Ice Melting in air and Cotton
Most recent answer: 10/22/2007
Q:
Why does ice melt faster in air than surrounded by cotton?
- Drew (age 7)
Val Verde Elementary, Perris, CA
- Drew (age 7)
Val Verde Elementary, Perris, CA
A:
Cotton is light and fluffy -- it is mostly air already! But cotton also
has lots of tiny fibers which are tangled up. These fibers keep air
from flowing freely through the cotton.
When ice melts in air, cold air sinks and warm air rises to take its place because cold air is more dense than warm air (stuff shrinks when it gets cold, even air!). This constant circulation of warm and cold air brings new batches of warm air in contact with the ice, allowing a faster heat transfer to the ice (the heat transfer rate is proportional to the difference in temperatures of the hot and cold objects - in this case, air and ice).
In the case with cotton, the air cannot flow, and the air trapped inside the cotton near the ice becomes as cold as the ice is and stays put. New hot air cannot replace it to melt the ice faster, and the ice stays cold longer.
The cotton may get wet, and there will still be some heat flow through it, so the ice will eventually melt.
Tom
When ice melts in air, cold air sinks and warm air rises to take its place because cold air is more dense than warm air (stuff shrinks when it gets cold, even air!). This constant circulation of warm and cold air brings new batches of warm air in contact with the ice, allowing a faster heat transfer to the ice (the heat transfer rate is proportional to the difference in temperatures of the hot and cold objects - in this case, air and ice).
In the case with cotton, the air cannot flow, and the air trapped inside the cotton near the ice becomes as cold as the ice is and stays put. New hot air cannot replace it to melt the ice faster, and the ice stays cold longer.
The cotton may get wet, and there will still be some heat flow through it, so the ice will eventually melt.
Tom
(published on 10/22/2007)