Chemical Tests and Practical Skills

Question 1

Test for primary or secondary alcohol

Answer 1

Add acidified potassium dichromate

Colour change for orange to green

Question 2

Test for aldehydes (Tollen’s Reagent)

Answer 2

• Tollens’ reagent [Ag(NH3)2]+.
• add silver(I) nitrate solution
• then a drop of sodium hydroxide solution to give a brown precipitate of silver(I) oxide
• add just enough dilute ammonia solution to redissolve precipitate
• add suspected aldehyde solution
• warm gently in a hot water bath for a few minutes
• aldehyde present if silver mirror precipitate forms

Question 3

Test for aldehyde (Fehling’s Test)

Answer 3

• Add Fehling’s solution to a test tube
• Add aldehyde and add in hot water bath for a few minutes
• Colour change from blue to brick red

Question 4

Test for carboxylic acid

Answer 4

• Add sodium hydrogen carbonate
• Produces effervescence/bubbles
  OR
• Universal Indicator/ blue litmus paper
• Turns red

Question 5

Suggest how you could distinguish between a primary an secondary alcohol

Answer 5

Acidified potassium dichromate turns from orange to green
Apparatus set up for distillation 
Tollens test (if silver mirror forms then primary)

Question 6

Suggest how the conditions for producing an aldehyde differ from producing a carboxylic acid

Answer 6

Aldehyde 
- (dilute H2SO4) 
- dilute potassium dichromate
- distillation IMMEDIATELY 
Carboxylic acid 
- (concentrated H2SO4)
- EXCESS potassium dichromate 
- reflux

Question 7

Investigating rate of reactions (gas produced)

Answer 7

• Add reactants together
• Measure change in mass over regular time intervals
  Or
• Measure volume of gas produced using a gas syringe over regular time intervals

Question 8

Suggest why deionised water is used over tap water

Answer 8

Does not contain any ions which can react with reaction mixture and give false results

Question 9

Suggest why an electric mantle is used over a Bunsen burner

Answer 9

• even heating
• more control over temperature
• less chance of overheating
• safer

Question 10

Accuracy

Answer 10

How close values are to true value - improved through correctly calibrated equipment

Question 11

Precision

Answer 11

How close values are to each other - improved through using high resolution equipment

Question 12

Test for NH4+

Answer 12

• add sodium hydroxide solution and heat gently
• ammonia gas given off
• pungent smell/turns damp red litmus paper blue

Question 13

Test for CO32-

Answer 13

• add dilute hydrochloric acid
• carbon dioxide gas given off
• turns limewater (calcium hydroxide) cloudy

Question 14

Test for OH-

Answer 14

Turns red litmus paper blue

Question 15

Test for NO3-

Answer 15

• add to NaOH solution
• then add powdered aluminium
• damp red litmus paper to test for ammonia gas (turns blue)

Question 16

Test for SO42- ions

Answer 16

• add hydrochloric/nitric acid to remove carbonate ions (forms carbon dioxide)
• add barium chloride to form barium sulphate (white precipitate)

Question 17

Suggest why barium chloride used to test for sulphate ions is acidified

Answer 17

Acid reacts with carbonate impurities often found in salts which would form a white barium carbonate precipitate and give a false positive result

Question 18

Suggest why barium chloride is used to test for sulphate ions

Answer 18

• forms BaSO4 precipitate

- insoluble in water

Question 19

Suggest why aluminium is added to solution when testing for nitrate ions

Answer 19

• aluminium reduces nitrate NO3- to NH4+
• NH4+ + OH- -> NH3 + H2O
• NH3 detected by red litmus paper

Question 20

Describe how to prepare a pure sample of silver bromide from a colourless solution containing a mixture of sodium chloride and sodium bromide

Answer 20

• add silver nitrate to form AgCl and AgBr (give equations)
• add DILUTE ammonia in excess to dissolve AgCl
• filter AgBr
• wash and dry to removed soluble impurities

Question 21

Test for halogenoalkane

Answer 21

• add NaOH then ACIDIFIED AgNO3

- forms white, cream or yellow precipitate based on which halogen is present