Topic 11 Chemical Tests + Topic 12 Reversible Reactions

Test for hydrogen

Test: collect gas in a test tube and test with a lit splint

Positive result: burns with a squeaky pop sound

Test for Oxygen

Test: collect gas in a test tube and test with a glowing splint

positive result: splint relights

Test for carbon dioxide

Test: bubble through limewater

Positive result: limewater becomes cloudy

Test for Chlorine

Test: test with damp blue litmus paper

Positive result: damp blue litmus paper turns red then bleaches

Test for Ammonia

Test: test with damp red litmus paper

Positive result: damp red litmus paper turns blue

Describe how to carry out a flame test

1. Clean nichrome wire in Bunsen

2. Dip wire in conc. HCl

3. Dip wire in salt containing positive ion

4. Put wire in edge of non-luminous Bunsen flame

5. observe colour of flame

Why do we use a non-luminous (blue) Bunsen flame?

Makes it easier to distinguish the colour of the result. / The bright orange luminous Bunsen flame makes it harder to distinguish results.

Colours of flame tests for cations(positive ions：lithium, sodium, potassium, calcium, copper)

Li : red/crimson

Na: yellow

K : lilac

Ca : orange-red

Cu : blue-green

Test for transition metals: Cu, Fe(II), Fe(III)

1. Add a few drops of NaOH sodium hydroxide to the solution

2. Observe colour of the precipitate

Cu → pale blue

Iron(II) →green

Iron(III) → brown

Test for ammonium ions

1. Add sodium hydroxide to the solution and heat it

2. test gas given off with damp red litmus paper

3. If ammonia (alkali) is given off, damp red litmus paper will turn blue, which means the solution contains ammonium ions

Test for halide ions (anions/negative ions： chlorine, bromine, iodine)

1. Add nitric acid and silver nitrate solution to the solution.

2. Observe colour of the precipitate

Chloride: white

Bromide: cream

Iodide : yellow

Test for sulfate

1. Add nitric acid and barium chloride

2. Observe colour of precipitate

3. if sulfate ion is present, it would be white

Why is it important that nitric acid is added to the solutions to be tested in the halide and sulfate tests?

It reacts with other negative ions present, which could cause false positives. Nitrate is always soluble.

Test for carbonate ions

1. Add a little HNO₃(nitric acid)

2. bubble through limewater

3. limewater will turn cloudy because carbon dioxide is given off

Physical test for water

Heat & Determine boiling point. Should be 100 degrees.

(also used to check whether water is pure)

Chemical test for water

• Anhydrous copper(II) sulfate turns from white to blue on the addition of water

• The equation is:

CuSO₄ (s) + 5H₂O (l) → CuSO₄.5H₂O (s)

Reversible reaction

products formed can react with each other to re-form the original reactants.