Water

Special Properties of Water

Water's polarity and its ability to form hydrogen bonds give it special properties, including adhesion, cohesion, and the ability to moderate temperature.
The polar nature (oppositely charged ends) of water allows for hydrogen bonding (attraction between polar molecules) between water molecules, which gives it several special properties that make life possible. While hydrogen bonds are weak compared with covalent bonds, the attraction is sufficient to cause cohesion, which is the tendency of two similar molecules to stick together. Adhesion, which is the tendency of two dissimilar molecules to stick together, is another property of water. Cohesion is responsible for water's high surface tension and is part of the reason why water beads up instead of spreading out over a surface. Surface tension also allows small organisms to rest or lay eggs on water. Cohesion and adhesion also allow trees and other tall plants to move water up against gravity from roots to leaves. In this case, the water molecules not only stick to each other, but also adhere to the sides of the tiny tubes inside of roots and stem. As water evaporates from leaves of the plant, the escaping water molecules pull others up through the stem from the roots.

Adhesion and Cohesion of Water Molecules

Water molecules use the properties of cohesion (attraction to other water molecules) and adhesion (attraction to surfaces) to move up a plant stem from the roots to the leaf tips. As water evaporates from leaves, the escaping water molecules pull on the attached strings of molecules stretching down to the plant's roots. These properties allow tall plants to exist outside of water.
Another special property of water is its ability to moderate temperature. Water has a high specific heat, which is the amount of energy in calories required to raise the temperature of 1 gram of a liquid by 1 degree Celsius. The hydrogen bonds between water molecules require energy to break, so the temperature of water does not change easily. If enough energy is put into water, the hydrogen bonds will eventually break. The separated water molecules will vibrate, but the heat does not immediately dissipate because the neighboring hydrogen bonds are still intact. This is why water boils much more slowly than alcohol, for example. Similarly, when water cools, heat is released from the molecules as each hydrogen bond is reformed. So, cooling is a slow process as well. This slow temperature change in response to the input or output of energy helps to keep Earth's oceans and atmosphere within a livable range of temperatures during the day or night and summer or winter. It also leads to a more gradual changing of the seasons as opposed to a sharp shift. Solar energy enters water but does not immediately break the hydrogen bonds. During the summer in some parts of the Northern Hemisphere, the air temperature may reach 30°C (86°F), but the water is always much colder, rarely reaching temperatures of 21°C (69.8°F) and not until late summer. Conversely, when air temperature starts to drop in the fall, the water retains its heat much longer, staying at 20°C (68°F) or so until late October.

When winter arrives, ice forms on the surface of ponds and lakes and remains there because ice floats. Hydrogen bonds lock into place in solid ice, holding water molecules apart, which makes ice less dense than liquid water. While the upper portions of the lake may be solid, the lower parts often stay liquid, allowing fish and other organisms to survive the colder months. During the spring thaw, the ice melts into dense, cold water, which sinks to the bottom, stirring up the body of water. This mixing allows oxygen and nutrients to be reintroduced, benefiting the organisms that live in the water.