Use of solvents in industry
When substances are made as a result of a chemical process, solvents are often used to extract the required chemical from the other products and/or remaining reactants. A solvent is normally chosen which dissolves the required product, but does not dissolve the other chemicals. This process is called solvent extraction, and can be used on a large or small scale, depending on the product concerned. The process can also be used for purification of a product.
The reasons for the differing solubilities are explained later in the 'Forces between molecules' topic, but an example of solvent extraction is shown below.
You are not required to learn the specifics of this section, but the solubility of a substance in two different solvents is actually another example of a (physical) equilibrium system. Therefore, we can use the context of solvent extraction to revise the concepts of solubility and equilibrium.
Solvent extraction
The partition (separation) of a chemical between two solvents can be carried out at home. Read the instructions carefully before you start.
If you are unable to carry out the activity, watch the video below. Click here to read the Solvent extraction experiment instructions.
- Add some water to a clear glass bottle with a screw-top so the bottle is about half full.
- Pour in a liquid that does not mix (immiscible) with the water. This could be a type of vegetable oil, or a mineral solvent such as white spirits or turpentine. Use about half as much oil as water.
- Check that the water and oil layers stay separate. (If not, choose another type of oil.)
- Identify the water (aqueous) layer. This will be the larger layer.
- Add a coloured solute such as ink or food colouring to the bottle, replace top, and shake to dissolve the solute.
- Allow the liquids in the bottle to settle and observe the results.
By comparing the colour of the two layers, you should be able to tell whether the solute was more soluble in one of the layers.
The video shows two liquids in a test tube. A coloured substance is added to the test tube and the liquids are shaken. The liquids form back into two and it can be seen that the coloured substance dissolves more in one layer than the other.
Click here to open the video in a new window.
In this experiment, there is a dynamic equilibrium situation. What do you think is happening to the particles of solute at the place where the oil and water are in contact with each other?
Solvent extraction works by choosing a solvent which dissolves one chemical well, but not the other chemical(s). This enables chemists and chemical engineers to design methods of separating and purifying products.
Access an activity that applies the idea of equilibrium constants to solvent partition. 
