Freezing Water

Most recent answer: 10/22/2007

Q:
What makes water freeze
- Jacob J. (age 13)
Liberty Christain MS, Bealeton, Va, USA
A:
Hi Jacob,

As you probably already know, ordinary matter is made of atoms and molecules. Water is made of molecules that are composed of one oxygen atom and two hydrogen atoms, and the molecules have a bent shape, kind of like a V (the oxygen atom is at the point of the V, and the hydrogen atoms are on the "legs" of the V).  If you examine a water molecule, you will find that it has a partial plus charge where the hydrogen atoms are and a partial negative charge where the oxygen atom is. The partial plus charge of the hydrogen attracts the partial negative charge of the oxygen, and this attraction (hydrogen bonding) makes the water molecules stick together. They stick together best when they form a regular pattern. That regular pattern is a rigid crystal: ice.

So why isn't water always frozen into ice?  If you could look at the individual molecules in any piece of matter, you would see that they sort of jiggle around, that is, they have kinetic energy. Molecules have less kinetic energy at lower temperatures and more kinetic energy at higher temperatures. At high enough temperatures, the molecules shake so much that they break away from the regular frozen pattern and melt. When they get even hotter, they can break away from sticking to each other at all and turn to a gas.


You may wonder why the change from liquid to solid is all-or -nothing. Why don’t the molecules just stick more and more as they get colder? We can sort of explain. In the solid, the molecules line up in a regular pattern, where each molecule just fits in a framework provided by its neighbors. A molecule can’t do that unless its neighbors are lined up properly. Once the neighbors are lined up, a molecule can lower its energy a lot by joining the pattern. So it becomes all-or-nothing.

Some materials, like window glass, are solid-feeling but don’t actually consist of regular patterns of lined up molecules. If my explanation is right, you would then not expect them to freeze abruptly but rather more gradually. That’s just what happens.


Thanks for your question!

(published on 10/22/2007)

Follow-Up #1: broken frozen pipes

Q:
When frozen water melts, does the warming water have enough energy to aid the damage of frozen pipes, due to a closed and pressurized compartment???---(pipes)

Plumbers of limited education believe this myth. I don’t.
please explain who is correct, and why.
- Rod Wellman (age 49)
Spokane, Washington,United States
A:
I guess you’re talking about the situation where just a little of the ice has melted. Once it’s all melted, the pressure should be back to normal. I’m not sure, but you could easily imagine that the first bit of liquid water, still under high pressure because the ice won’t fit in the pipes without being pressurized, might accelerate the rupture of pipes because the liquid can easily flow out into any crack as it forms. So I’m not positive, but it sounds like the ’plumbers of limited education’ are right.

BTW, the plumbers I’ve dealt with around here are very smart guys, with broad interests. Also, the one time my pipes froze and burst one was gracious enough to fix them right away, even though it was Christmas Eve. Also, they make a lot of money. One of the oddities of our culture is that people shy away from jobs like that. OK, admittedly that rant was off topic.

Mike W.

Water accelerates cracks in glass -- somehow the molecules get in between the rupturing surfaces and help the growth of a pre-existing crack.  I don’t know if the same effect happens in metals -- could be.

Tom

(published on 10/22/2007)