Half-life
MCAT glossary · Chemical & Physical Foundations of Biological Systems
Definition
The time required for a quantity to fall to half its initial value.
Half-life in plain English
Half-life is simply the time it takes for half of something to disappear or decay away. If a medicine has a 4-hour half-life, then 4 hours after you take it, half the dose is gone; after another 4 hours, half of THAT is gone (so a quarter remains), and so on. The neat, slightly sneaky part is that it never quite hits zero — each half-life just chops the remaining amount in half again. This one idea shows up everywhere on the MCAT: radioactive atoms decaying, drugs clearing your bloodstream, even chemicals reacting, because they all thin out by the same steady halving rhythm.
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