What Water Is
Water is a molecule consisting of two hydrogen atoms bonded to a single oxygen atom. Its chemical formula — H₂O — is possibly the most widely known formula in science, learned by children and chemists alike. But the simplicity of the formula conceals the strangeness of the substance. Water is, by almost every chemical measure, an anomaly.
Most molecules of similar size and weight are gases at room temperature. Water is a liquid. It has an unusually high melting point, boiling point, and surface tension. It absorbs and releases heat more effectively than almost any other common substance. Each of these properties is a consequence of hydrogen bonding — the weak but numerous attractions between water molecules — and each of them is, in some way, responsible for life as we know it.
Why Ice Floats
The water you drink may have passed through the bodies of dinosaurs, fallen as rain in the Carboniferous period, or been frozen in a glacier for a hundred thousand years.
When most substances freeze, their molecules pack more closely together and the solid sinks in the liquid. Water does the opposite. As water cools toward freezing, it initially contracts as expected. But below 4 degrees Celsius, it begins to expand, as the hydrogen bonds between molecules lock into the hexagonal lattice structure of ice. Ice is less dense than liquid water. It floats.
This is not a minor curiosity. If ice sank, lakes and oceans would freeze from the bottom up. The frozen layer would insulate nothing below it; the water above it would freeze too. Bodies of water in cold climates would freeze solid. The aquatic ecosystems that exist beneath the ice of frozen lakes — ecosystems that support enormous chains of life — would not exist.
Water also has at least twenty known solid phases — different crystal structures that form under different conditions of pressure and temperature. The ice familiar to us, ice Ih, is the form that exists at ordinary atmospheric pressure. Ice VI, which forms under pressures exceeding ten thousand atmospheres, is denser than liquid water and would, in fact, sink. Somewhere in the interiors of large icy moons, this kind of ice may exist in bulk.
Water and Life
Life, as far as we know it, requires liquid water. This is not simply a statement about the chemistry of Earth life; it reflects the specific properties of water that make it useful as a solvent for biological processes. Water dissolves an enormous range of substances, transporting nutrients into cells and waste products out of them. It participates directly in many biochemical reactions. It regulates temperature in living organisms through sweating and evaporative cooling.
The search for life elsewhere in the universe is, to a great extent, a search for liquid water. Wherever liquid water exists on other worlds — in the subsurface oceans of Europa and Enceladus, in the ancient riverbeds of Mars, perhaps in the clouds of Venus — the possibility of life is considered worth investigating.
The Water Cycle
On Earth, water moves in a continuous cycle between the oceans, the atmosphere, and the land. The Sun heats the surface of the oceans, causing water to evaporate. Water vapour rises into the atmosphere, cools, condenses into clouds, and falls back to the surface as precipitation — rain, snow, hail, or sleet. It flows across the land in rivers, percolates into the ground to become groundwater, and eventually returns to the oceans.
This cycle has been operating for billions of years. The water you drink may have passed through the bodies of dinosaurs, fallen as rain in the Carboniferous period, or been frozen in a glacier for a hundred thousand years. Water is neither created nor destroyed in ordinary conditions on Earth; it is only moved, transformed between its phases, and moved again.
Scarcity
Approximately 71 per cent of Earth's surface is covered with water. Of this, 96.5 per cent is the salt water of the oceans. Of the remaining 3.5 per cent, most is locked in ice caps and glaciers. Less than one per cent of all water on Earth is accessible freshwater — in rivers, lakes, and shallow groundwater — and this fraction supports almost all of humanity's agricultural, industrial, and domestic water use.
Water scarcity is among the most serious challenges facing the coming century. Population growth, agricultural demand, industrial use, and climate change are altering the distribution and availability of freshwater. Aquifers built up over thousands of years are being depleted in decades. Rivers that once reached the sea no longer do. The Colorado River, which carved the Grand Canyon, now rarely reaches the Gulf of California.