Chemical tests

Question 1

test for carboxylic acid

Answer 1

add sodium carbonate. should be able to see bubbles (carbon dioxide)

Question 2

Test for aldehydes

Answer 2

Tollens reagent. Forms silver precipitate on the side of the test tube. doesn’t react with ketones. doesn’t distinguish between different aldehydes

Question 3

what’s a test for unsaturation

Answer 3

add the hydrocarbon to a solution of bromine water. the solution will turn from orange to colourless if the hydrocarbon is unsaturated

Question 4

describe the test for halides

Answer 4

add nitric acid to remove carbonate ions that give a false positive, add silver nitrate. silver chloride is white,
bromide is cream, iodide is yellow. to distinguish between these add dilute ammonia solution. silver chloride will dissolve. then add concentrated ammonia solution and silver bromide will dissolve. Silver iodide dissolves in neither

Question 5

describe the test for sulphate ions

Answer 5

add nitric acid to remove carbonate ions that give a false positive. add barium nitrate. barium sulphate is insoluble so a white precipitate will form

Question 6

describe the test for carbonate ions

Answer 6

add dilute nitric acid. use a delivery tube to bubble the gas through lime water. it will turn cloudy if carbon dioxide is present. this proves it contained carbonate ions

Question 7

describe the test for hydroxide ions

Answer 7

damp red litmus paper turns blue

Question 8

describe the test for ammonia

Answer 8

add dilute sodium hydroxide. heat the solution and place damp red litmus paper near the mouth of the boiling tube. it will turn blue if ammonia is present

Question 9

describe how to distinguish between group 2 metals

Answer 9

group 2 hydroxides solubility increases down the group. group 2 sulphate solubility decreases down the group. using this information, adding sulphuric acid or sodium hydroxide can distinguish between them

Question 10

test for 1° or 2° alcohols

Answer 10

heat with acidified potassium dichromate. colour changes from orange to green. doesn’t work for 3° as they cant be oxidised

Question 11

test for carbonyl group (C=O)

Answer 11

add DNP in methanol and concentrated H2SO4. a bright orange precipitate forms. only works for ketones and aldehydes

Question 12

test for haloalkanes

Answer 12

warm with ethanolic aqueous silver nitrate solution. chloro-alkane forms white precipitate. Bromo-alkane forms cream precipitate. Iodo-alkane forms yellow precipitate.