Tests

Question 1

Alkenes

Answer 1

• Bromine water

- Goes from orange, reddish brown to colourless

Question 2

Chlorine Gas

Answer 2

• Test tube with sample gas
• Blue litmus paper
• DAMPEN IT
• Put damp blue litmus paper in test tube
• If chlorine is present, it turns from BLUE to WHITE
• Could turn red first because when the Cl dissolves into the water it forms HCL which is acidic but then the paper is bleached so turns white
• Chlorine is poisonous so one wants to do it wearing a mask or in a fume cupboard

Question 3

Oxygen Gas

Answer 3

• Glowing splint
• Sample of gas in test tube
• Put splint in test tube
• If the splint relights, there is oxygen present
• Because burning requires oxygen

Question 4

Hydrogen Gas

Answer 4

• Test tube of hydrogen
• Lit splint
• Put lit splint in test tube, and a ‘pop’ will be heard
• Where the flame ignites the hydrogen

Question 5

CO2 Gas

Answer 5

• Gas sample in one tube
• Aqueous solution of calcium hydroxide (limewater) in another
• Bubble gas sample through the aqueous solution of calcium hydroxide
• If the gas is CO2, the solution turns CLOUDY
• CO2 and Calcium Hydroxide react to form calcium carbonate and water

Question 6

Test Result for Lithium

Answer 6

Crimson Flame

Question 7

Test result for Sodium

Answer 7

Yellow Flame

Question 8

Test result for Potassium

Answer 8

Lilac Flame

Question 9

Test result for Calcium

Answer 9

Orange-Red Flame

Question 10

Test result for Copper Ion

Answer 10

Green Flame

Question 11

Limitations with Flame Tests

Answer 11

• Colour of a flame test may be difficult to distinguish between. This is especially true if there is a LOW CONCENTRATION of the metal compound
• Sometimes a sample contains a mixture of metal ions which can mask the colour of the flame
• Scientists may use FLAME EMISSION SPECTROSCOPY instead of this to fix these problems

Question 12

What is Flame Emission Spectroscopy?

Answer 12

• Sample of metal ion in solution is placed into a flame
• The light given out is then passed into a machine called a spectroscope
• Spectroscope converts light into a line spectrum
• The positions of the lines in the spectrum are specific for a given metal ion
• So it can be used to identify a metal ion in the sample
• They can be compared to a data book to see the metal ions which the solution is composed of
• It can also tell us the concentration of the metal ion
• This is because the lines become more intense at a higher concentration

Question 13

Advantages of instrumental methods

Answer 13

• Rapid
• More sensitive, will work on tiny samples of metal compounds
• More accurate

Question 14

Test for Carbonates

Answer 14

• Add dilute HCL to sample
• Gas produced bubbled through limewater
• If it turns cloudy, there is CO2 present
• So it is a positive result

Question 15

Test for Sulphates

Answer 15

• Add dilute HCl to remove any carbonate or sulphite ions (they could be impurities and could react with the barium ions to give a white precipitate which reduces reliability of experiment or confuse results)
• Add BaCl2
• If BaSO4 is produced then a white precipitate is formed, which is a positive result

Question 16

Test for Halide Ions

Answer 16

• Add dilute nitric acid (gets rid of impurities of carbonate ions and sulphite ions
• Add Silver nitrate
• Silver reacts with halide
• If CHLORINE is present, a WHITE PRECIPITATE is formed
• If BROMINE is present, a CREAM PRECIPITATE is formed
• If IODINE is present, a YELLOW PRECIPITATE is formed