Observations and tests

Question 1

How can you test for carbonates (CO₃^2-) ?

Answer 1

Add Dilute acid (HCl) and carbon dioxide will form.

Limewater turns cloudy.

Question 2

How can you test for sulfates (SO₄^2-) ?

Answer 2

Add dilute HCl followed by barium chloride (BaCl₂).

White precipitate of barium sulfate would form.

Question 3

How can you test for halides and separate them?

Answer 3

Add nitric acid, then silver nitrate solution.

Precipitate of:

Silver chloride - white precipitate

Silver bromide - cream precipitate

Silver iodide - yellow precipitate

Question 4

How can you test for ammonium (NH₄⁺) compounds?

Answer 4

Add sodium hydroxide (NaOH) and gently heat the mixture.

Put litmus paper where the gas is given off when heated.

If litmus paper turns blue, ammonia (NH₃) is given off.

Question 5

How can you test for alkenes?

Answer 5

Bromine water turns colourless when added to alkene.

Question 6

How can you test for haloalkanes?

Answer 6

Use silver nitrate.

Chloroalkane - white precipitate

bromoalkane - pale cream precipitate

iodoalkane - pale yellow precipitate

Question 7

How can you test for phenols?

Answer 7

Phenol is solid at room temperature.

Add sodium hydroxide (NaOH) and the phenol will dissolve.

Add carbonate (NaCO₃) to the solution. Nothing should happen. This means the original substance is phenol.

Question 8

How can you test for carboxylic acids?

Answer 8

Add solid carbonate to the solution (NaCO₃).

The solution will fizz, carbon dioxide released will turn limewater cloudy.

Question 9

How can you test for Carbonyl groups?

Answer 9

Dissolve 2,4-DNP (2,4-dinitrophenylhydrazine) in sulfuric acid, water and methanol.

Drop the substance into the solution and shake well.

If bright orange precipitate form, carbonyl group is present.

Question 10

How can you test for aldehydes?

Answer 10

Put tollens’ reagent into the solution you suspect to be aldehyde.

When heated, the aldehyde is oxidised and the silver ions in the tollens’ reagent are reduced.

Silver mirror is formed.

Question 11

How can you test for primary and secondary alcohols?

Answer 11

Add acidified dichromate (K₂Cr₂O₇) and heat.

Primary - orange solution turns green as aldehyde forms.

Secondary - orange solution turns green as ketone forms.

Tertiary - nothing happens.