl)

• Bromide (Br⁻) → Cream precipitate (AgBr)

• Iodide (I⁻) → Yellow precipitate (AgI)

Why is nitric acid added before silver nitrate?

To remove carbonate impurities, which could also form a precipitate.

what is the general ionic equation for the formation of the precipitate from halide ion

Ag⁺ + X^- ——> AgX(s)

how to test for a carbonate

1. add dilute acid

2. bubble the gas through limewater

3. if CO₂ is present, effervescence then CO₂ gas is formed turning the limewater cloudy

- make sure you connect the test tube of the suspected ion to the test tube of limewater quickly so that none of the CO₂ escapes

How do you carry out a flame test?

Dip a clean nichrome wire loop into a sample of the solid, then place it in a blue Bunsen burner flame and observe the colour.

why must a blue bunsen burner flame be used

because a yellow flame isn’t hot enough and it may mask the colours being made by some ions

Why must the wire loop be cleaned before each test?

To remove contamination that could affect the flame colour by masking colours produced by ions or by two or more ions being present on the wire and so the colours mix

how to clean the wire before each flame test

place it in acid first to avoid contamination

lithium colour change flame test

Lithium (Li⁺) → Crimson red

sodium colour change flame test

Sodium (Na⁺) → Yellow

potassium colour change flame test

Potassium (K⁺) → Lilac

calcium colour change flame test

Calcium (Ca²⁺) → Orange-red

copper colour change flame test

Copper (Cu²⁺) → Green

How do you test for metal cations using sodium hydroxide?

Add sodium hydroxide (NaOH) solution to the sample and observe the colour of any precipitate formed.

Ca NaOH test results

Calcium (Ca²⁺) → White precipitate

Mg NaOH test results

Magnesium (Mg²⁺) → White precipitate

Al NaOH test results

Aluminium (Al³⁺) → White precipitate (dissolves in excess NaOH)

Cu NaOH test results

• Copper(II) (Cu²⁺) → Blue precipitate

Fe²⁺ NaOH test results

Iron(II) (Fe²⁺) → Green precipitate³⁺

Fe3+ NaOH test results

Iron(III) (Fe³⁺) → Brown precipitate

How can you distinguish between calcium and magnesium ions in the NaOH test?

Use a flame test – calcium gives an orange-red flame, while magnesium has no flame colour.

Why does aluminium hydroxide dissolve in excess NaOH but not calcium or magnesium?

Aluminium hydroxide is amphoteric, meaning it reacts with excess alkali to form a soluble complex.

How do you test for sulfate ions?

Add dilute hydrochloric acid (HCl) followed by barium chloride (BaCl₂) solution.

What is the positive result for sulfates?

A white precipitate of barium sulfate (BaSO₄) forms.

Why is hydrochloric acid added before barium chloride?

To remove carbonate impurities, which could also form a white precipitate.