The idea that the rate of reaction simply describes the ratio between one quantity and another is worth expanding on with some students.

We know that rate can be expressed as: 

rate = evolution of product/time

For example if the rate of reaction between calcium carbonate and hydrochloric acid is measured as 10 cm³ of CO₂ gas per minute, then students should be encouraged to consider that if the rate remained constant how much product would be produced in 5 min, 10 min or 30 seconds?

Ths idea needs to be further developed when looking at the gradient of the graphs obtained by experiment.