Discuss why the particles in a colloid are prevented from settling out. Introduce the concept of polarity and charged atoms. Use diagrams to show how areas on the surface of the colloid molecule attract positive or negative charged areas of the dispersing medium. The layers repel each other and prevent settling out. Indicate why heating and stirring will interfere with the colloid particles and cause settling out. See diagrams to explain the Tyndall effect and the electrostatic layers.