Testing for Ions

Question 1

What do chemists use to identify parts, usually ions, in substances or to rule them out?

Answer 1

Simple tests, called qualitative tests

Question 2

What do qualitative tests tell you?

Answer 2

Which ions are present, but not how much

Question 3

What do many qualitative tests involve in terms of reactions?

Answer 3

Precipitation reaction

Question 4

What is a precipitate reaction?

Answer 4

Insoluble solid forms, usually from 2 aqueous solutions

Question 5

How are qualitative tests usually carried out?

Answer 5

On a small scale, in a test tube with only 1–2 cm³ of unknown substance, especially important if not much of substance is available

Question 6

What do you want to be careful of in identification tests?

Answer 6

Not wasting too much of substance available

Question 7

What can anions be used to test for?

Answer 7

• Carbonate ions
• Sulfate ions
• Halide ions

Question 8

What do carbonate ions react with?

Answer 8

Acids

Question 9

What is the method to identifying a solution with carbonate ions?

Answer 9

1. Add a dilute strong acid to suspected carbonate
2. Collect any gas formed and pass it through limewater

Question 10

What would the positive test observations be if carbonate ions are present in a solution?

Answer 10

• Fizzing/colourless gas is produced
• Gas turns limewater cloudy

Question 11

What do sulphate ions react with?

Answer 11

Barium ions to form an insoluble salt, barium sulfate

Question 12

What is the method to identifying a solution with sulphate ions?

Answer 12

Add dilute hydrochloric acid and barium chloride to suspected sulfate

Question 13

What would the positive test observations be if sulphate ions are present in a solution?

Answer 13

White precipitate of barium sulfate is produced

Question 14

Which order is it important to carry out tests for unknown substances?

Answer 14

1. Carbonate test
2. Sulfate test
3. Halide test

Question 15

What would happen in an identification reaction if you hadn’t already ruled out carbonate ions?

Answer 15

You wouldn’t know if precipitate was carbonate or barium sulfate

Question 16

What do silver ions form?

Answer 16

Insoluble precipitate

Question 17

What would happen in an identification reaction if you hadn’t already ruled out sulphate ions?

Answer 17

You wouldn’t know if precipitate was silver sulfate or a silver halide

Question 18

What do halide ions react with silver ions to form?

Answer 18

Different-coloured silver halide precipitates

Question 19

What is the method to identifying a solution with halide ions?

Answer 19

• Dissolve suspected halide in water
• Add an aqueous solution of silver nitrate
• Note colour of any precipitate formed
• If colour is hard to distinguish, add aqueous ammonia (first dilute, then concentrated)
• Note solubility of precipitate in aqueous ammonia

Question 20

What would the positive test observations be if halide ions are present in a solution?

Answer 20

• Silver chloride, white precipitate, soluble in dilute aqueous ammonia
• Silver bromide, cream precipitate, soluble in concentrated aqueous ammonia only
• Silver iodide, yellow precipitate, insoluble in dilute and concentrated aqueous ammonia

Question 21

What are nitrate ions known as in ionic equations?

Answer 21

Spectator ions and don’t need to be included in ionic equation as they don’t react

Question 22

What do ammonium ions react with to produce ammonia and water?

Answer 22

Hydroxide ions

Question 23

What is the method to identifying a solution with ammonium ions?

Answer 23

• Add sodium hydroxide solution to suspected ammonium compound and warm very gently
• Test any gas evolved with red litmus paper

Question 24

What would the positive test observations be if ammonium ions are present in a solution?

Answer 24

• Ammonia gas will turn red litmus paper blue
• Ammonia gas has a distinctive smell

Question 25

Why do you need to be careful with ammonia gas?

Answer 25

It's hazardous and should be tested with care