What does 'reactivity' refer to in chemistry?

In chemistry, 'reactivity' specifically refers to the tendency of a substance to undergo chemical reactions with other substances. This property is crucial in understanding how substances interact, combine, or transform into new substances when exposed to different conditions or reactants. For instance, highly reactive elements, such as alkali metals, readily react with water or halogens, whereas inert gases like neon are characterized by their lack of reactivity under standard conditions.

The first option relates to phase changes, which describe physical transformations rather than chemical reactions. The third option, concerning stability, implies the resistance of a compound to chemical change, which is an inverse measure of reactivity. The fourth option, viscosity, pertains to the flow characteristics of a liquid and is not related to its chemical behavior. Thus, the definition of reactivity as the tendency of a substance to engage in chemical reactions accurately reflects its role in the study of chemical properties and behaviors.