Final answer:
Reactivity is the rate of a chemical reaction and can be measured by observing changes in physical properties like gas volume or solution conductivity. The rate of reaction varies with the nature of reactants, their state, temperature, and concentration, as well as catalyst presence.
Step-by-step explanation:
The term reactivity refers to the rate at which a substance undergoes a chemical change. Acid, when combined with metals like magnesium or zinc, exhibits high reactivity, which means that the rate of reaction is high. The rate of reaction is quantified as the change in the concentration of a reactant or product over a given time period. This rate can be measured in various ways, such as by observing changes in volume or pressure for reactions that produce or consume gases, monitoring light absorption for reactions involving colored substances, or measuring changes in conductivity for reactions with aqueous electrolytes.
The rate at which a reaction occurs can depend on factors such as the chemical nature of the reactants, their state of subdivision, the temperature, the concentration of the reactants, and the presence of a catalyst.