separation of substances

Question 1

What is purity?

Answer 1

Can be a single element. Not mixed with any other elements.

i.e “pure” silicon

Question 2

How do we describe if a substance is pure?

Answer 2

One way is to measure its melting and boiling point.

A pure substance melts at a specific FIXED temperature. A pure substance also has a specific fixed boiling point.

(straight horizontal line for melting and boiling point on a graph)

while impure substances melt and boil over a RANGE of temperatures.

Question 3

What are the methods of purification?

Answer 3

• Filtration
• Crystallisation
• Distillation
• Fractional distillation
• Paper chromatography

These are all physical separation techniques. So they are used to separate MIXTUREs only. They cannot be used to separate compounds.

//Evaporation is also one, but it is not required for C1 chemistry

Question 4

Explain filtration

Answer 4

It is used to separate an INSOLUBLE solid from a liquid.

Equipment includes; filter funnel, filter paper and a beaker.

1. Pour mixture into filter paper
   // the liquid passes through while the solid remains

End of experiment

Question 5

Explain Crystallisation (/evaporation)

Answer 5

It is used to separate a SOUBLE solid from a liquid.

Equipment; beaker

1. Gently use heat to cause some of the solution to evaporate
   - however you must be sure that the heating does not affect the chemical we are trying to crystallise
   //this will leave behind crystals

Question 6

Explain Distillation

Answer 6

It is used to separate a SOUBLE solid from a liquid. Where you can get both products, the solid and liquid separately (like crystallisation, where the liquid evaporates)

1. evaporate liquid by heating
2. condense vapour by cooling

equipment/apparatus:

• flask (where the solution is placed
• connected to a continuous glass tube
• glass tube is surrounded by a CONDENSER
  //cold water continuously runs through the condenser, keeping the glass tube cold (causing the vapour to turn into liquid)
• bunsen burner for heating
• thermometer

Question 7

Explain Distillation

Answer 7

it is used to separate a mixture of DIFFERENT liquids. The liquids must have different boiling points.

equipment/apparatus:

• Fractionating column: the flask is attached to a long column containing hundreds of different glass beads

by gently heating the liquid, both liquids will evaporate. But the one with a lower boiling point will evaporate more easily. When the vapours hit the fractionating column, they condense and turn back into a liquid. The lower boiling point liquid will make its way through the FC faster.

everything else, and the process is the same as distillation.

Question 8

Explain paper chromatography

Answer 8

Allows us to separate substances based on their solubility.

• take a sheet of chromatography paper, draw a line (with pencil, because pen would dissolve too) near the bottom and put dots of the substances you will be testing on the line.
• Place this line in a beaker filled with a solvent.
• The solvent travels up the paper and carries the substances along with it.

If the substance dissolves into the same constant colour–it is a pure substance, while if it separates into multiple colours it is not pure.

Rf value = distance travelled by substance/distance travelled by solvent

paper chromatography can be used to identify unknown substances. By searching up the rf value in data sheets.

additionally, if another substance is used to find commonalities in the unknown substance–if the colours/dots line up on the paper the same substance is present in the unknown substance.