There are a few ways to increase the rate of a reaction. when the amount of a substance in a given volume is increased, this is altering the

If the volume remains the same, but the amount (number of moles) is increased, this is an increase in concentration, which is defined as amount per volume (usually moles/liter).
For some reactions, increasing the concentrations also causes the rate of reaction to increase.

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D. most models represent a real world object in a simpler way