Question

You can make a snowball by packing snow together using hand pressure. The effect of this pressure increase is to partially liquefy the ice, which then re-freezes to hold the snow together. By comparing the phase diagrams of water and CO₂, determine whether it is possible to make analogous "dry ice balls."

Answer 1

No; it is not possible to make analogous dry ice balls.

The answer to this lies in the phase diagram of water and CO2. The phase diagram shows the behaviour of the three phases of a substance when they are in equilibrium.

From the phase diagram of water; it can be seen that the solid liquid equilibrium line has a negative slope. In other words; if we increase the pressure; the freezing point of water will decrease; means it will freeze below 0 oC. That is why; when we apply pressure by our hands, it manifests itself to partially liquefy the ice, which then re-freezes to hold the snow together.

But this is not the case with CO2. In this case; the solid-liquid equilibrium line is positively sloped. This means that increasing the pressure will increase the melting point and hence solid CO2 will not liquefy. As a result; moulding it as per our wish will not be possible and hence dry ice balls cannot be made.