Physics · Thermal Properties Of Matter · NEET
No. Evaporation is a liquid slowly turning into vapour at its surface. Sublimation skips the liquid stage completely: the solid turns directly into vapour. In sublimation the solid and vapour states can coexist in thermal equilibrium, just like solid and liquid coexist during normal melting.
Dry ice is solid carbon dioxide (CO2). At normal atmospheric pressure, liquid CO2 cannot exist. So when solid CO2 is warmed at room pressure, it goes straight to vapour. That is why it is called dry ice, and this direct solid-to-vapour change is sublimation. Iodine behaves the same way.
In the NCERT activity, a wire with heavy blocks on both ends is hung over an ice slab. The wire presses hard on a thin line of ice. High pressure lowers the melting point of ice, so the ice right under the wire melts and the wire sinks in. Above the wire the pressure is normal again, so the water re-freezes. The wire slowly cuts through, yet the slab stays a single solid block. This melting under pressure and re-freezing is called regelation.
For ice, higher pressure LOWERS the melting point. This is special to water because ice is less dense than water (water expands on freezing). So squeezing ice helps it melt. For most other substances, higher pressure instead raises the melting point.
It is one common textbook reason. The thin blade puts a large pressure on a small area of ice, which can lower the melting point and form a thin water layer that acts as a lubricant. When the skate moves on, the pressure drops and the water re-freezes. This melt-then-refreeze cycle is regelation. (Friction heating also plays a role.)
Sublimation is a change of STATE: solid to vapour directly, driven by heat, with no liquid stage. Regelation is a PRESSURE effect on ice: pressure melts the ice, then removing the pressure re-freezes it. Sublimation does not need extra pressure; regelation is all about pressure changing the melting point of ice.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Dry ice (solid CO2) turning into CO2 vapour, and solid iodine turning into purple iodine vapour on gentle heating. Naphthalene balls (used against moths) also shrink by sublimation.
Yes. Just as solid and liquid coexist at the melting point, during sublimation both the solid and the vapour states of the substance coexist in thermal equilibrium.
On the pressure-temperature (P-T) phase diagram, the sublimation curve separates the solid region from the vapour region. Points on this curve represent states where solid and vapour phases coexist.
Because ice is less dense than liquid water, higher pressure lowers ice's melting point, so pressure can melt it. Most substances become denser as solids, so pressure raises their melting point and regelation does not happen the same way.
Yes. To break solid bonds and go straight to vapour, the substance must absorb latent heat, just like melting and boiling need latent heat. The temperature stays constant while the solid sublimes.
