Separating Mixtures

 Separating solids from liquids – filtration

 

If a substance does not dissolve in a solvent, we say that it is insoluble. For example, sand does not dissolve in water – it is insoluble.

Filtration is a method for separating an insoluble solid from a liquid.

When a mixture of sand and water is filtered:

  • The sand stays behind in the filter paper ( the residue)
  • The water passes through the filter paper (the filtrate)

How filtration works:


Separating solids from liquids – evaporation

Evaporation is used to separate a soluble solid from a liquid.

For example, copper sulfate is soluble in water – its crystals dissolve in water to form copper sulfate solution.

During evaporation, the water evaporates away leaving solid copper sulfate crystals behind.


Separating the solvent from a solution – simple distillation

Simple distillation:

It is a method for separating the solvent from a solution.

For example, water can be separated from salt solution by simple distillation.

This method works because water has a much lower boiling point than salt.

Boiling point: the temperature at which a substance rapidly changes from a liquid to a gas

process:

  1. Heating the solution causes a part of solution(with the lowest boiling point) to evaporate.
  2. The vapor will then be cooled until it condenses and turns back into a liquid, then it will be collected.
  3. The reminder of the solution will be left behind in the flask or container.

 

Miscible and Immiscible liquids

Mixtures of liquids can be separated according to their properties (whether the liquids dissolve in each other ).

  1. Miscible liquids: liquids dissolve in each other.
  2. Immiscible liquids: liquids don’t dissolve in each other.

 

Separation of liquids

The separation technique used for each liquid depends on the properties of the liquids.

 

Immiscible liquids:

Immiscible means that the liquids don’t dissolve in each other.

Example: oil and water

How to separate immiscible liquids?     use a separating funnel

  1. Put the two liquids into funnel
  2. Left them for short time to settle out and form two layers.
  3. The tap of the funnel is opened and the bottom liquid is allowed to run.
  4. The two liquids are now separate.

 

Miscible liquids:

Miscible liquids are harder to separate as they dissolve in each other.

Miscible liquids are often separated using fractional distillation.

This is possible as miscible liquids have different boiling points.

 

Separating a liquid from a mixture – fractional distillation

Fractional distillation is a method for separating a liquid from a mixture of two or more liquids.

This method works because the liquids in the mixture have different boiling points.

When the mixture is heated one liquid evaporates before the other one.

Example : liquid ethanol can be separated from a mixture of ethanol and water by fractional distillation.

 


Separating coloured substances dissolved in a liquid – Chromatography

Chromatography is used to separate different substances(coloured substances) dissolved in a liquid.

Examples:  inks, food colourings and plant dyes

It works because some of the coloured substances dissolve in the solvent used better than others, so they travel further up the paper.

 

Process:

1. Draw a line near the bottom of a sheet of filter paper, using pencil marks

( are insoluble and will not react with the solvent).

2. Add spots of each different dye to the line at regular intervals.

3. Roll up the sheet of paper and put it in the beaker of the solvent.

Make sure the dyes do not touch the solvent as they could dissolve

in it if this happens.

5.  The solvent will then travel up the paper, carrying the dyes with it.

6. Each different dye that is present within the solvent will move up

the paper at a different rate and form a mark in a different place.

7. The end result will be a pattern of spots known as a chromatogram.

 

 


 

Separating Mixtures Sheet ( Note) (PDF)

Separating Mixtures Sheet  answers (PDF)

 

 

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