Chemical Analysis

Question 1

What is a cation?

Answer 1

A positively charged ion

Question 2

What test is used to identify metal ions?

Answer 2

Flame tests

Question 3

How do you carry out a flame test?

Answer 3

• clean a platinum wire loop by dipping it into dilute HCl and holding it into a flame
• if it burns without any colour, the you can dip it into the sample you want to test and put it back in the blue part of the Bunsen flame

Question 4

What colour is formed when lithium is flame tested?

Answer 4

Red

Question 5

What colour is formed when sodium is flame tested?

Answer 5

Yellow

Question 6

What colour is formed when potassium is flame tested?

Answer 6

Lilac

Question 7

What colour is formed when calcium is flame tested?

Answer 7

Orange-red

Question 8

What colour is formed when copper is flame tested?

Answer 8

Blue-green

Question 9

Describe tests for Cu2+, Fe2+ and Fe3+ using sodium hydroxide solution

Answer 9

Many metal hydroxides are insoluble and leave a precipitate, some of these have a distinctive colour. To test for this, add sodium hydroxide solution to the compound.

Cu2+ leaves a blue precipitate
Fe2+ leaves a sludgy green precipitate
Fe3+ leaves a red-brown precipitate

Question 10

Describe the test for NH4+ using sodium hydroxide

Answer 10

Add sodium hydroxide to solution and if damp red litmus paper turns blue (gas given off), then ammonia is present and the solution contained ammonium. (Litmus paper must be damp so that gas can dissolve and make the colour change)

Question 11

What is an anion?

Answer 11

A negative ion

Question 12

Describe the test for carbonates

Answer 12

Add dilute HCl to sample, if carbonates are present then CO2 will be released (test with lime water)

Carbonate + acid -> carbon dioxide + water
CO3 + 2H ->CO2 + H20

Question 13

Describe the test for Cl-, Br- and I-

Answer 13

To test for chlorine, bromine and iodine ions add dilute nitric acid followed by silver nitrate solution (acidified silver nitrate solution). Acid is added to get rid of carbonate or sulfate ions as these would give a precipitate and affect results.
Chloride will give a white precipitate of silver chloride
Bromide will give a cream precipitate of silver bromide
Iodide will give a yellow precipitate of silver iodide

Question 14

Describe the test for sulfate ions

Answer 14

Add dilute HCl and barium chloride solution to sample (acidified barium chloride). A white precipitate means sulfate ions are present

Question 15

What is the test for chlorine?

Answer 15

Bleaches damp blue litmus paper white

Question 16

What is the test for oxygen?

Answer 16

Relights a glowing splint/spill

Question 17

What is the test for carbon dioxide?

Answer 17

Turns limewater cloudy

Question 18

What is the test for hydrogen?

Answer 18

Makes a squeaky pop sound with a lighted splint (H burns with O to form H2O)

Question 19

What is the test for ammonia?

Answer 19

Ammonia turn damp red litmus paper blue and smells

Question 20

What is the test for water using anhydrous copper sulfate?

Answer 20

Add sample to anhydrous copper sulfate and if white powder turns blue to form hydrated copper sulfate then water is present. This tells you if water is present but not if it’s pure.

Question 21

What is the physical test for pure water?

Answer 21

Freeze and boil sample while recording the freezing and boiling points. If these aren’t 0 and 100°C, then the sample is not pure water. This is because a pure sample of water has defined physical properties including its fpt and bpt