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Chemistry revision Topic 2:Separating and Purifying substances (Rf Values,…
Chemistry revision Topic 2:Separating and Purifying substances
Purity/Pure substances
Everyday
Clean or natural
Chemistry
Made up of a single element or compound
If made up of different elements that aren't all part of a simple compound, it is called a
mixture
Has a sharp specific boiling or melting point
How to find out if a substance is pure
Use a melting point apparatus to measure the melting point
If the melting point matches the expected, the substance is pure, if not it is a mixture
Separating techniques:
Distillation
Simple distillation
Separating a liquid from a mixture which has a boiling point very far apart from the other
Heat mixture in a distillation flask until it is the correct temperature for the liquid you want to evaporate
It will evaporate into the condenser which will cool and condense the gas into liquid which will then slide into the beaker
Fractional distillation
Separating a mixture of different liquids with different boiling points
Heat the flask with all the mixtures in it to the boiling point of the first liquid
This will then evaporate up the fractional column and into the condenser, into its container
Then the heat could increase and the next liquid will be separated
If other liquids start evaporating, they will condense before reaching the condenser as it gets cooler near the top of the column and will condense on the glass rods in the column
Used to separate mixtures that include a liquid
Filtration:
Used if desired product is an insoluble solid
Used for purification
Pour mixture including insoluble solid into filter funnel containing filter paper
Make sure solid doesn't go over the top or down the side of the filter paper
Liquid will pass through and insoluble solid will be left in the funnel
Crystallisation
Used to separate a soluble product from a solution
Pour solution into evaporating dish over a Bunsen burner and heat until some water has evaporated,
When you see some crystals/ when some water has evaporated (point of crystallisation), remove dish from heat and allow to cool
Dissolved compound should start to form crystals as it becomes insoluble in the cold, highly concentrated solution
Filter crystals from solution and leave in a warm place to dry/ drying oven or desiccator
Makes the solution more concentrated(more crystals)
Chromatography
Used to separate a mixture of soluble substances and identify them. Can separate more than 1 ink at a time, just make sure they are far enough so they won't run into eachother
Mobile phase
When the molecules can move (liquid or gas)
Stationary phase
When the molecules can't move (solid or really thick liquid)
Chemicals in mixture spends different amounts of time dissolved in mobile phase and stuck on the stationary phase
How fast chemical moves to the stationary phase depends on how it distributes itself between each phase.
The more time it spends in the mobile phase, the faster it will move
Spot ink on baseline (pencil as it won't move with solvent and is insoluble).
Put the paper upright in solvent (water). Solvent depends how well the compounds dissolve in that substance
Put a watch glass on top of the beaker to prevent evaporating
Solvent will start to move up the paper
Once the chemicals in the ink dissolve in the ink, they will move up the paper at different rates so the dyes will separate.
If any dyes in the ink are insoluble in the solvent, they will stay on the baseline
The point the solvent has reached as it moves up the paper is the solvent front. When it has nearly reached the top, take the paper out and draw a line with pencil along the solvent front and leave to dry
Make sure ink doesn't touch solvent at the start as it would wash away
Factors effecting the distance a compound moves:
How soluble they are in the solvent
How attracted they are to the paper
Higher solubility and less attracted to paper will spend more time in mobile phase so will move further
A pure substance won't separate but will only have one spot
An impure substance will separate into at least 2 spots
Rf Values
Result of chromatography is called a chromatogram
Ratio between distance between between the dissolved substance (solute(ink)) and the distance travelled by the dissolved solvent.
Further through the stationary phase a substance move, the larger the Rf value.
Distance travelled by solute/distance travelled by solvent
Chromatography of a pure substance with the same Rf value of one of your spots in the mixture, the substance may be present
If they match once, you can repeat in a different solvent to see if the chemicals are the same. If they match again, there is more of a chance they are the same