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.