Lesson 3

Solvents and solutions

Some important terms

Water is a very important solvent but it's not the only one. Acetone is used to remove nail polish, turpentine to dissolve oil paints and good perfumes are solutions of the fragrance dissolved in ethanol. Dry cleaning fluid is an organic solvent used to clean clothes which would be damaged by detergent and water.

A solution is said to be saturated when no more solute can be dissolved in the solvent.

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 (it becomes the residue)
the water passes through the filter paper (it becomes the filtrate)

The slideshow shows how filtration works:

Shows a beaker with a mixture of solid and liquid in it. Another beaker has a funnel with some filter paper in

A beaker containing a mixture of insoluble solid and liquid. There is filter paper in a filter funnel above another beaker

The solid and liquid mxture is poured into the filter funnel

The mixture of insoluble solid and liquid is poured into the filter funnel

The liquid can drip through the filter paper into the beaker below, but the solid particles can't. They are caught in the filter paper.

The liquid particles are small enough to pass through the filter paper as a filtrate. The solid particles are too large to pass through the filter paper and stay behind as a residue.

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.

A solution is placed in an evaporating basin and heated with a Bunsen burner.

The amount of the solution has reduced as some has evaporated. Small particles can be seen at the bottom of the basin containing the solution

The volume of the solution has decreased because some of the water has evaporated. Solid particles begin to form in the basin.

The solution has evaporated, leaving a crystalised solute

All the water has evaporated, leaving solid crystals behind.

Chromatography

Simple chromatography is carried out on paper. A spot of the mixture is placed near the bottom of a piece of chromatography paper and the paper is then placed upright in a suitable solvent, eg water. As the solvent soaks up the paper, it carries the mixtures with it. Different components of the mixture will move at different rates. This separates the mixture out.Chromatography can be used to separate mixtures of coloured compounds. Mixtures that are suitable for separation by chromatography include inks, dyes and colouring agents in food.

The colours separate and move up the paper at different rates.

The link below shows how chromatography works.

https://www.youtube.com/watch?v=PvHvx7k7UPU

Page updated

Report abuse