When exposed to sunlight, which one of the following common laboratory liquids slowly gets converted to the highly poisonous gas phosgene?

Introduction / Context:
Chemical safety is an important aspect of laboratory practice. Some seemingly harmless liquids can slowly decompose into hazardous substances under certain conditions, such as exposure to light or air. This question asks which common organic liquid turns into phosgene, a highly toxic gas, when exposed to sunlight. Knowing this helps students understand why some chemicals are stored in dark bottles and handled with care.

Given Data / Assumptions:
- The transformation in question occurs when a liquid is exposed to sunlight over time.
- The product formed is phosgene, a poisonous gas historically used as a chemical warfare agent.
- The options include chloroform, acetone, benzene, and propylene.
- Standard textbook information about stability and photochemical reactions of these compounds is assumed.

Concept / Approach:
Chloroform, with the formula CHCl3, is a chlorinated solvent that can undergo slow oxidation in the presence of oxygen and sunlight to form phosgene, COCl2, along with hydrogen chloride. Because phosgene is extremely toxic, chloroform is often stored in dark bottles to limit light exposure and may contain stabilisers such as ethanol to inhibit this decomposition. In contrast, acetone, benzene, and propylene do not typically produce phosgene under normal laboratory sunlight exposure in the same way. Therefore, chloroform is the substance of concern in this question.

Step-by-Step Solution:
Step 1: Recall that phosgene, COCl2, is a toxic gas that can form from the oxidation of certain chlorinated hydrocarbons. Step 2: Identify chloroform, CHCl3, as a chlorinated solvent often mentioned in safety notes regarding its slow conversion to phosgene in the presence of light and air. Step 3: Note that chloroform bottles are commonly amber coloured or kept in dark places, and sometimes contain small amounts of ethanol as a stabiliser. Step 4: Consider acetone, benzene, and propylene, none of which are routinely described as precursors for phosgene in simple sunlight exposure conditions in school level chemistry. Step 5: Conclude that chloroform is the correct answer since it is known to give phosgene upon prolonged exposure to light and oxygen.

Verification / Alternative check:
Safety data sheets and standard organic chemistry textbooks specifically warn that chloroform can decompose to phosgene on exposure to air and light. They usually recommend storage in tightly closed, light resistant containers and may advise the presence of ethanol to react with any phosgene formed. No similar widely publicised warning exists for acetone, benzene, or propylene producing phosgene under ordinary sunlight. This consistent treatment in safety literature confirms chloroform as the substance named in the question.

Why Other Options Are Wrong:
Acetone: A common solvent that can be flammable but is not noted for forming phosgene under simple sunlight exposure in air.
Benzene: An aromatic hydrocarbon and known carcinogen, but its photochemical reactions do not typically involve formation of phosgene in the way described for chloroform.
Propylene: An unsaturated hydrocarbon gas used in industry, not a chlorinated liquid that would readily yield phosgene by simple oxidation in sunlight.

Common Pitfalls:
Students may confuse different chlorinated solvents or may not remember which specific compound is linked to phosgene formation. Another mistake is to focus only on toxicity and choose benzene because it is a known carcinogen, forgetting that the question is about conversion to phosgene. To avoid such confusion, associate chloroform with the safety advice about phosgene formation and storage in dark bottles, and keep benzene separate in memory as a carcinogenic aromatic hydrocarbon without that particular photochemical hazard.

Final Answer:
When exposed to sunlight, Chloroform can slowly decompose in the presence of oxygen to form the poisonous gas phosgene, which is why chloroform is stored in dark, well closed bottles with stabilisers.