The molecules in the ice tend to separate and fly off, leaving the ice cubes to become smaller and smaller over time.
Everyone knows that ice melts, especially on a hot summer day when the ice pools at the bottom of the glass, before you can say ‘it’s hot’. However, what most people don’t realize is that ice melts in the freezer as well. Well technically the process is called ‘sublimation’.
Sublimation is a process in which a substance goes directly from the solid phase to a gaseous phase, completely skipping the middle phase, liquid. What we learn in basic science is that a substance goes from solid to liquid to gas, and vice versa. However, there are situations where sublimation occurs.
Ice forms when the water molecules are pressured enough to pack themselves closer together in order to eventually become the densely packed ice cubes. During this process, the molecules lose their kinetic energy which is what allows them to slow down and pack themselves in. The temperature of the freezers and the pressure of the cold air allow them to do this.
Freezers, especially in frost-free refrigerators are often extremely dry and completely devoid of moisture and humidity. These kinds of refrigerators operate by blowing cold air into the refrigerator and freezer boxes, in order to keep the air and the ice cubes evenly cool.
Now, when the ice is frozen and is below freezing temperature, there are times when some of the surface molecules of the ice gain enough kinetic energy to break free from the structure and fly off, i.e. sublimation. Usually, these molecules would stay close to the ice, lacking the energy levels required to go far. Also, they are usually locked in a box.
Under normal circumstances, these molecules would lose their energy and rejoin the ice. However, the air blowing in these frost-free refrigerators tend to dissipate these molecules. Hence, instead of rejoining the ice cube, they tend to fly to the other parts of the freezer and stick there. This is why there is random ice on the inside of the freezer, as well as ice crystals on nearly everything in there. Also, these molecules often escape when the freezer door is opened. Thus, the blast of cold air every time the door is open.
So, in short, the reason that ice cubes shrink in the freezer is due to sublimation and displacement of water molecules.