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Explain in detail why some medicines are stored in dark-coloured bottles.

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Hint: What would a dark coloured bottle limit the exposure of the medicine inside to? Think of the possible effect said exposure could have on the medicine inside.

Complete step by step answer:
> Many chemicals and medicines undergo photocatalysis i.e. decomposition in the presence of light and therefore decompose in the presence of sunlight as they are vulnerable to its UV rays, completely voiding their usability as a result which can have drastic economic and health effects.
- For instance, when Hydrogen Peroxide, a medicine often used to treat minor injuries such as cuts and bruises, is stored in the presence of light it decomposes much faster.
\[2{{H}_{2}}{{O}_{2}}\xrightarrow{\text{UV rays}}2{{H}_{2}}O+{{O}_{2}}\]
- Other examples chemical compounds which are affected by the presence of light are chloroform $(CHC{{l}_{3}})$, which results in the production of a poisonous gas called phosgene $(COC{{l}_{3}})$ in the presence of UV rays, and silver nitrate $(AgN{{O}_{3}})$, which decomposes into silver, nitrogen and oxygen when subjected to light.
- Therefore, so as to ensure that such a decomposition does not occur, the medicines which involve chemicals which can undergo photocatalysis are stored in dark-coloured bottles.

Note: While the answer to this question is fairly intuitive, no answer is satisfactory without an abundance of examples. Therefore, it is advisable to remember as many decomposition reactions which take place in the presence of light as possible.
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