RP4

Carry out simple test-tube reactions to identify cations and anions.

What reagent do you use to test for group 2 metal cations higher up the group?

Sodium hydroxide (NaOH) (or any other species containing hydroxide ions).

Give the ionic equation for the reaction between a group 2 metal cation-containing substance and sodium hydroxide.

M²⁺ + 2OH^- → M(OH)₂

What is the positive result for the test for group 2 metal cations with sodium hydroxide? Give the general formula of the species responsible for this observation.

Formation of white precipitate. M(OH)₂ is the white precipitate.

Talk me through how to carry out the test for group 2 metal cations with sodium hydroxide.

1) Add ~10 drops of the sample to a clean test tube.

2) Add ~10 drops of 0.6 mol dm^-3 sodium hydroxide to the same test tube.

3) Gently swirl to mix.

What reagent do you initially use to test for the presence of halide anions?

Concentrated sulfuric acid (H₂SO₄).

What is the positive result for the test for chloride ions with concentrated sulfuric acid? How can you further test this? What is the species responsible for this observation?

• White misty fumes + effervescence.

• Blue litmus paper turns red (indicates acidic gas).

• HCl (hydrogen chloride gas).

What is the positive result for the test for bromide ions with concentrated sulfuric acid? Which species are responsible for this observation?

• White misty fumes + effervescence + brown vapour + choking gas.

• White misty fumes and effervescence from HBr (hydrogen bromide gas), brown vapour from Br₂ (bromine), choking gas is SO₂ (sulfur dioxide).

True or false? Sulfur dioxide (SO₂) is a choking gas.

True.

True or false? Blue litmus paper turns yellow in acidic conditions.

False. BLUE LITMUS PAPER TURNS RED IN ACIDIC CONDITIONS.

Talk me through how to carry out the test for halide ions with concentrated sulfuric acid.

1) Add ~10 drops of the sample to a clean test tube.

2) Add sulfuric acid in drops and continue shaking the test tube until it is in excess.

What reagent do you use to test for ammonium ions?

Warm sodium hydroxide (NaOH).

What reagent do you use to test for group 2 metal cations further down the group?

Dilute sulfuric acid (H₂SO₄).

Give the balanced symbol equation for the reaction between ammonium ions and hydroxide ions.

NH₄ + OH^- → NH₃ + H₂O

What colour does red litmus paper turn in the presence of a base?

Blue.

Talk me through how to test for ammonium ions using sodium hydroxide.

1) Add ~10 drops of the sample to a clean test tube.

2) Add ~10 drops of sodium hydroxide to the same test tube.

3) Gently swirl to mix the test tube contents.

4) Leave the test tube in a warm water bath.

What reagent do you use to test for hydroxide ions?

Magnesium chloride (MgCl₂).

What is the positive result for the test for hydroxide ions with magnesium chloride? What species is responsible for this observation?

Formation of white precipitate, this is Mg(OH)₂ (magnesium hydroxide).

What reagent do you use to test for carbonate ions?

Hydrochloric acid (or any acid).

What is the positive result for the test for carbonate ions with hydrochloric acid? What is the species that causes this result?

Effervescence (that when bubbled into limewater makes the lime water go milky), caused by carbon dioxide gas.

What is lime water?

Calcium hydroxide solution.

Give and explain any safety precautions you must take when testing for halide ions with ammonia.

Carry out the reaction in a fume cupboard as ammonia is a respiratory irritant.