Chemistry - Tests for Cations, Gases and Anions

How do you test for cations?

Flame tests

What colour flame do you get from lithium (Li+)?

Red

What colour flame do you get from sodium (Na+)?

yellow

What colour flame do you get from potassium (K+)?

lilac

What colour flame do you get from copper (Cu2+)?

blue green

What colour flame do you get from calcium (Ca2+)?

orange red

Describe the flame test.

1. Clean a nichrome or platinum wire to get rid of impurities, do this by dipping it in concentrated hydrochloric acid and heating it in a Bunsen burner flame. 

2. Apply the sample:

   Dip the clean wire into the unknown salt and then heat it in the blue flame of a Bunsen burner. 

How do you test for chlorine?

1. Take your sample and a dampened piece of blue litmus paper

2. Put this litmus paper into the test tube

If chlorine is present, paper will turn from blue to red to white as it bleaches paper.

How do you test for oxygen?

1. Take a glowing splint and place it into the sample of the gas.

If gas is oxygen, the glowing splint will relight

How do you test for hydrogen?

1. Take a burning splint and the sample.

2. Move them closer

There will be a squeaky pop if hydrogen is present.

How do you test for carbon dioxide?

1. Take your gas sample and an aqueous solution of limewater

2. Bubble gas sample through limewater

If carbon dioxide is present, the limewater will go cloudy.

How do you test for carbonates?

1. Add dilute hydrochloric acid to test sample

2. Take gas produced and run it though limewater.

This is because Acid + carbonate → salt + carbon dioxide + water

So it will turn limewater cloudy.

How do you test for Sulfates?

1. Add dilute hydrochloric acid (to remove any carbonates or sulphite ions)

2. Add barium chloride solution to test sample

This forms barium Sulfate, which is white precipitate.

How do you test for chloride ions?

1. Add dilute nitric acid to sample (removes carbonate and sulphite impurities)

2. Add silver nitrate

If the sample are chloride ions, it will form a white precipitate.

How do you test for bromide ions?

1. Add dilute nitric acid to sample (removes carbonate and sulphite impurities)

2. Add silver nitrate

If the sample are bromide ions, it will form a cream precipitate.

How do you test for iodine ions?

1. Add dilute nitric acid to sample (removes carbonate and sulphite impurities)

2. Add silver nitrate

If the sample are chloride ions, it will form a yellow precipitate.

How do you test for ammonium?

1. Add dilute sodium hydroxide to sample.

2. Heat it

3. Hold Damp red litmus paper over test tube.

If the solution contains ammonium ions, ammonia gas will be produced, turning the damp red litmus paper blue.