Chemical Analysis (tests)

Question 1

How do you test for hydrogen?

Answer 1

• test tube with unknown gas
• remove bung and insert burning splint
• hydrogen gas burns rapidly and produces a pop sound

Question 2

How do you test for oxygen?

Answer 2

• test tube with unknown gas
• remove bung
• place glowing split into the test tube
• if it’s oxygen, it should relight

Question 3

How do you test for carbon dioxide?

Answer 3

• test tube of unknown gas and the test tube of limewater
• drew some of the gas into a plastic pipette
• bubble the gas through limewater
• repeat this several times, the limewater will turn cloudy.

Question 4

What is limewater?

Answer 4

Aqueous solution of calcium hydroxide.

Question 5

How do you test for chlorine?

Answer 5

• insert damp litmus paper into the mouth of the test tube with the gas inside
• chlorine bleaches the litmus paper and turns it white

Question 6

Describe how a flame test works:

Answer 6

• place a small amount of the chemical onto wire mounted in a handle
• place the end of this onto a blue bunsen burner flame
• colour of flame identifies the metal ion present

Question 7

Flame test for lithium?

Answer 7

Crimson flame

Question 8

Flame test for sodium?

Answer 8

Yellow flame

Question 9

Flame test for potassium?

Answer 9

Lilac flame

Question 10

Flame test for calcium?

Answer 10

Orange-red flame.

Question 11

Flame test for copper?

Answer 11

Green flame

Question 12

What are the disadvantages of flame tests?

Answer 12

• colour of the flame can be difficult to distinguish
• especially is there is a low concentration of the compound
• sample could contain a mixture of metal ions
• masks the colour of the flame

Question 13

Describe how flame emission electroscope works:

Answer 13

• a sample of the metal ion in solution is placed into a flame
• the light given out is then passed into a spectroscope
• the spectroscope converts the light into a line spectrum
• the positions of the lines in the spectrum are specific for a given metal ion
• lines become more intense at higher concentrations

Question 14

What are the advantages of instrumental methods?

Answer 14

• more rapid than flame tests
• sensitive so they will work even on a tiny sample of the metal compound
• accurate so they are more likely to identify the metal ion correctly than flame tests

Question 15

What happens if we add sodium hydroxide solution to a solution of calcium/magnesium/aluminium ions?

Answer 15

White precipitate is formed.

Question 16

What happens if you add excess sodium hydroxide solution to these solutions?

Answer 16

The aluminium precipitate will redissolve.

Question 17

How do you work out which one of the precipitates are calcium?

Answer 17

Flame test

Question 18

What happens if you add sodium hydroxide solution to a solution of copper (II) ions?

Answer 18

A blue precipitate of copper (II) hydroxide forms.

Question 19

What happens if you add sodium hydroxide solution to a solution of iron (II) ions?

Answer 19

A green precipitate of iron (II) hydroxide is formed.

Question 20

What happens if you add sodium hydroxide solution to a solution of iron (III) ions?

Answer 20

A brown precipitate of iron (III) hydroxide is formed.

Question 21

How do you test for carbonates?

Answer 21

1. Add dilute acid to the sample.
2. The acid will react with the carbonates to make carbon dioxide gas. Fizzing. This doesn’t prove the presence of carbon dioxide.
3. Bubble the gas through limewater. If it goes cloudy, then this proves that we have carbon dioxide.
4. That means we started with the carbonate ion.

Question 22

How do you test for halide ions?

Answer 22

1. Add dilute nitric acid to the sample.
2. Add silver nitrate solution.
3. Halide ions produce a precipitate of the silver halide. Each halide makes a different coloured precipitate.

Question 23

What do the chloride ions produce?

Answer 23

A white precipitate of silver chloride.

Question 24

What do the bromide ions produce?

Answer 24

A cream precipitate of silver bromide.

Question 25

What do the iodide ions produce?

Answer 25

A yellow precipitate of silver iodide.

Question 26

How do you test for sulphate ions?

Answer 26

1. Add dilute hydrochloric acid to our sample.
2. Add barium chloride solution.
3. If sulphate ions are present, there will be a white precipitate.