Water H2O
The commonest molecular compound on Earth; a liquid, freezing to ice at 0°C and boiling to steam at 100°C. It covers about 75% of the Earth's surface, and dissolves almost everything to some extent. It is essential to life, and occurs in all living organisms. It is strongly hydrogen-bonded in the liquid phase, and co-ordinates to dissolved ions. Unusually, the solid is less dense than the liquid; this results in ice floating on ponds, and accounts for the destructiveness of continued freezing and thawing. Water containing substantial concentrations of calcium and magnesium ions is called 'hard', and is 'softened' by replacing these ions with sodium or potassium, which do not form insoluble products with soaps.
Water Pollution
Water pollution affects oceans, streams, rivers, lakes, ponds, and groundwater, and can be caused by natural impurities or human activities that pollute the nearby water or water supplies.
Natural impurities in water are sometimes, but not always, pollutants. They are divided into three categories of particles: suspended particles that absorb light and make water cloudy, such as beach sand, coal dust, and bacteria; colloidal particles, such as soot and some viruses, which cannot be removed from water by ordinary filtration and cause the water to look cloudy when observed at right angles to a beam of light; and dissolved matter, which are the 'smallest impurities in water, including molecules and ions of various substances, such as chloride or sodium ions or carbon dioxide molecules.
Human activities are often the cause of localized water pollution, as water becomes contaminated with heavy metals, toxic chemicals, and bacteria. Rivers may experience oil and chemical spills, untreated sewage runoff from homes and industry, and nonpoint source pollution, such as contaminated runoff from highways, parking lots, and agricultural fields. Groundwater (or subsurface water) may be contaminated by the infiltration of pollutants from landfills and septic tanks, or by percolation of water containing contaminated runoff. Parts of the ocean are sometimes polluted by oil tanker spills and garbage dumping.
Some 1.2 billion people lack access to clean water, twice that number have no sanitation, and most of the world will not have enough water within 30 years. This combination of scarcity and bad management affects food supplies, health, education, nature and economic development. It means women spend long periods collecting it, families spend up to half their daily income on it, farmers lose their land, and infants die.
Global consumption of freshwater is doubling every 20 years and new sources are becoming scarcer and more expensive to develop and treat.
In 1996, says the UN, humanity used about 54% of all the accessible freshwater contained in rivers, lakes and underground aquifers. This is conservatively projected to climb to at least 70% by 2025, reflecting population growth alone, and by much more if per capita consumption rises at its current pace.
Some 70% of all the world's fresh water used by man goes to grow food, and in parts of the US, North Africa and Asia, farmers can take up to 95%. Unavoidable population increases in the next 20 years will mean that agriculture alone will need at least 17% more water than it does now just to grow the extra food these people will need.