SUSPENSIONS AND COLLOIDS

We have seen that a homogeneous mixture is one in which it is not possible to distinguish its components with the naked eye. However, sometimes we may mistakenly think that a mixture is homogeneous when, in fact, we are dealing with a suspension or a colloid.

5.1 Suspensions

Consider, for example, a finely divided solid (powder), which is dispersed in a liquid or gas. In the absence of agitation, and given enough time, the solid settles and falls.

In this case, it is a heterogeneous mixture known as a suspension. The most common suspensions are of solids in gases or liquids. The phase in which the dispersion occurs is called the dispersing phase and the solid that is suspended is known as the phase separation.

5.2 Colloids

A mixture with intermediate characteristics between a suspension (heterogeneous) and a solution (homogeneous) is known as a colloid.

In this case, the particles do not settle at the bottom after some time. These are particles smaller than 100 nm, which are very difficult to separate from the dispersing phase.

Strictly speaking, a colloid is neither a solution nor a heterogeneous mixture. However, in many cases, the terminology used for solutions is used for colloids. For example, we speak of solute to refer to the substance that is in the smallest proportion in this mixture.

There are colloids formed by the dispersion of a solid or liquid in a gas or liquid, or by condensation of a gas in another gas phase, as in the case of clouds.

Everyday examples of suspensions and colloids

Antibiotics for children are not soluble in water. They are prepared by making a suspension of the medicine in water, so that they can be administered in liquid form.

An example of a colloid is homemade gelatine. There are many examples of colloids in the kitchen, as not all substances dissolve in the solvent.

Designing an experiment

Based on what you have learnt about the Tyndall effect, can you design an experiment to demonstrate that jelly is really a colloid? Describe your experiment.

The solid particles that form a colloid are larger than those of the solute that is dissolved in a solution. These particles are therefore able to change the direction of propagation of light, scattering it. This is because each of the small particles that make up the scattered phase acts as a light emitter, causing its direction to change, so that the light rays can be seen. This phenomenon is known as the Tyndall effect. It can be observed clearly in partly cloudy skies, as clouds are an example of a colloid.

Understand, think, investigate a) Salt water. b) Dust cloud. c) Milk. d) A blood sample. e) Wall painting.

19 Check. Which technique would you use to separate the components of these mixtures? Are they solutions, suspensions or colloids?

20 Write a definition for the following terms: suspension and colloid. Look up the term ‘emulsion’ and explain its meaning in your own words. Give an example of each using material systems that are present in your home.

21 Assembly of ideas. Mayonnaise, like milk, is an emulsion. One version of this sauce is made from milk and olive oil. Why do you think it is necessary to whisk the mixture to obtain the sauce? What happens when we say that the sauce has been “curdled”?

22 Find the following materials at home and classify them as homogeneous, heterogeneous or colloid mixtures: a) Juice made from concentrate. b) Whole milk. c) Deodorant spray. d) Chamomile tea. e) Hydrogen peroxide. f) Body lotion. g) Cream.