A student was given liquid and told that it contained a mixture. How could the student determine whether or not the mixture was a solution, suspension, or colloid?
A combination of visual inspection and some stability checks....
Visual inspection will give you a good idea as to whether the mixture is a solution or not. In general terms solutions tend to be transparent - thats not a 100% guarantee, as some more complex solutions, such as polymer solutions can exhibit opalescence. However, as a general guide lack of any turbidity or cloudiness is a good guide that you have a solution.
Telling the difference between a colloid and a suspension? Well, colloids are suspensions, albeit ones where the dispersed phase is of specific dimensions. But another feature fairly typical of liquid colloids is high stability. Other suspensions may have a tendency to settle and phase-separate on standing.
If your liquid is milky or cloudy in appearance, and after 5 minutes or so in a centrifuge you end up with two separate layers, then you likely have a suspension that is not a stable colloid. If centrifugation doesn't have any effect on your liquid mixture, then its probably a colloid.