the answer files

Why is the sky blue?

What is lightning?

What are the states of matter?

Does hot water freeze faster than cold?

Why does a can of Coke sink while a can of Diet Coke floats?

Why do you close your eyes when you sneeze?

Why do mosquitoes bite me more than my friends?

Why do your feet stink?

How does gravity work?

How fast does the space shuttle move in orbit?

What is escape velocity?

What would happen if you turned on your headlights while moving at the speed of light?

Does hot water freeze faster than cold?
No. But it could seem that way. Why? Because it takes a lot more cooling to make ice from water that's at the freezing point (0 degrees Celsius) than to cool water from, say, 1 degree to 0 degrees.

are you sure?
Notice how the graph stabilizes at 0 degrees Celsius. Water does not freeze until the heat of crystallization is removed.

To change matter from the one state to another, you must add or remove heat energy. To cool one gram of water by one degree Celsius, you must remove one calorie of heat energy. But after the water reaches 0 degrees Celsius, freezing requires the removal of almost 80 calories -- the heat of fusion. Similarly, you must add almost 80 calories to melt one gram of ice at 0 degrees Celsius.

Why can it seem that hot water freezes faster than cold water? Because the bulk of the cooling serves to remove the heat of crystallization. So to freeze one gram of water at 80 degrees C, you must remove 160 calories, only twice as much as you need to freeze one gram at 0 degrees.

back more
The Why Files
.
There are 1 2 3 4 5 6 7 8 9 10 11 12 pages in this feature.
Credits | Feedback | Search

©1998, University of Wisconsin, Board of Regents.