A Sweet Briar College Learning Resource

H2O - The Mystery, Art, and Science of Water

The Chemistry of Water
Professor Jill Granger

Properties

IMAGE SOURCE: "Chemistry in Context" Wm C Brown Publishers, Dubuque Iowa, 2nd edition, A project of the American Chemical Society, ed: A. Truman Schwartz et al., 1997, Chapter 5 "The Wonder of Water"


Water is Weird !?

Chemically speaking, water is very weird. It doesn't behave at all like it should. Let's consider somethings you know:


Ice Floats. That's not weird.... is it?

That's very weird. The solid state of most things are much denser than the liquid state and therefore sink. Usually what happens when a solid is formed is that the molecules become more tightly packed together. When things melt, the molecules move apart and get liquid. But water is weird - the solid state is less dense than the liquid. To understand why we'll have to take a close-up look at the molecular arrangement of solid water (ice) and liquid water.


The Structure of Ice

IMAGE SOURCE: "Biochemistry", second edition, by D. Voet and J.G. Voet, John Wiley and Sons, Somerset NJ, 1995, Chapter 2 "Aqueous Solutions", pg 31


Water boils at 100°C
and freezes at 0°C.

That's certainly not unusual.

Oh, Yes it is! Did you know that the Celcius temperature scale was based on the two physical changes of water? That wasn't done because water has typical chemical behavior, only because water is a familiar substance. You may have realized too that the boiling point of water is not always "100 °C". Ever read Cake Mix Directions? They give different directions for "High Altitudes". That's because many physical changes depend on pressure. The boiling point of water depends on the pressure of the air around it:

Let's compare the boiling of water with some other chemically similar substances.


IMAGE SOURCE: "Chemistry in Context" Wm C Brown Publishers, Dubuque Iowa, 2nd edition, A project of the American Chemical Society, ed: A. Truman Schwartz et al., 1997, Chapter 5 "The Wonder of Water"

Water is way out of line! It boils at an extremely high temperature for its size. Why? Because of the extensive network of Hydrogen bonds. Those H-bonds are cohesive forces - they want to hold the water molecules together - and there are a lot of them! The process of boiling requires that the molecules come apart: a process that takes a lot more energy than expected.


What's unusual about the freezing point?

The freezing point is much higher than expected again because of the hydrogen bonding. To get the water molecules to undergo the transition from liquid to solid is relatively easy. Liquid water has only 15 percent more H-bonds than solid water.


How am I affected by these temperature - phase relationships?

If water were "normal", it would be a gas at room temperature. No lakes, no rain, no body fluids!


Is there anything else?

Another result of the Hydrogen bonding network is that water has a very high Specific Heat. This is like the baked potato effect. Once heated, water takes a very long time to cool off. Or in reverse, it takes a lot of heat to make water hot. Compare the specific heat of water to some other common substances:

You've noticed and used water's high heat capacity yourself.


How's that?

  • Lake Effect Snow
  • Sweating
  • Coolant for engines

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CONTENTS INTRODUCTION PURPOSE SCHEDULE REQUIREMENTS PARTICIPANTS


H20 - The Mystery, Art, and Science of Water
Chris Witcombe and Sang Hwang
Sweet Briar College