Time

When we talk about rates, we always talk about concentration per unit time. Why is this? Well, time is a great reference tool that we have to gauge how far a reaction has proceeded. Because we can closely monitor how much time has passed (using a stopwatch or a clock), we can measure the concentration at two different times and find the rate.

Here, we see that the concentration dropped one gallon in one hour and forty-five minutes. This is our rate. If we drank two gallons of orange juice, and it took one hour and forty-five minutes to finish one gallon of orange juice, we would say that our rate is independent of the concentration. Why? Because it took the same amount of time to finish one gallon as it did to finish two gallons. However, if it took twice as long to finish two gallons as it did to finish one gallon, then we can say that our rate is dependent on how much we drink.

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