A mixture of water and starch is colloidal because it forms a shell of firmly bound molecules of water that stops the starch particles from aggregating with the molecules of water when they collide. However, when heated, aggregation occurs because the particles collide with each other with greater energy and this destroys the protective shell.
A colloidal mixture occurs when insoluble matter exists within another substance. These mixtures normally bridge the gap between suspensions and solutions, meaning that they form a mixture that is an intermediate between a heterogeneous mixture and a homogeneous mixture (solution). Their characteristics are also intermediate between these two types of mixtures.