Gases are least likely to behave in an ideal fashion when the individual molecules experience intermolecular interactions. This is most likely to happen in conditions that force the molecules into close proximity—such as high pressures and small volumes—and when the gas particles do not have enough speed and energy to disperse and overcome IMF with other molecules—such as in low temperature situations. High pressure conditions would result in molecules in close proximity due to a decrease in volume. Low temperature conditions would also result in the molecules being in close proximity, as temperature is directly proportional to volume in the ideal gas equation and having less speed would allow more time for molecules to interact. Thus, the correct answer is (D).
Expert Approach:
An "ideal gas" is assumed to be composed of particles that themselves have no volume, and do not attract one another. Real gases approximate ideal behavior except under certain extreme conditions, when they deviate. For example, at super-high pressures molecules are squeezed very closely together, and start running out of room; it becomes a lot harder than expected to further compress such a gas. Conversely, attractive forces between molecules are relatively insignificant when velocities are high, but at low temperatures, when molecules move relatively slowly, attractive forces between them become more noticeable, and they may even begin to clump together. Because I'm familiar with these ideas, I look proactively for an option that says high pressure and low temperature and pick (D).If my content knowledge were somewhat lacking, I wouldn't just guess randomly though. Instead I would try to leverage the knowledge that I did have. For example, I might think about the fact that if temperature is sufficiently low, a gas spontaneously transforms into a liquid (or solid), which definitely doesn't obey the ideal gas law. As temperature approaches this condensation point, therefore, a gas should deviate increasingly from ideal behavior. This insight eliminates (B) and (C). Alternatively, I could imagine trying to compress a real gas to the point that its molecules were virtually touching each other. Doubling the pressure in that situation clearly wouldn't cut the volume in half, and this realization eliminates (A) and (B).One last observation: I know a lot of people will get this question right because they've memorized that "gases behave most ideally at high temperature and low pressure." And some things (not this one!) just have to be memorized. Whenever possible, though, I try to avoid learning content that way because:
- It's a lot easier to remember (or even reconstruct) concepts you understand than stuff you've merely memorized.
- Memorized knowledge can be brittle—it's harder to apply it to new situations and easier to misremember.
- Memorized knowledge has little context—it doesn't fit within a matrix of related concepts that reinforce each other.
- In the long run, memorization usually uses up more brain power than understanding, not less.
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