REQUIRED PRACTICAL 4 - ID tests

Question 1

Calcium Ca2+ flame colour

Answer 1

brick red

Question 2

Strontium Sr2+ flame colour

Answer 2

Red

Question 3

Barium Ba2+ flame colour

Answer 3

Pale green

Question 4

Observation of halides with Acidified silver nitrate

Answer 4

• Flouride = no ppt too soluble
• Chloride = white ppt
• Bromide = cream ppt
• Iodide = yellow ppt

Question 5

How to test for halides

Answer 5

1. Dissolve spatula in measure of sample of dilute HNO3 and add few cm3 of dilute or conc ammonia

Question 6

Observations of halides with DILUTE ammonia

Answer 6

• Chloride = dissolves to form colourless solution
• Bromide = ppt does not dissolve
• Iodide = ppt does not dissolve

Question 7

Observations of halides with CONC ammonia

Answer 7

• Chloride dissolves to form colourless solution
• Bromide = dissolves to form colourless solution
• Iodide = ppt does not dissolve

Question 8

Ionic equations for halides reactions with acidified silver nitrate

Answer 8

• Ag+(aq) + Cl-(aq) —> AgCl(s)
• Ag+(aq) + Br-(aq) —> AgBr(s)
• Ag+(aq) + I-(aq) —> AgI(s)

Question 9

Why does the silver nitrate solution have to be acidified?

Answer 9

To remove other ions which would interfere with the test like OH- or sulphite ions

Question 10

Why is HCl acid not used to acidify the AgNO3 reaction

Answer 10

Cl- would react with silver ion Ag+ to form white ppt and form flase positive result

Question 11

How to get pure dry sample of salt

Answer 11

1. Filter to remove the solid from soluble products
2. Wash to remove any insoluble purities
3. Dry to remove water

Question 12

Purity test

Answer 12

• Melting point test
• Pure substances melt at fixed melting point
• Impure substances melt over range of temperatures

Question 13

Trend down group 2 solubility with NaOH

Answer 13

Increases down group 2 (white ppt-> slight white ppt -> colourless solution)

Mg(OH)2 is insoluble

Question 14

Barium chloride with any amount of NaOH

Answer 14

remains colourless

Question 15

Magnesium chloride with NaOH

Answer 15

• Colourless to slightly white ppt when limited NaOH used
• Colourless to white ppt when excess used

Question 16

Calcium bromide with any amount of NaOH

Answer 16

colourless –> slightly white ppt

Question 17

Strontium chloride with any amount of NaOH

Answer 17

• Colourless to slightly white ppt

Question 18

Trend down group 2 solubility with H2SO4

Answer 18

• solubility decreases
• BaSO4 is white ppt and solubility increases down the group

Question 19

Barium chloride with any amount of H2SO4

Answer 19

White ppt

Question 20

Strontium chloride with any amount of H2SO4

Answer 20

White ppt

Question 21

Calcium bromide with any amount of H2SO4

Answer 21

Slight white ppt

Question 22

Magnesium chloride with H2SO4

Answer 22

• With limited H2SO4 slightly white ppt
• With excess remains colourless solution

Question 23

Test for ammonia solution

Answer 23

• Place ammonium chloride + NaOH + warm releases NH3
• Forms ammonia gas which is a weak base and turns damp red litmus paper blue

Question 24

Test for strong base (NaOH) and weak base (NH3 solution)

Answer 24

• For both damp red litmus paper goes blue
• Using a pH meter you will get a different reading NaOH = pH 13/14 and NH3 pH = 8-10

Question 25

Test for carbonate ions CO3(2-) or HCO3-

Answer 25

* Add sample of hydrogen carbonate salt to acid and ensure the gas is bubbled into limewater * Effervescence and solid disappears * Turns cloudy if CO2 given off

Question 26

Test for sulfates

Answer 26

* Dissolve sulfate salt in dilute HNO3 and add few cm3 of barium chloride * White ppt forms = BaSO4

Question 27

KCL, KBr and KI with conc sulfuric acid

Answer 27

* KCl effervescence * KBr effervescence and brown gas produced, solution turns deep brown * KI solution goes deep brown immediately and brown gas produced

Question 28

Paper test for KCl

Answer 28

Damp litmus paper turns red

Question 29

Paper test for KBr

Answer 29

Damp K2Cr2O7 paper orange changes to green in presence of SO2 gas

Question 30

Paper test for KI

Answer 30

Damp lead nitrate paper turns black in presence of H2S