Practicals and Testing for Substances

Question 1

Testing for Group 2 hydroxides

Answer 1

Add a hydroxide such as NaOH which forms M(OH)2, observe what happens - higher up group 2 a white precipitate forms.
Add NaOH in excess to see if the precipitate dissolves.
Mg2+, Ca2+, Sr2+ leave a white precipitate
Ba2+ leaves a colourless solution
White (Mg2+), slight white (Ca2+), slight white (Sr2+), colourless (Ba2+)
Considerations - excess NaOH added because Ca2+ and Sr2+ are slightly soluble so i=we can distinguish between them and Ba2+

Question 2

Testing for group 2 sulfate

Answer 2

Add sulfuric acid to form a metal sulfate and HCl and observe white precipitates.
Add H2SO4 in excess and see what dissolves.
Mg2+ will leave a colourless solution in excess
Colourless, slight white, white, white.
Considerations - excess H2SO4 added to distinguish between Mg2+ and Ca2+

Question 3

Testing for ammonium ions

Answer 3

Add ammonium to test tube, add equal vol of dilute NaOH and stir and warm in a water bath.
Hold damp red litmus paper in test tube and it will turn blue because ammonium and hydroxide produce ammonia gas

Question 4

Testing for hydroxide ions

Answer 4

Place sample in test tube, dip red litmus paper/ universal indicator paper into liquid.
Litmus paper turns blue
Universal indicator turns dark blue

Question 5

Testing for carbonate ions

Answer 5

Place Na2CO3 in test tube and connect delivery tube to test tube containing Ca(OH)2 solution (limewater)
Add dilute HCl to the Na2CO3 test tube.
Na2CO3 bubbles (effervescence) and Ca(OH)2 turns from colourless to cloudy

Question 6

Testing for sulfate ions

Answer 6

Place MgSO4 to test tube and add equal vols of dilute HCl and BaCl2.
White precipitates forms

Question 7

Testing for halide ions

Answer 7

Place KCl, KBr, KI in test tubes.
Add dilute nitric acid and silver nitrate
add dilute ammonia and mix.
Then add conc ammonia and mix.
Acidified silver nitrate forms precipitates white, cream and yellow.
Dilute ammonia. - colourless, cream, yellow.
Conc ammonia - colourless, colourless, yellow

Question 8

Flame test result for Na+

Answer 8

Yellow-orange

Question 9

Flame test result for K+

Answer 9

Lilac

Question 10

Flame test result for Ca2+

Answer 10

Orange-red

Question 11

Flame test result for Cu2+

Answer 11

Green-blue

Question 12

Flame test result for Li+

Answer 12

Crimson red

Question 13

Flame test result for Ba2+

Answer 13

Apple green

Question 14

Flame test result for Sr2+

Answer 14

Red

Question 15

Which combination of reagents can be used to oxidise alcohols?

Answer 15

Potassium dichromate (K2Cr2O7) and sulfuric acid H2SO4

Question 16

Primary alcohols are partially oxidised to

Answer 16

Aldehydes

Question 17

Secondary alcohols are partially oxidised to

Answer 17

Ketones

Question 18

Tertiary alcohols are partially oxidised to

Answer 18

Nothing, they can’t be oxidised

Question 19

Aldehydes are oxidised to

Answer 19

Carboxylic acid

Question 20

Ketones are oxidised to

Answer 20

Nothing, they can’t be oxidised

Question 21

Hexanol - hexanal

Answer 21

Distillation

Question 22

Propanol - propanoic acid

Answer 22

Reflux

Question 23

Pentan-2-ol - 2pentanone

Answer 23

Reflux

Question 24

Adding potassium dichromate to alcohol solution produces what colour change in 1º and 2º alcohol

Answer 24

Orange - green

Question 25

Test to see if the product of oxidation is an aldehyde or ketone

Answer 25

Add Tollens reagent, if silver mirror forms we started a 1º alcohol and are left with an aldehyde. No change means we started with a 2º alcohol and are left with a ketone

Question 26

Half equation for oxidation of alcohols (Cr2O7 2-)

Answer 26

Cr2O7 2- -> Cr3+ Balance atoms using H+ and H2O Balance charges using electrons

Question 27

How to test if a sample has a 1º 2º or 3º alcohol

Answer 27

Add acidified potassium dichromate and heat. If there is a colour change from orange to green, there is either a 1º or 2º alcohol. No change means 3º. Colour change means that oxidation has taken place.

Question 28

How to test for sulfate anions

Answer 28

Acidify solution with dilute HCl and add BaCl2, white precipitate will form because of the BaSO4

Question 29

How to make up a standard solution

Answer 29

Weigh solid Dissolve solid in beaker using distilled water and glass rod Place dissolved substance into volumetric flask Rinse beaker by swirling using distilled water and pour into volumetric flask Add distilled water up to 250cm line

Question 30

Mass spectrometry

Answer 30

Ionisation - gaseous molecule injected into mass spectrometer and an electron gun knocks off an electron leaving +1 ions Acceleration - ions accelerated towards negatively charged detection plate Deflection - ions deflected by magnetic field into curved path which is dependent on their mass and charge Detection - positive ions hit detection plate and gain an electron producing a flow of charge. Current depends on abundance