Of all substances on Earth, water is the most common.
Water is also the only substance on Earth that exists in all three states: solid, liquid and gas. The totality of these three states – all the earthly bodies of gas, liquid and solid – is known as the hydrosphere. The hydrosphere comprises the oceans, lakes, rivers, underground aquifers and groundwater, along with the North Pole and South Pole and all forms of ice and snow therein, plus gaseous water vapor found in the atmosphere.
There are numerous substances on Earth that dissolve easily into water.
Oceans, lakes, rivers, ice and other forms of water dissolve chemicals. One such chemical is table salt, which is composed of chlorine and sodium, and abbreviated NaCl. When table salt is added to water, the positive parts of the water molecule embrace the chlorine and the negative parts embrace the sodium. As a result, the salt molecule disappears and is dissolved. The opposite process – removing the water molecules from the salt – is known as desalination. In nature, this happens naturally when seawater evaporates, becomes clouds in the atmosphere and the salt remains behind in its molecular form. People use a similar means to make saltwater drinkable by boiling it to remove the salt crystals, then collecting the steam.
Almost all (approximately 97%) of the water in the hydrosphere is saltwater.
As mentioned above, the process of removing the salt from saltwater is known as desalination. One of the oldest means of making drinkable water involves evaporating saltwater and then collecting the steam, which condenses to form droplets of water. This process is called solar distillation, since it requires the sun to evaporate the water. From around 400 A.D. forward, the process evolved – from passive solar heating to actually boiling the water and using sponges to sop up the freshwater from the air above the pot.
The word “freshwater” means water that is less salty than the oceans.
There are two kinds of freshwater. One is moving water (e.g. rivers) and the other is stationary (e.g. lakes). Stationary freshwater bodies change dynamically over the seasons, influencing the lives of their plants and animals. In spring, after the ice has melted, water from the bottom of the lake warms and moves upward to mix with water at the surface, which is then made available to surface-dwelling plants such as phytoplankton. Toward summer, the phytoplankton is also supported by the lake’s decomposers, like fungi and bacteria, which begin breaking down the lake’s dead animals to provide more nutrients. Toward fall, daylight decreases, the lake cools off and plant life grows slower. In winter, the lake continues to cool, but the ice forming on the surface forms a kind of insulation for plant and animal life in the lower regions of the lake.
Water activity changes dramatically over the seasons for people.
Even frozen bodies of water attract fun seekers in winter – in the form of skating, skiing and ice fishing, to name a few. In ice fishing, the surface ice is cut to make a hole through which a fishing line is dangled below the ice. In warmer seasons, other fishing takes over. Fly fishing is popular on rivers and streams, wherein the fishing line is flicked back and forth across the river in an almost hypnotic manner both for the fisherperson and the fish. Spin fishing, often done from boats or other watercraft, involves attaching to the line weights that draw the hook and lure down deep into the water. Rowboats, sailboats and kayaks are propelled through the water by oars or sails that use wind energy to push them across the water. Speedboats travel across the water at high velocity due to the power of their motors. Another form of aquatic recreation is the enjoyment of thermal springs, which are created by the natural flow of groundwater that is heated geothermally – literally, by the natural warmth of the Earth. Also, shallow volcanoes can heat groundwater to make hot springs hot. One country famous for its hot volcanic springs is, ironically, Iceland.