Question

Why can `CO_2` be easily solidified?

Answer 1

The process of making `CO_2` actually speeds up the process of turning it from the liquid to the solid state.

Explanation:

In the dry ice industry (I hope there's one), before `CO_2` can be made into solid form (which is dry ice), it first has to become a liquid.

In reality, there are two ways of making dry ice:

The cheaper and more efficient way is by pressurising and cooling `CO_2` into a liquid. As the `CO_2` liquefies, the ambient pressure is reduced, allowing some of the liquid `CO_2` to vaporize.
As the `CO_2` vaporizes it causes a rapid decrease in temperature. This inverse temperature-pressure relationship allows the liquid `CO_2` that doesn’t evaporate to turn into a snowy substance that is compressed into dry ice blocks/pellets that you see.

The not-so-cheap but faster way is by increasing the pressure much higher than the above method and cooling the `CO_2` rapidly. This causes the `CO_2` to deposit, that is, from gaseous state directly to a solid state.

As for your statement about why `CO_2` is easily solidified, I believe it's just because humans have had more practice and done more research into solidifying `CO_2`.

Answer 2

It's not that easy to solidify it... In fact, I would say it is HARDER to solidify `"CO"_2` than water...

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Let's ACTUALLY look at the phase diagram:

How dry ice is usually made is that it is pressurized into a liquid at typical cold temperatures (`0^@ "C"` is doable), and then cooled down to cryogenic temperatures to get it into a solid.

That means we first move UP the phase diagram from a gas to a liquid, and then LEFT from a liquid to a solid.

It's not to say that is easy... but after releasing the dry ice into normal atmospheric pressure conditions, it spontaneously will sublime, crossing the solid-gas coexistence curve rightwards at the `(P,T) = (1color(white)(.)atm,-78.5^@ C)` point.