If a radioactive source has a half life of 2 hours how long will it take for the activity to go from 200 to 25?

It takes 6h to the radioactive source decrease its activity from 200 to 25

Explanation:

The equation that describes the readioactive decay is:

Ln [A] = -kt + Ln [A]₀

Where [A] could be taken as activity of the source after time t, [A]₀ is initial activity and k is decay constant.

Decay constant could be written as:

k = ln2 / Half-life

k = ln 2 / 2h

k = 0.3466h⁻¹

Replacing in decay equation:

Ln [25] = -0.3466h⁻¹t + Ln [200]

-2.0794 = -0.3466h⁻¹t

6h = t

It takes 6h to the radioactive source decrease its activity from 200 to 25

answer : one can design an experiment using a liquid and an additive that can be added to the liquid and observe the chemical changes in the reaction species.

one would need a beaker which is filled with a known amount of the liquid in which thermometer is placed to measure the temperature changes; after addition of the additive to the liquid solution one can note down the temperature change and find whether the reaction is endothermic or exothermic in nature. by subtracting the initial temperature value with the final temperature one can find the temperature changes.

various combinations of liquid and additive can be used. liquid in this case can be a known quantity of water and additives can be epsom salt or without epsom salt the experiment can be conducted to notice the changes. water and ice, hydrogen peroxide and dry yeast, vinegar and baking soda,etc.