What does Half-Life measure?

Half-Life measures the amount of time it takes for half of a substance to dissipate, decay, or undergo a transformation. This concept is most commonly associated with radioactive substances, where it defines the time required for half of a radioactive material to decay into a different substance. Additionally, half-life is also applicable in pharmacology, where it helps in understanding how long a drug remains active in the body before its concentration is reduced by half.

Other options address related but distinct concepts. The time it takes for a substance to double, for instance, pertains to growth or accumulation, which is quite different from the decay process described by half-life. The speed of chemical reactions is associated with kinetics, focusing on how quickly reactions occur rather than how much of a substance remains over time. Lastly, the effectiveness of a pesticide may relate indirectly to half-life, particularly in assessing how long a pesticide remains active, but it does not directly measure the decay or dissipation of the substance itself. Thus, the correct interpretation of half-life is specifically tied to the time required for half of a substance to dissipate.