U3S4 tests for functional groups

Question 1

Testing for primary, secondary and tertiary alcohols

Answer 1

1. Add 10 drops of alcohol to 2cm3 of acidified potassium dichromate solution in a test tube
2. Warm the mixture gently in a hot water bath
3. Watch for a colour change, orange to green in primary and secondary

Question 2

Test for primary alcoholds

Answer 2

• oxidise under reflux and tests positive for carboxylic acid
• oxidise under distillation and tests positive for aldehyde

Question 3

Test for secondary alcohols

Answer 3

• oxidise under reflux and tests positive positive for ketone

Question 4

Fehling’s solution

Answer 4

1. Add 2cm3 to test tube
2. Add 5 drops of aldehyde or ketone
3. Put in water bath for a few minutes
   Aldehyde = brick red, ketone = stays blue

Question 5

Tollen’s reagent

Answer 5

1. Put 2cm3 of 0.1 mole dm-3 silver nitrate solution in a test tube
2. Add afew drops of dilute sodium hydroxide solution, light brown precipitate should form
3. Add drops of dilute ammonia solution until brown precipitate dissolves completely
   Aldehyde = silver ions reduced to silver metal, ketone = no change

Question 6

Carboxylic acid test

Answer 6

1. Add 2cm3 of solution being tested to a test tube
2. Add 1 small spatula of solid sodium carbonate
3. If solutions begin to fizz, bubble the gas it produces through limewater
4. Will turn cloudy if acid present

Question 7

Test for alkenes

Answer 7

1. Add 2cm3 of solution to test tube
2. Add 2cm3 bromine water to test tube
3. Shake test tube
4. If present, bromine water turns from orange to colourless