Qualitative Analysis

Question 1

Describe a flame test.

Answer 1

Dip a piece of nichrome wire into hydrochloric acid to remove previous tested samples, then dip it in the non-luminous flame of the Bunsen Burner.

Question 2

What are the ions and their colors tested in a flame test?

Answer 2

Lithium, red
Sodium, yellow
Potassium, lilac
Calcium, orange-red
Copper, blue-green

Question 3

How do you identify a positive ion (cation)?

Answer 3

Add sodium hydroxide solution.

Question 4

Test for ammonium

Answer 4

Add NaOH solution, and the gas produced will turn red litmus paper blue

Question 5

Test for Cu2+

Answer 5

Add NaOH solution, a blue precipitate Cu(OH)2 will form

Question 6

Test for Fe2+

Answer 6

Add NaOH solution, a green precipitate Fe(OH)2 will form

Question 7

Test for Fe3+

Answer 7

Add NaOH solution, a orange-brown precipitate Fe(OH)3 will form

Question 8

Why might a Fe2+ solution already tested with NaOH, have its green precipitate turn brown after a while?

Answer 8

Fe2+ salts will be oxidised by the oxygen in air to form Fe3+, so the green precipitate will turn brown.

Question 9

Test for Cl-

Answer 9

Add nitric acid then silver nitrate solution. A white precipitate (AgCl) will form.

Question 10

Test for Br-

Answer 10

Add nitric acid then silver nitrate solution. An off-white precipitate (AgBr) will form.

Question 11

Test for I-

Answer 11

Add nitric acid then silver nitrate solution. A yellow precipitate (AgI) will form.

Question 12

Test for CO3 2-

Answer 12

Add hydrochloric acid then test gas produced with limewater. Fizzing and limewater turns cloudy

Question 13

Test for SO4 2-

Answer 13

Add hydrochloric acid then barium chloride solution. White precipitate (BaSO4) will be formed.

Question 14

Why can’t you use HCl to test for halogens?

Answer 14

The chloride ion in HCl will react with silver nitrate and produce a false positive result.

Question 15

Test for hydrogen

Answer 15

Place a lit splint to the end of the test tube, squeaky pop produced

Question 16

Test for oxygen

Answer 16

Place a glowing splint to the end of the test tube, relights it

Question 17

Test for chlorine

Answer 17

Place litmus paper near the gas, bleaches the litmus (blue, red, white)

Question 18

Test for ammonia gas

Answer 18

Place red litmus paper near the end of the test tube, turns blue

Question 19

Test for carbon dioxide

Answer 19

Bubble through limewater, white precipitate formed

Question 20

Test for water

Answer 20

Add to anhydrous copper sulfate (white), turns it blue

Question 21

Test for pure water

Answer 21

Test its boiling point, if boils at 100C then only water is present