Chloroform (CHCl3) used as an anaesthetic must be stored in tightly stoppered dark bottles. Otherwise, what harmful change can occur on exposure to air and light?

Introduction / Context:
Chloroform is a volatile organic compound that was historically used as an anaesthetic. However, it is chemically reactive when exposed to oxygen and light. Understanding why chloroform must be stored in dark, tightly stoppered bottles links basic organic chemistry with practical safety. Exam questions often ask why certain chemicals require special storage conditions and what dangerous products may form if they are not handled correctly.

Given Data / Assumptions:

• The substance is chloroform, CHCl3.
• It is used as an anaesthetic and therefore must be safe when administered.
• The storage recommendation is to keep it in tightly closed, dark bottles.
• The question asks what happens if it is not protected from air and light.

Concept / Approach:
When chloroform is exposed to air and sunlight, it can undergo a slow oxidation reaction with oxygen. This reaction produces phosgene, COCl2, a very toxic gas that was used as a chemical warfare agent. Phosgene can severely damage lungs if inhaled. To minimise this risk, chloroform is stored in dark containers, sometimes with a small amount of ethanol to inhibit phosgene formation. The key point is that the danger is not simple evaporation or harmless colour change, but chemical oxidation to a poisonous compound.

Step-by-Step Solution:
Step 1: Recall the formula of chloroform, CHCl3, and its use as a solvent and anaesthetic.Step 2: Remember that in the presence of oxygen and sunlight, chloroform can react to form new products.Step 3: One of the main products of this oxidation is phosgene, with the formula COCl2, which is highly toxic.Step 4: Phosgene is a gas that can cause severe lung damage and is unfit for any medical use.Step 5: To prevent this, chloroform is stored in dark, well stoppered bottles so that air and light do not easily reach it.Step 6: Therefore, the correct description of what happens if chloroform is not protected is that it gets oxidised to phosgene.

Verification / Alternative check:
Safety guidelines for handling chloroform mention the risk of phosgene formation and recommend storing it in amber glass bottles, away from sunlight. Some formulations include ethanol as a stabiliser because ethanol can react with phosgene and reduce its concentration. These details confirm that oxidation to phosgene is a recognised hazard and that the storage advice is designed to prevent exactly this reaction.

Why Other Options Are Wrong:
Option a, that chloroform simply becomes coloured, is incomplete and misleading because it omits the formation of toxic phosgene. Option b describes only evaporation, which does occur but is not the main danger described in textbooks. Option d suggests decomposition to chloropicrin, which is not the standard reaction of chloroform under these conditions. Option e, polymerisation to a harmless solid, has no basis in the usual chemistry of chloroform. Thus, these options do not match the known oxidation to phosgene.

Common Pitfalls:
Students may wrongly assume that all volatile organic solvents behave similarly and only worry about flammability or evaporation. In this case, the main risk is chemical transformation into a very toxic gas. Another pitfall is confusing different chlorinated compounds and their hazards. To avoid these mistakes, it is helpful to memorise that chloroform can produce phosgene on exposure to air and light, which is why dark, tightly stoppered bottles and stabilisers are used.

Final Answer:
If chloroform is not stored properly, it can get oxidised to toxic phosgene (COCl2) in the presence of air and light, which is why it is preserved in dark, tightly closed bottles.