SC2 Separating Substances Flashcards
What is chromatography?
A process to separate the constituents of a mixture
How can chromatography show the difference between pure and impure substances?
Pure ones will not separate into a number of spots
How is the Rf value calculated?
distance moved by spot/distance moved by solvent
What does a substance’s Rf value depend on?
How soluble it is in the solvent
In chromatography, why must the substances be placed on a pencil line?
Pencil will not dissolve in the solvent
In chromatography why must the solvent height be lower than the pencil line?
So that the substances do not dissolve into the solvent off the paper
How is the melting point of a pure substance different from that of an impure substance?
Pure substance has one specific melting point. Impure substance has a melting range.
What is simple distillation used to separate?
A single liquid from a mixture
What is fractional distillation used to separate?
Multiple liquids of different boiling points
What is filtration used to separate?
An insoluble solid from a liquid
What is crystallisation?
Forming crystals of a soluble solid from a solution.
What is potable water?
Drinking water
How can sea water be made potable?
By distillation
What are the stages used to make water potable.
Sedimentation, filtration and chlorination
What is a pure substance?
A substance made of one type of atom or molecule only.
How would you separate water from salt water?
Simple distillation
How would you separate salt from salt water?
Crystallisation
Give instructions for producing a pure, dry sample of a salt from a solution of that salt.
- Heat the solution until it is saturated
- Leave in a shallow dish for the rest of the water to evaporate
Why must chemical analysis be done with pure water?
Because dissolved salts in non pure water could give false positive results.
How is the melting point of a mixture different from a pure substance?
A pure substance has a single melting temperature, a mixture has a range of melting points.
How do you calculate Rf value?
Distance moved by substance/distance moved by water
H
How can you identify if a clear liquid is a pure substance?
Heat the liquid. Once it begins boiling measure the temperature. If the temperature stays constant it is a pure substance but if the temperature carries on increasing as it boils it is a mixture.
