Separating and Analysing

Question 1

How do you collect pure water from sea water?

Answer 1

1. Simple Distillation
2. The solution is heated and the part of the solution, that has the lowest boiling point evaporates
3. The vapour is then cooled, condenses and is collected
4. The rest of the solution is left behind in the flask
   (the salt remains in the flask)

Question 2

What is fractional distillation used for?

Answer 2

-Fractional distillation is used to separate liquids such as alcohol (ethanol) and water. Both are completely miscible with each other
-The different liquids have different boiling points so will evaporate at different temperatures
-The liquid with the lowest boiling point evaporates first. When the temperature on the thermometer matches the boiling point of this liquid it will reach the top of the column
-Liquids with higher boiling points may also start to evaporate. But the column is cooler towards the top. So they will only get part the way up before condensing and running back down towards the flask
-When the first liquid has been collected, you raise the temperature until the next one reaches the top
E.G. Crude oil at a refinery

Question 3

How do you do paper chromatography?

Answer 3

1. Draw a line near the bottom of the sheet of filter paper, use pencil as is insoluble and won’t react with the solvent
2. Add spots to the line at regular intervals
3. Loosely roll the sheet and place it in a beaker of solvent e.g. water
4. Make sure dyes aren’t touching the solvent as you don’t want them to dissolve into the solvent
5. Place a lid on the top of the container to stop the solvent evaporating
6. The solvent seeps up the paper carrying the dyes with it
7. Each different dye will move up the paper at different rates and form s spot in different places
8. The end result is a pattern of spots called a chromatogram

Question 4

How does chromatography separate mixtures?

Answer 4

• Chromatography works because different dyes will move up the paper at different rates
• Some will stick to the paper and others will dissolve more readily in the solvent and travel more quickly
• The distance the dyes travel up the paper depends on the paper and solvent that you use

Question 5

How do you separate petrol and water?

Answer 5

• Immiscible and so they won’t mix with each other and petrol floats on top of the water
• Use a separating funnel

Question 6

How is filtration used?

Answer 6

• To separate an insoluble solid from a liquid

- And used for purification

Question 7

How is crystallisation used?

Answer 7

• To separate a soluble solid from a solution
  1. Place in evaporating dish and slowly heat solution, some of solvent will evaporate and the solution will get more concentrated . Stop heating when crystals start to form
  2. Remove dish and place in warm place for rest of solvent to slowly evaporate to get big crystals
  3. Dry product by using a drying oven or a desiccator

Question 8

How do you separate rock salt?

Answer 8

• Rock salt is a mixture of salt and sand
  1. Grind up the rock salt using a pestle and mortar
  2. Dissolve in beaker and stir
  3. Filter through filter paper in a funnel
  4. Evaporate in an evaporating dish
• The sand is insoluble so stays in big grains which won’t fit through the tiny holes in the filter paper, so it collects on the filter paper
• The salt is dissolved in the solution so it does go through and when the water is evaporated the salt forms crystals in the evaporating dish

Question 9

What happens when you add water to cobalt chloride paper?

Answer 9

Paper blue to pink

Question 10

What happens when you add water to anhydrous copper sulphate?

Answer 10

Turns blue

Question 11

How is chlorine collected?

Answer 11

• Denser than air, so usually collected in downwards into test tube or gas jar
• Chlorine too soluble to collect it satisfactorily over water, but collect over concentrated salt solution instead
• Chlorine is a green gas which bleaches damp litmus paper

Question 12

How is ammonia collected?

Answer 12

• Less dense than air and extremely soluble in water, so can only be collected upwards into a test tube or gas jar
• Ammonia turns damp red litmus paper blue

Question 13

How can chromatography help you identify dyes?

Answer 13

1. Work out what dyes are present in an unknown substance e.g. ink, you can use chromatography to find out
2. First make chromatograms for your unknown substance and for some reference material (dyes that you think might be the ink)
3. Now compare the chromatograms to work out what dyes are in your unknown substance, spots on the chromatograms for the unknown substance will match spots on the chromatograms of the reference materials when the dyes are the same

Question 14

What do you use simple distillation for?

Answer 14

-To separate out solutions
(FD: to separate a mixture of liquids)
-Problem with simple distillation is that you can only use to separate things with very different boiling points

Question 15

How is oxygen collected?

Answer 15

• Almost same density as air and is only slightly soluble in water
• Normally collect over water
• Relights a glowing splint

Question 16

How is hydrogen collected?

Answer 16

• Less dense than air and is almost insoluble in water
• Collect it over water or upwards into a test tube or gas jar
• Hydrogen pops when a lighted splint is held to the mouth of the tube. The hydrogen combines explosively with oxygen in the air to make water

Question 17

How is carbon dioxide collected?

Answer 17

• Denser than air and only slightly soluble in water and so it can be collected over water as well
• Collected downwards into a test tube or gas jar
• Turns limewater milky

Question 18

How can you check the purity of water?

Answer 18

• Anhydrous copper sulphate does not show that the water is pure only shows that water is present
• Freezes at exactly 0 degrees and boils at exactly 100 degrees at 1 atmosphere pressure

Question 19

What does a flame test do?

Answer 19

Shows the presence of certain metal ions in a compound

Question 20

Describe a flame test

Answer 20

• Heating the substance and seeing whether it burns with a distinctive colour flame
• Clean a platinum wire loop by dipping it in some dilute HCL and then holding it in a flame
• Once you hold the loop in the flame and it burns without any colour you can dip it into the sample you want to test and then put it back in the flame

Question 21

What colour does lithium ions burn with?

Answer 21

Crimson-red flame

Question 22

What colour does sodium ions burn with?

Answer 22

Yellow-orange flame

Question 23

What colour does potassium ions burn with?

Answer 23

Lilac flame

Question 24

What colour does calcium ions burn with?

Answer 24

Brick-red flame

Question 25

What is another way, apart from flame tests, to identify metal ions?

Answer 25

1. Many metal hydroxides are insoluble and precipitate out of solution when formed and some of these hydroxides have a characteristic colour
2. So in this test you add a few drops of sodium hydroxide to a solution of your mystery compound in a test tube, in the hope of forming an insoluble hydroxide
3. If you get a coloured insoluble hydroxide then you can tell which metal was in the compound

Question 26

What colour precipitates are formed when adding sodium hydroxide? For Cu 2+, Fe 2+, Fe 3+, NH4+?

Answer 26

Copper (II) : blue
Fe 2+ : sludgy green (green), slowly turns brown
Fe 3+ : Reddish brown (orange-brown)
-Ammonium ions, add sodium hydroxide and if ammonia given off this means there are ammonium ions in your mystery substance (no precipitate but a smell of ammonia)
-Ammonia turns red litmus paper blue

Question 27

How do you test for anions, carbonates, using HCL?

Answer 27

1. To test for carbonates add dilute HCL to your test sample
2. If carbonates are present, then carbon dioxide will be released, as carbonates give off CO2 with HCL
3. Test for CO2 using limewater, turns limewater milky

Question 28

How do you test for anions, sulphate ions, using HCL and Barium Chloride?

Answer 28

• Sulphate ions produce a white precipitate
  1. To test for sulphates add dilute HCL followed by barium chloride solution
  2. If a white precipitate of barium sulphate forms it means the original compound was a sulphate
  3. Hydrochloric acid is added to remove any traces of carbonate or sulphate ions (to remove impurities) before you do the test as both of these would produce a precipitate, so they would confuse the results.

Question 29

How do you test for halides with Nitric Acid and Silver Nitrate?

Answer 29

1. To test for chloride, bromide and iodide ions add dilute nitric acid followed by silver nitrate solution
   2.
   -A Chloride ion gives a white precipitate of silver chloride
   -A Bromide ion gives a cream precipitate of silver bromide
   -An Iodide ion gives a yellow precipitate of silver iodide
2. The acid is added to get rid of carbonate or sulfite ions before the test, but you use nitric acid and not HCL

Question 30

Describe how copper (II) sulphate crystals can be used to test for water

Answer 30

1. When copper (II) sulphate is bound to water it forms blue crystals
2. If you heat the blue hydrated copper (II) sulphate crystals it evaporates the water off
3. This leaves white anhydrous copper (II) sulphate powder which does not have water bound to it
4. If you then add a couple drops of water to the white powder you get the blue crystals back again
   - Hydrated means with water and Anhydrous means without water

Question 31

What does it mean if a substance is pure?

Answer 31

• It means it is only made up of one substance and so has set defined physical properties like boiling point and freezing point
• So physical test to show if water is pure to see if it boils always at 100 degrees Celsius and freezes at 0 degrees Celsius

Question 32

What is simple distillation used for?

Answer 32

For separating out a liquid from a solution