Chemistry

Matter in Our Surroundings

To Study the Properties of a Colloid

If we shake some soap powder with water in a beaker, we get a colloidal soap solution which is not perfectly transparent, it is somewhat translucent. The soap particles cannot be seen by us. If the soap solution is kept for some time, the soap particles do not settle down (showing that it is quite stable). If we filter the soap solution, the whole solution passes through the filter paper and no residue is left behind (showing that it cannot be separated by filtration). All these observations indicate that soap and water mixture is a true solution. The scattering of light by a soap solution and the examination of soap solution under a high power microscope, however, show that soap solution is not a true solution. This point will become more clear from the following discussion.
In a true solution (like sugar solution), the solute particles are so small that they cannot scatter (or reflect) light rays falling on them. For example, if a beam of light (from a torch) is put on a true solution (say, sugar solution) kept in a beaker in a dark room, the path of light beam is invisible inside the solution when seen from the side (see Figure 32). The beam of light can become visible only when the solute particles are big enough to reflect light falling on them. Since the particles of a true solution do not scatter light, we conclude that they must be very, very small.
In a colloidal solution (or colloid), the particles are big enough to scatter light. This can be shown as follows. If a beam of light is put on a colloidal solution (say, soap solution), kept in a beaker in a dark room, the path of light beam is illuminated and becomes visible when seen from the side (see Figure 33).The path of light beam becomes visible because the colloidal particles are big enough to scatter light falling on them in all the directions. This scattered light enters our eyes and we are able to see the path of light beam.
The scattering of light by colloidal particles is known as Tyndall effect. The scattering of light by colloidal solutions tells us that the colloidal particles are much bigger than the particles of a true solution and hence colloidal solutions are not true solutions. So, a true solution can be distinguished from a colloidal solution by the fact that a true solution does not scatter a beam of light passing through it but a colloidal solution scatters a beam of light passing through it and renders its path visible. In other words, a true solution does not show Tyndall effect but a colloidal solution shows Tyndall effect.
The particles of some of the colloidal solutions can be seen through a high power microscope. For example, if a drop of milk is examined under microscope, we can see the small particles of fat floating in the liquid. This observation shows that colloids are heterogeneous in nature, though they appear to be homogeneous. Let us write down the properties of colloidal solutions now.

Notes

Matter in Our Surroundings - Notes

1. 9. Non-Metals Have Low Densities.