November/December 2008: What are water's special qualities, and how do they work?

Ice floats and expands when it freezes. Why? Because ice crystals stretch water's normal 105° angle and "open it up" a bit when trying for the perfect 120° hexagonal angle. This reduces the "closeness" (density) of the water molecules so ice is lighter than water. This is also why ice expands when it freezes and why snowflakes always have six sides.

Salt water boils at a higher temperature and freezes at a lower temperature than fresh water. Why? Because when salt is dissolved in water it reduces the fraction of water in solution and hence the natural random tendency of the water molecules to fly out into the air (vapour pressure). You need to raise the temperature to make the water boil (vapour pressure = air pressure). So salted water boils about 5-10° hotter and cooks things faster.

Salt melts ice for the same reason. You need to lower the temperature to get salty water to freeze.

Water tension makes water elastic. Each water molecule is one oxygen atom connected to two hydrogen atoms. The oxygen is slightly negatively charged while the hydrogens are positive. Opposites attract, so this makes water molecules more "sticky" towards each other. The "front" of one molecule is attracted to the "back" of another. It makes water elastic so that it can form bubbles and drops or be pulled up by plant roots and blood vessels.

Creatures can live under ice! Ice has a density about 93% of water so it floats on top. Deeper water below the ice stays at 4°C because the colder water rises due to its lower density. So at the bottom of a frozen lake, stream or pond, it's 4°C, warm enough for life-or hibernation at least.

-originally posted here by Dr. Barry Shell, Simon Fraser University