Question

1. what is the definition of half - life

the time it takes for a substance to double in amount
the time it takes for a substance to reduce to half of its original amount?
the time it takes for a substance to completely decay
the time it takes for a substance to increase its amount by 1/2 of the original

Explanation:

Half - life is a fundamental concept in radioactive decay. It specifically refers to the time duration during which a radioactive substance (or any substance undergoing a similar decay - like process) reduces its quantity to half of what it was initially. For example, if there is \(N_0\) amount of a radioactive isotope at time \(t = 0\), after one half - life (\(t = T_{1/2}\)), the amount of the isotope will be \(\frac{N_0}{2}\).
The first option ("The time it takes for a substance to double in amount") is incorrect as it describes a growth process (like in some population growth models, but not related to half - life). The third option ("The time it takes for a substance to completely decay") is wrong because in radioactive decay (and most exponential decay processes), a substance never completely decays in a finite time. The fourth option ("The time it takes for a substance to increase its amount by 1/2 of the original") is also incorrect as it implies an increase, while half - life is about decay (reduction).

Answer:

The time it takes for a substance to reduce to half of its original amount