Students frequently misunderstand the concept of  surface area and the rate of reactions. Yet the effect of increasing surface area can be easily seen in the video of burning milk powder. Watch the video before answering the follwoing question.

What is the reason that the milk powder burns so readily?

Oxygen, which is needed for combustion, is able to make contact with the fine particles of milk powder. This means that the there are more successful collisions per unit of time and consequently the rate of reaction is very high. The surface area to volume ratio of the milk powder is very high.

The development of the idea of surface area to volume ratio can be used here to aid understanding of a concept which is typically something that students find difficult.

Imagine that you are given  a plasticine block and repeatedly asked to cut this  plasticine block into smaller and smaller pieces.

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What is the key learning point in this activity?

The key  point here is that the mass of the original “reactant” has not changed but the surface area has increased fourfold at each stage of cutting.

If we continue this we see that by constantly dividing up the “reactant” particle up the reactant surface area available increases. The end point of these divisions would be at a molecular level as  would be found in liquid, aqueous or gaseous form.

This is the ultimate “small particle” to ensure a rapid rate of reaction.

Students could then go on to look at the mathematical relationship between surface area and volume.