Pressure Melting of Ice: While-U-Wait
Context for Use
- 2-liter soda bottle
- 32-gauge rust-resistant craft wire
- Ice cube
- C-clamp (to mount fork to table or desk)
- Drainage collection pan
- Fill bottle with water.
- Make loop of wire to hang bottle on ice cube.
- Arrange mount on table to suspend forl above pan.
- Place ice cube (near room temperature) on fork.
- Hang full bottle from ice cube by fine wire loop.
- Ask students to post multiple working hypotheses as to the likely outcome.
- Wait ~15 minutes while discussing principles of pressure melting.
- Wire cuts through ice cube (pressure melting).
- Ice refreezes behind it (regelation).
- Melting cube can be picked up and held by one end.
- A septum of bubble-free regelation ice marks the healed track of the wire.
Teaching Notes and Tips
It requires about 30 minutes for the wire to cut through a standard ice cube. Finer wire would cut faster (but break even more frequently) as would a heavier load. Although the ice cube shrinks in the process, there is typically enough left to do a second pass. This might be described as testing the hypothesis that the ice heals itself because it is still cold from the freezer; the ice must be isothermal at 0°C after 30+ minutes exposed in a classroom!
The time while the demonstration is proceeding is best used in:
- posing multiple working hypotheses as to the outcome of the experiment,
- discussion of the various temperature regimes in glaciers and
- discussion of the mechanics of ice deformation (often demonstrated itself with Silly Putty).
- Is the ice too weak to resist the wire (100 kPa sheer strength)? If so, why did the ice heal?
- Does the ice heal as refreezing ecause of "residual cold" from the freezer? If so, why did it melt in the first place?
- Based on the wire diameter, ice cube width, and bottle height, what is the actual stress on the ice as a result of the attached weight?