Tests for substances & ions

Question 1

Outline how you would test for ammonium (NH4+) ions

Answer 1

Ammonium cations can be tested for by mixing the unknown compound with warm, dilute NaOH (aq).
If ammonium ions are present, ammonia gas will be given off, turning damp red litmus paper blue.

NH4+ + OH– → NH3 + H2O
(NH4Br + NaOH → NaBr + NH3 + H2O)

Question 2

Outline how you would test for an alkene

Answer 2

Add bromine water
Turns brown -> colourless

Question 3

Test for tertiary alcohol

Answer 3

Add acidified K2Cr2O7
Sample remains orange

Question 4

Test for secondary alcohol

Answer 4

Add acidified K2Cr2O7
-Sample turns orange -> green
Add Tollens’ reagent
- No change occurs

Question 5

Test for primary alcohol

Answer 5

Add acidified K2Cr2O7
-Sample turns orange -> green
Add Tollens’ reagent
- Silver mirror forms

Question 6

Test for carboxylic acids

Answer 6

Add carbonate (e.g. Na2CO3)
Bubbles form

Question 7

Test for carbonyl compounds e.g. aldehydes/ketones

Answer 7

Add 2,4-DNPH (Brady’s reagent)
- Deep orange precipitate forms
- Precipitate purified by recrystallisation
- The melting point of the formed precipitate can then be measured and compared to literature values to find out which specific aldehyde or ketone had reacted with 2,4-DNPH

The 2,4-DNPH test is especially useful as other carbonyl compounds such as carboxylic acids and esters do not give a positive result

Question 8

Test for whether substance is aldehyde or ketone

Answer 8

• Tollens’ reagent
• If silver mirror forms, aldehyde.
• Reduction equation = Ag+(aq) + e- -> Ag(s)

Question 9

Test for carbonate ions

Answer 9

• Add HCl (aq) to test tube
• Add carbonate (e.g. sodium carbonate)
• A bung with a delivery tube should be attached to the test tube.
  The delivery tube should transfer the gas which is formed into a different test tube which contains a small amount of limewater.
• Carbonate ions will react with hydrogen ions from the acid to produce carbon dioxide gas.
• Carbon dioxide gas will turn the limewater milky.

Question 10

Test for sulfate ions

Answer 10

• Acidify the sample with dilute hydrochloric acid and then add a few drops of aqueous barium chloride
• If a sulfate is present then a white precipitate of barium sulfate is formed:
  Ba2+ (aq) + SO42- (aq) → BaSO4 (s)

Question 11

Outline how to test for halide ions

Answer 11

• Add nitric acid
• Add silver nitrate solution
  A precipitate will form, either white, cream or yellow, if a halide ion is present in the sample

White ppt = chloride ions
Cream ppt = bromide ions
Yellow ppt = iodide ions

• Could then add ammonia solution
  Chloride = dissolves in dilute NH3 (aq)
  Bromide = dissolves in conc NH3 (aq)
  Iodide = doesn’t dissolve in NH3 (aq)

Question 12

Sequence of tests

Answer 12

1) Carbonate
2) Sulfate
3) Halide

Question 13

How to test for metals

Answer 13

Dilute NaOH solution reacts with some metal ions in solution, forming metal hydroxides. Some of these metal hydroxides are insoluble, so they appear as precipitates.

For example, copper sulfate solution reacts with a few drops of NaOH:

Copper sulfate + sodium hydroxide → sodium sulfate + copper hydroxide

CuSO4(aq) + 2NaOH(aq) → Na2SO4(aq) + Cu(OH)2(s)

Copper hydroxide forms a blue precipitate.

Question 14

How can you test for group 1 and 2 ions?

Answer 14

Group 1 and 2 ions can be tested for by using a flame test. Different metals will give differently colored flames.