# Video: Identifying the Name of the Temperature and Pressure at Which Gas, Liquid, and Solid Phases Coexist in Equilibrium in a Set of Names

Which of the following is the temperature and pressure at which the gas, liquid, and solid phases of a particular substance can coexist in equilibrium? [A] Sublimation [B] Condensation [C] Critical point [D] Deposition [E] Triple point

02:55

### Video Transcript

Which of the following is the temperature and pressure at which the gas, liquid, and solid phases of a particular substance can coexist in equilibrium? (A) Sublimation, (B) condensation, (C) critical point, (D) deposition, or (E) triple point.

This question is asking us about when the gas, liquid, and solid phases of a substance are in equilibrium with each other. This might occur if we had a sealed container of water that contained both liquid water, solid water or ice, and water vapor above it. When we have all three phases, solid, liquid, and gas, in equilibrium with each other like this, it’s called the triple point. The triple point for water occurs at a temperature of 273 kelvin, or zero degree Celsius, and a pressure of 0.006 atmospheres.

We can determine the triple point of a substance by looking at the temperature–pressure phase diagram for a substance, which tells us which conditions, in terms of pressure and temperature, a substance will be a solid, liquid, or a gas. This is the temperature–pressure phase diagram for water. We can see the triple point occurs where the solid, liquid, and gas phases all intersect. So the correct answer for this question is answer choice (E), the triple point. But let’s go ahead and look through our other answer choices just so we know what they are.

Answer choice (A) or sublimation is the process of going directly from a solid to a gas. On our phase diagram, the process of sublimation would be crossing from the solid region of the phase diagram to the gaseous region of the phase diagram, like I’ve marked with this green arrow. Condensation is the process of going from a gaseous substance to a liquid substance, which we can see on the phase diagram is going from the gaseous region to the liquid region, somewhere along the line that I’ve marked in pink.

The critical point is the end point on the pressure–temperature phase diagram between the gas and the liquid phases. Past this point, the liquid phase and the gas phase have the same density. You can no longer compress a gas into a liquid. The substance here usually has strange properties and is often referred to as a supercritical fluid. Our final answer choice is deposition, which is the process of going from a gas to a solid. This is the opposite of sublimation, so it would occur by crossing the gas–liquid interface in the opposite way on the phase diagram, like I’ve marked in blue.

But as we’ve discussed, this question was interested in the temperature and pressure where a gas, liquid, and solid phase of a substance can all exist in equilibrium, which is the triple point.