What is the difference between a sol and a gel?

Hint:Both sol and gel are colloidal solutions containing uniformly dispersed solute particles. The solute particles cannot pass through a semipermeable membrane.

Complete answer:
Colloidal solutions are heterogeneous in nature and are stable. The solute particles in a colloidal solution have the particle size ranging from one nanometer to one thousand nanometer.
Sol is a liquid state of colloidal solution whereas gel is a solid or semisolid state of colloidal solution. No definite structure is present for sols whereas generally a honeycomb like structure is present for gel.
A sol constitutes a uniformly dispersed solid in a fluid. A gel constitutes a liquid uniformly dispersed in a solid. The disperse phase in a sol is a solid. The dispersed phase in a gel is a liquid. The dispersion medium in a sol is a liquid. The dispersion medium in a gel is a solid.
The examples of sol include blood, mud water, arsenic sulphide sol​, sol of sulphur in water, paints and milk of magnesia.
The examples of gel include butter, cheese, hair gel and fruit jellies. Gels are stable at comparatively low temperature.
Thus, in the sol of mud water, the solid disperse phase constitutes mud particles and the liquid dispersion medium constitutes water.
If you cool a sol, you can convert it to a gel. Similarly, if you heat a gel, you can obtain a sol.

Note:
Colloidal solution differs from the true solution in terms of particle size of solute particles. The solute particles of a colloidal solution cannot pass through a semipermeable membrane whereas the solute particles of a true solution can pass through the semipermeable membrane. Two phases are present in the colloidal solution whereas only one phase is present in true solution.