Chemistry Question #70

Anonymous, a from the Internet asks on September 26, 1999,

What are water's special qualities, and how do they work?

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The answer

Barry Shell answered on September 26, 1999

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.

The geometry of oxygen's outer electron orbits bends water's two hydrogen atom bonds at about 105°. As water crystallizes, neighbouring molecules link together with hydrogen bonds between the O's of one and H's of another water molecule to form hexagonal 6-sided crystals.

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. Water is most dense at 4°C. 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. Did you know:

  • Water is the only naturally occurring substance that exists in all three states-gas, liquid and solid-at normal Earth temperatures.
  • Water dissolves more things than any other liquid.
  • Water's polarity dissolves salts very well and is the reason the sea is salty and the basis for blood, nerves, roots, sea shells, rock formations and a lot more.
  • Blood is exactly as salty as sea water. That's 3.47% salt.
  • Fresh water freezes at 0° Celsius. Really salty water (a saturated solution) freezes at 0° Fahrenheit.
  • The Pacific ocean is higher than the Atlantic because the Atlantic ocean is saltier, hence heavier and therefore lower.

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