Answer:
Change in concentration / Change in time
Explanation:
Whenever the "rate" of something is being spoken about, "rate" always refers to a change in something divided by time.
If the rate of a reaction is increased, that means the reaction is going faster, and hence would take less time. So mathematically, if the rate is "something" divided by time, and time decreases, the overall fraction is going to be larger, meaning the rate is higher.
In a chemical reaction, there is always reactants and products. A lot of the time, the reactants are being converted to products, and during this process, the concentration of reactants decrease, while the concentration of the products increase. The only way that we can tell how fast the reaction is going, is by calculating the concentration of either the reactants or the products.
Hence, the rate of a reaction is the change in concentration / change in time.
