What is reaction rate ?

Explanation

**Step-by-Step Solution:**

1. **Definition of Reaction Rate**: The reaction rate, or rate of reaction, refers to the speed at which reactants are converted into products in a chemical reaction. It quantifies how quickly the concentration of a reactant decreases or the concentration of a product increases over time.

2. **Change in Concentration**: When considering a reactant (let's denote it as A), the rate of reaction can be expressed in terms of the change in concentration of A over a specific time period. For example, if the initial concentration of A at time \( t = 0 \) is \( [A]_0 \) and at time \( t \) it is \( [A]_t \), the change in concentration (\( \Delta A \)) can be calculated as:
\[ \Delta A = [A]_t - [A]_0 \]
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