Chemistry Notes on H2O: Essential Properties of Water

Water is the only substance known whose solid form is less dense than its liquid form. As it is cooled, it is most dense at 4 degrees celcius, and as it turns to ice the particles spread further apart, resulting in ice. It is very important that ice is less dense than water, because it prevents lakes from freezing solid and killing everything in them. The layer of ice on top insulates the lake from the extreme cold of the air.

Hydrogen Bonds

The structure of water is that of two hydrogen atoms bound to an oxygen atom in a bent formation. The oxygen is more electronegative than the hydrogens, so the electrons spend more time near the oxygen. As a result, the oxygen has a partial negative charge, while the hydrogens have partial positive charges. The name for this time of molecule is polar. These are weak bonds, but their large number can overcome their strength.

Water molecules can stick to other polar substances through a process called adhesion. This explains why water forms a meniscus in a glass graduated cylinder. The molecules bind to the glass, which is also polar, and the edges move upward so that more molecules can bind to the glass. Plastic is non-polar, so a meniscus will not form in a plastic graduated cylinder.

Water molecules also bind to others in a process called cohesion. This explains how insects can float on top of lakes and rivers, because the molecules are held together to form surface tension. Cohesion also accounts for the ability of trees to pull water from the roots all the way to the top of the tree. The cohesion of the particles makes it like pulling a string, because they are all joined to each other.

Cooling by Sweating

The reason that you are cooled by sweating is the high heat of vaporization of water. Heat from your skin is used to heat the sweat enough to make it evaporate, or vaporize, so you are cooled in the process. The reason that high humidity makes it feel hotter is that there is already so much water vapor in the air that less of it evaporates from your skin. This results in less evaporative cooling.

Cooler by the Lake

Large lakes and oceans help to keep places cooler because it takes a lot of energy to change the temperature of water. This is called the specific heat, or the energy required to change a given amount of it by one degree. The only compound with a higher specific heat than water is ammonia.