Does dry ice melt or evaporate?

Dec 1, 2016

At room temperature and atmospheric pressure, neither one! It skips past the liquid phase at room temperature.

Here's what the phase diagram of ${\text{CO}}_{2}$ looks like:

At $\text{298.15 K}$, room temperature, and $\text{1 bar}$ of pressure, we can see that ${\text{CO}}_{2}$ is a gas. Thus, when we take dry ice, we had stored it below its sublimation point, $\text{199.65 K}$ ($- {78.5}^{\circ} \text{C}$).

When exposed to room-temperature conditions, it thus sublimates, i.e. it spontaneously crosses the solid-gas equilibrium line on the above phase diagram and becomes a gas.

MELTING?

If we were to want dry ice to melt at room temperature, we would have to first increase the pressure to at least $\text{100 bars}$ or so. In those conditions, it would be just to the left of the critical point, but it would still be a liquid.

But if one raises the temperature just a few more degrees, one would obtain a supercritical fluid, which means the liquid and gas phases are no longer distinguishable.

That is, initially one would see bubbles in a liquid, but eventually, the bubbles would disappear as the fluid becomes uniform.

EVAPORATING?

if we were to want dry ice to evaporate at room temperature, we would have to decrease the pressure slightly from $\text{100 bars}$, where it was a liquid at room temperature previously.

So, under normal conditions (i.e. $\text{1 bar}$ and $\text{298.15 K}$), we do not see dry ice evaporate or melt, only sublime.