Water

The strong attractions that water molecules have for each other are responsible for many of water's highly unusual properties, as compared with other liquids of about the same molecular weight. Among these are:

1. Its unusually high boiling point (if it were similar to the other liquids, it would be a gas at room temperature).
2. Its high heat of vaporization (the amount of heat it takes to change the liquid to a gas).
3. Its high heat of fusion (the amount of heat it takes to melt solid ice).
4. Its high heat capacity (the amount of heat it takes to raise its temperature by a certain amount).
5. Its world-champion rank among liquids as a solvent (it has been called the universal solvent because it dissolves so many different substances).
6. The low density (the lightness) of ice, which makes it float on the surface of liquid water. As water is cooled to make ice, it gets slightly denser, like all liquids. But at 39.2°F (4°C) it reaches its maximum density; when cooled below that temperature, it gets less dense until it reaches 32°F (0°C), at which time it freezes and suddenly decreases to 91.7% of the density of the water. Being less dense than the water, the ice floats.

The normal boiling point of water is 212°F (100°C), and its freezing point is 32°F (0°C). In fact, zero and 100 degrees on the Celsius scale are defined as the freezing and boiling points of water. Water is also the standard by which many other quantities are measured. For example, the density of a material is often expressed as its specific gravity or specific weight: how many times denser it is than water.

In pure water, one out of every 555 million molecules is broken down—dissociated—into a hydrogen ion and a hydroxide ion:

These ions are enough to make water a slight conductor of electricity. That is why water is dangerous when there is electricity around. The slight dissociation of water is responsible for the acid and base balances of all of the chemical reactions that take place in water, and that includes almost all the chemical reactions that take place anywhere, including those in the human body. Acid-base balance is probably the most important single factor that affects chemical reactions.