Water, as we know, is polar with 2 positive points and 2 negative points. In the liquid phase, the attractions (hydrogen bonds) are constantly forming and breaking as the molecules move around each other.
As the temperature drops, the attractions become more effective (South Street Effect). Eventually, each water molecule forms 4 attractions with 4 neighbors. This is shown in the graphic below from the Lawrence Berkely National Laboratory (www.lbl.gov) on this page.
It is clear to see that in the ice crystal (on the right) the molecules are farther apart from each other than they are in the liquid phase. As a result the solid is LESS dense than the liquid.
If you are having trouble wrapping your head around that, imagine it this way. If you got a group of friends (lets say at least 6) you could hold hands in a big circle. If all of you were relaxed, you could come together in a tight clump while still holding hands. However, if each of you held your arms out at 110o, then you would be forced apart into an open circle.
Water works the same way. When the temperature is higher, the attractions are looser and the water molecules can move around. If the temperature drops, the attractions force the molecules to move into positions that allow "correct" angles. This forces the freezing water apart, making it less dense.
That means that if solid ice is placed under enough pressure, the molecules can be forced out of the crystal structure (ice) and pushed closer together becoming a liquid.
No comments:
Post a Comment