Qualitative Analysis

Question 1

Describe the method for a flame test

Answer 1

• clean a metal loop with concentrated acid
• check the loop is clean, pass through a blue flame, until no colour is present.
• repeat step 1 if necessary until the loop is clean
• place the clean loop in the sample until the loop is covered
• record the colour observed when the loop is held in the blue flame

Question 2

Why must the test for any ion be unique?

Answer 2

Different ions react differently with different substances. For example when anions are placed in flames they go certain colours (each a unique one)

Question 3

What colour flame does copper go?

Answer 3

Green-blue

Question 4

What colour flame does sodium go?

Answer 4

Yellow

Question 5

What colour flame does potassium go?

Answer 5

Lilac

Question 6

What colour flame does calcium make?

Answer 6

Orange-red

Question 7

What do,our flame does lithium give?

Answer 7

Red

Question 8

Why can the use of machines be more effective then humans for testing samples?

Answer 8

• Machines are faster. Multiple repeats means more reliable.
• machines are more sensitive (for testing things like toxicity)
• machines are more accurate (offer quantitative data)

Question 9

Why would people be better then machines for testing samples?

Answer 9

Machines can be expensive, and require further expenses for training, calibration and maintenance.

Question 10

How do you carry out a precipitate test?

Answer 10

A few drops at a time of sodium hydroxide solution are added to the test solution. Different metal ions produce different coloured hydroxide precipitates.
Sodium hydroxide is then added in excess to observe any further change.

Question 11

How do you test for the aluminium ion?

Answer 11

Add a few drops of sodium hydroxide solution to an aluminium solution (e.g aluminium nitrate) and a white precipitate will form. When you add a few more the precipitate will dissolve

Question 12

How do you test for a calcium ion?

Answer 12

Add a few drops of sodium hydroxide solution to a sample. If calcium is present a white precipitate will form. When you add more sodium hydroxide no change will occur.

Question 13

How do you test for copper (II) ions?

Answer 13

Add a few drops of sodium hydroxide to a sample. If copper (II) is present a blue precipitate will form, there will be no change when more is added.

Question 14

How do you test for iron (II) ions?

Answer 14

Add a few drops of sodium hydroxide to a sample, if iron (II) is present a dirty green precipitate will form. There is no change when ,ore sodium hydroxide is added.

Question 15

How do you test for iron (III) ions?

Answer 15

Add a few drops of sodium hydroxide solution to a sample. If iron (iii) is present a red/brown precipitate will form. No change when added in excess.

Question 16

Describe the chemical test for ammonium ions?

Answer 16

You use sodium hydroxide solution to detect them. Ammonia gas NH3 is produced when the mixture containing ammonium ions is warmed, ammonia has a characteristic sharp smell. But you also use a confirmatory test, ammonia changes damp red litmus paper blue.

Question 17

Describe the test to identify carbonate ions?

Answer 17

Add acid to ionic substance, if carbonate ions are present carbon dioxide gas will turn limewater from clear and colourless to milky/cloudy.

Question 18

Describe the test to identify sulphate ions?

Answer 18

Add HCl to ionic substance to remove contaminants (e.g carbonates). Add barium chloride solution, if sulphate ions are present a white precipitate will form.

Question 19

Describe the test to identify halide ions? (Chloride in particular)

Answer 19

Add nitric acid to ionic substance to remove contaminants (e.g carbonates). Add silver nitrate solution, if chloride ions are present a grey-white precipitate will form.