During a chemical reaction, reactants change into products. The speed at which this takes place is called the rate.

We can define this as being the 'appearance' of products or 'disappearance' of reactants in a set unit of time.
This can be represented as a mathematical relationship:

rate of reaction = loss of reactant / time

Sometimes this judgement is made on the basis of the completion of the reaction and the 'disappearance' of a reactant.

To some this may suggest we are looking at how far the change has gone rather than how fast. In fact in this case we would be measuring the average rate over the course of the reactions.

In chemistry we sometimes do this with reactions such as that between magnesium and hydrochloric acid.

[INSERT RATE_CI_02_vid]

How might we judge the rate of reaction here?

Measurement or rate of bubble formation, mass loss, change in pH or 'disappearance' of magnesium.

Which of the above responses might lead students to think about how far the reaction has gone rather than how fast?

The apparent 'disappearance' of the magnesium is problematic here as students are waiting for the reaction to finish.

The key here is the use of a timing device. This gives students the chance to measure change of one quantity vs time.