Which of the following gases is a greenhouse gas and is also known to deplete the ozone layer because it contains chlorine and fluorine atoms?

Introduction / Context:
Some man made gases not only trap heat in the lower atmosphere, acting as greenhouse gases, but also reach the stratosphere where they release chlorine or bromine atoms that destroy ozone. These compounds were widely used as refrigerants and aerosol propellants. This question asks you to pick the gas that is both a greenhouse gas and a potential ozone depleting substance due to its chlorine and fluorine content.

Given Data / Assumptions:

• The options include CH2ClF, C2H2, PF5, COCl2, and CH4.
• We focus on compounds that contain halogens such as chlorine and fluorine, because they are associated with ozone depletion when stable enough to reach the stratosphere.
• We also consider greenhouse behaviour, especially for halogenated hydrocarbons.

Concept / Approach:
Halogenated hydrocarbons such as chlorofluorocarbons (CFCs) and some related compounds are potent greenhouse gases and also break down ozone in the stratosphere by releasing chlorine radicals. CH2ClF is a chlorofluorocarbon type compound that contains both chlorine and fluorine. Such compounds have been used as refrigerants and have known environmental impacts. In contrast, pure hydrocarbons like C2H2 and CH4 may act as greenhouse gases, but only CH4 is important in that role and does not contain chlorine or fluorine. PF5 and COCl2 have different uses and chemical behaviours and are not primary ozone depleting halocarbon greenhouse gases in the context tested here.

Step-by-Step Solution:
Step 1: Look for compounds that include chlorine (Cl) and fluorine (F), because such halogenated compounds are typical ozone depleting substances. Step 2: Among the options, CH2ClF clearly contains both chlorine and fluorine atoms. Step 3: Recognise that CH2ClF belongs to a family of halogenated hydrocarbons used as refrigerants and propellants, known to be greenhouse gases. Step 4: Note that C2H2 (acetylene) contains only carbon and hydrogen and is not a major ozone depleting gas. Step 5: Understand that PF5 and COCl2, while toxic and reactive, are not standard examples of ozone depleting greenhouse gases in high school environmental chemistry. Step 6: Conclude that CH2ClF is the correct answer.

Verification / Alternative check:
Environmental regulations, such as those under the Montreal Protocol, target substances like CFCs, HCFCs, and some related halogenated gases because they both trap heat and erode the ozone layer. These molecules often have formulas containing carbon, chlorine, and fluorine. CH2ClF fits this pattern. While methane (CH4) is a very important greenhouse gas, it does not contain halogens and does not directly release chlorine in the stratosphere. Thus only CH2ClF combines both key features highlighted in the question: greenhouse behaviour and ozone depletion potential associated with halogen content.

Why Other Options Are Wrong:
- C2H2: This is acetylene, a hydrocarbon used as a fuel gas in welding; it can influence local air chemistry but is not a typical ozone depleting greenhouse gas in the same sense as halocarbons.
- PF5: Phosphorus pentafluoride is a reactive gas used in specific chemical processes but is not a major greenhouse and ozone depleting gas taught in school level environmental chemistry.
- COCl2: Phosgene is a toxic gas used historically as a chemical weapon and in industry; it does not match the classic pattern of halogenated greenhouse gases driving ozone depletion.
- CH4: Methane is a strong greenhouse gas but contains no chlorine or fluorine and is not classed as a halogenated ozone depleting substance.

Common Pitfalls:
Students sometimes focus only on greenhouse gas status and might choose methane (CH4) because it is well known in that context, ignoring the ozone depletion part of the question. Others may look at any chlorine containing compound, such as COCl2, without noticing the emphasis on typical greenhouse and ozone depleting behaviour of halocarbons. Carefully reading for both clues, greenhouse effect and ozone depletion with halogen content, leads to the correct selection of CH2ClF.

Final Answer:
The gas that is a greenhouse gas and also depletes the ozone layer is CH2ClF.