Unit 1 Topic 7 Tests For Ions

Question 1

What is a precipitate?

Answer 1

A solid that can be formed on mixing two solutions

Question 2

Two ways in which cations can be tested for

Answer 2

Flame tests and using sodium hydroxide solution

Question 3

Describe how a flame test is carried out. [5]

Answer 3

1. Dip a flame test rod in deionised water (or nichrome wire in concentrated hydrochloric acid)
2. Place the rod in a non-luminous Bunsen flame to clean it
3. Put the flame test rod into the sample
4. Heat the sample in a non-luminous Bunsen flame
5. Observe the colour of the flame

Question 4

Why is it sometimes difficult to see the lilac colour of a potassium ion?
What can be done to see the lilac colour?

Answer 4

Many potassium compounds are contaminated with sodium compounds and the sodium ion gives a strong yellow colour.

Hold the sample at the edge of the flame to see the lilac colour.

Question 4

Why are metal chlorides used for flame tests?

Answer 4

The chloride ion does not affect the flame test colour in any way

Question 5

Colour of flame if Na+ is present

Answer 5

Golden yellow

Question 6

Colour of flame if K+ is present

Answer 6

Lilac

Question 7

Colour of flame if Ca2+ is present

Answer 7

Brick red

Question 8

Colour of flame if Ba2+ is present

Answer 8

Apple green

Question 9

Colour of flame if Cu2+ is present

Answer 9

Blue-green

Question 10

Describe how you could use sodium hydroxide solution to identify a cation present in a solution [4]

Answer 10

1. Add a few drops of sodium hydroxide solution to the test solution containing the cation
2. Note any observations
3. Continue to add the sodium hydroxide solution until it is in excess
4. The colour of the precipitate and whether or not it redissolves in excess sodium hydroxide solution identifies the cation present.

Question 11

Colour of precipitate when Mg2+ is present using sodium hydroxide solution

Answer 11

White

Question 12

Solubility of precipitate in excess sodium hydroxide solution when Mg2+ is present

Answer 12

Insoluble/does not redissolve

Question 13

Colour of precipitate when Cu2+ is present using sodium hydroxide solution

Answer 13

Blue

Question 14

Solubility of precipitate in excess sodium hydroxide solution when Cu2+ is present

Answer 14

Insoluble/does not redissolve

Question 15

Colour of precipitate when Al3+ is present using sodium hydroxide solution

Answer 15

White

Question 16

Solubility of precipitate in excess sodium hydroxide solution when Al3+ is present

Answer 16

Soluble/redissolves forming a colourless solution

Question 17

Colour of precipitate when Fe2+ is present using sodium hydroxide solution

Answer 17

Pale green

Question 18

Solubility of precipitate in excess sodium hydroxide solution when Fe2+ is present

Answer 18

Insoluble/does not redissolve

Question 19

Colour of precipitate when Fe3+ is present using sodium hydroxide solution

Answer 19

Red-brown

Question 20

Solubility of precipitate in excess sodium hydroxide solution when Fe3+ is present

Answer 20

Insoluble/does not redissolve

Question 21

Colour of precipitate when Zn2+ is present using sodium hydroxide solution

Answer 21

White

Question 22

Solubility of precipitate in excess sodium hydroxide solution when Zn2+ is present

Answer 22

Soluble/redissolves forming a colourless solution

Question 23

If more than a few drops of sodium hydroxide solution are added initially in the Al3+ and Zn2+ tests, what may happen?

Answer 23

No precipitate may be observed because it may have already dissolved.

Question 24

Explain why the Al3+ and Zn2+ tests with NaOH solution are inconclusive.
What test must be done instead?

Answer 24

Both form a white precipitate that is soluble in excess NaOH solution, forming a colourless solution.
Aqueous ammonia must be used to distinguish between them.