# Video: Identifying the Name of the Lowest Temperature and Pressure at Which the Liquid and Vapor of a Substance Have the Same Density in a Set of Names

Which of the following is the lowest temperature and pressure at which the liquid and vapor of a substance have the same density? [A] Sublimation [B] Condensation [C] Critical point [D] Deposition [E] Triple point

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### Video Transcript

Which of the following is the lowest temperature and pressure at which the liquid and vapor of a substance have the same density? (A) Sublimation, (B) condensation, (C) critical point, (D) deposition, or (E) triple point.

This question is asking us when liquid and vapor of a substance have the same density. Specifically, it’s asking us about the temperature and pressure when this occurs. So to answer this question, let’s take a look at the temperature–pressure phase diagram for a substance. This particular phase diagram is for water or H2O. A phase diagram shows us which temperature and pressure a substance will be a solid, a liquid, or a gas. There is an endpoint on the line between the liquid and the gas phase on the phase diagram here, which is called the critical point. Past this point, the liquid and the gas of the substance will have the same density. And the gas is no longer able to be compressed into a liquid.

When a substance exist past this critical point, it’s referred to as a supercritical fluid. Supercritical fluids often have unusual properties for that substance. For example, H2O when it exists as a supercritical fluid is nonpolar, even though H2O in any other phase is a polar substance. And supercritical carbon dioxide is used to make decaffeinated coffee. So it looks like answer choice (C), the critical point, is the correct answer to this question. But let’s take a look at the other answer choices just so we know what there are.

Sublimation is the process of going from a solid to a gas. In our pressure–temperature phase diagram, this would occur when the substance crosses the line in between the solid and the gaseous phase like I’ve marked with this green arrow. Condensation is the process of going from a gas to liquid. On our phase diagram, that would occur when the substance crosses the line between the gas and the liquid phases as I have marked in blue.

Deposition is the opposite of sublimation. It’s when a substance goes from a gas to a solid. On our phase diagram, this would occur when the substance crosses the line in the opposite direction as we did in sublimation. The last answer choice is the triple point, which is when the solid, liquid, and gaseous phases are all in equilibrium with each other. On the phase diagram, this occurs when the lines between the solid, liquid, and gaseous phase all intersect.

But this question was asking about the lowest temperature and pressure where a liquid and vapor of a substance will have the same density, which occurs at the critical point.