A solution contains one substance mixed in with another in such a way that the individual particles of the mixture cannot be seen. This depends on the two substances in the mixture being able to combine together.

However, some substance do not combine. This is called insoluble. Oil and water are insoluble, and as oil is less heavy than water, it floats on the surface.

Insoluble things can be a problem, for example when you want to wash up used dinner plates. What scientists have to do is to fool the insoluble substances into breaking up into little bits and then staying in suspension. This is what a washing-up liquid does. It is made of minute chemical rods (called molecules). One end of the rods attracts water, while the other end attracts oil and grease. As a result, when you push the grease about with a washing-up brush, the grease will appear to dissolve in the water and can be washed away. However, it is just suspended.