Separating Techniques

Question 1

When can you use filtration?

Answer 1

• if a product of a reaction is an insoluble solid you can use filtration to separate it from the liquid reaction mixture

Question 2

How do you do filtration?

Answer 2

• put some filter paper into a funnel and pour your mixture into it
• liquid part of the mixture runs through the paper
• which leaves a solid residue

Question 3

How do you do crystallisation to separate a soluble solid from a solution?

Answer 3

• pour the solution into an evaporating dish and gently heat the solution
• some of the water will evaporate and the solution will become more concentrated
• when some of the water has evaporated crystals will start to form
• remove dish from heat and leave the solution to cool
• salt should start to form crystals as it becomes insoluble in the cold, highly concentrated solution
• filter crystals out of solution
• leave them in a warm place to dry
• you could also use a drying oven

Question 4

How can you use filtration and crystallisation to separate rock salt?

Answer 4

1 Grinding- grind the rock
2 Dissolving- dissolve in beaker and stir
3 Filtering- filter through filter paper in a funnel
4 Crystallisation- evaporate in an evaporating dish

Question 5

What is observed when you use filtration and crystallisation to separate rock salt?

Answer 5

• the sand does not dissolve so it collects in the filter paper
• the salt is dissolved in the solution- so it does go through
• when the waters evaporated the salt forms as crystals in the evaporating dish

Question 6

How can you do paper chromatography?

Answer 6

• draw a line near the bottom of a sheet of filter paper (baseline)
• add spots of different inks to the line at regular intervals
• role the sheet and put it in the beaker of solvent (water)
• level of solute is below the baseline
• place a lid on the top of the container to stop the solvent evaporating
• solvent seeps up the paper carrying the inks with it
• each different dye in the inks will move up the paper at different rates
• and form a spot in a different place
• leave the paper to dry when the solvent has reached the top

Question 7

Why do you use a pencil when doing chromatography?

Answer 7

• pencil marks are insoluble
• so it wont dissolve in the solvent

Question 8

During chromatography, why is the level of solvent below the baseline?

Answer 8

• you don’t want the inks to dissolve into the solvent

Question 9

How do you calculate Rf for each chemical?

Answer 9

Rf= distance travelled by solute/distance travelled by solvent

Question 10

How can you carry out simple distillation to separate out solutions?

Answer 10

• used for separating out a liquid from a solution
• solution is heated and the part of the solution that has the lowest boiling point evaporates
• vapour is then cooled, condenses back into a liquid and is collected
• rest of solution is left behind in a flask
• you can use SD to get pure water from sea water
• water evaporates and is condensed and collected
• you will end up with just salt left in the flask

Question 11

Why is simple distillation different to fractional distillation?

Answer 11

• if you have a mixture of liquids with similar boiling points you cannot use simple distillation

Question 12

How can you do fractional distillation to separate a mixture of liquids?

Answer 12

• put your mixture into a flask and put a fractionating column on top, then heat it
• different liquids have different boiling points, so they will evaporate at different temperatures
• 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 the highest boiling points might also start to evaporate
• the column in cooler towards the top
• so you will only get part of the way up before condensing and running back down the flask
• when the first liquid has been collected you raise the temperature until the next one reaches the top