Water
Water
Water is a liquid that is essential to life and plays many important roles. Water molecules (small particles) are the most common molecules in the body of every organism. Water is the ideal medium for transporting essential nutrients into the body and for transporting waste products out of the body.
Water is a chemical compound made up of one oxygen atom bonded to two hydrogen atoms (H2O). Without water, it is safe to say that there would probably be no life on Earth. Virtually all of the chemical reactions that occur in every organism take place in some sort of a watery environment. Because it is an excellent solvent (meaning that it has the ability to dissolve many different substances), water plays a key role in life. Water is the best way to move substances into and out of cells because of its ability to be a "universal solvent." Since many substances dissolve in water, they are able to cross cell membranes and enter cells. Water also is the major constituent of all living things. If all the molecules in an organism were sorted and counted, the majority would be water molecules. In some living things, this percentage is as low as 60 percent water, but in some ocean animals like the jellyfish, the percentage is closer to 90 percent. The human body is about 65 percent water.
Humans know that they cannot live without drinking water. Although some people can go as long as six weeks without any food, without water they would die in a matter of days. Water is the primary component of blood, lymph (a colorless liquid that bathes the tissues of the body), and the fluids in all of the body's tissues. The body regulates its temperature with water by perspiring, and eliminate wastes with water.
Water is just as essential to plants as it is to all animals. Water is a key raw material that plants use during photosynthesis (when they make their own food). During photosynthesis, plants use the hydrogen atoms from water molecules (H2O) and add them to carbon atoms to make carbohydrates (a group of naturally occurring compounds that are essential sources of energy for all living things) and other forms of food. The oxygen molecules (O2) given off during photosynthesis come from water molecules.
Water is a unique substance with special properties. Water is able to absorb a great deal of heat, and on a global scale (that is, on something the size of Earth), it can act as an enormous moderating influence. When Earth's surface temperatures begin to rise too high, the oceans act like thermal sponges and soak up much of the excessive heat. Water also has a high surface tension, which means that its molecules closely stick together. This explains why it is able to support a broad, flat object, like a boat, and why certain insects can glide across its surface.
Water is also the only common, natural substance that is able to be found in three different physical states. Ice is its solid state, and water assumes a rigid crystalline structure at or below 32°F (0°C). It is found as snow, hail, frost, glaciers, and ice caps. Its liquid state has no particular shape and exists up to a temperature of 148°F (64.4°C). Water in this form covers three-fourths of the Earth's surface in the form of oceans, lakes, rivers, swamps, as well as rain clouds, dew, and ground water. The gaseous state of water occurs above 148°F and does not have a definite volume. This means it takes on the exact shape and volume of its container. It occurs naturally as fog, steam, and clouds.
Because of water's unique hydrogen bonding, ice is less dense than water. If this were not so and ice did not float, then all bodies of water would freeze from the bottom up, becoming solid masses of ice and destroying all life in them. Water also is a major force for changing the Earth's surface, as erosion regularly carves away softer parts and can cause major alterations and changes in the Earth's natural features. The major use of water by humans is for crop irrigation, although a great deal is used industrially, as well as to keep individuals, homes, and communities clean. Since much of the water used to irrigate crops either evaporates or is given off by plants, humans are a major factor in the hydrologic or water cycle. The water cycle describes the process in which water circulates between the sea, the atmosphere, living things, and the land in a continuous cycle