Essential for Life on Earth







What are the physical and chemical properties of water that make it so unique and necessary for living things? When you look at water, taste and smell it - well, what could be more boring? Pure water is virtually colorless and has no taste or smell. But the hidden qualities of water make it a most interesting subject.



You probably know water's chemical description is H2O. As the diagram to the left shows, that is one atom of oxygen bound to two atoms of hydrogen. The hydrogen atoms are "attached" to one side of the oxygen atom, resulting in a water molecule having a positive charge on the side where the hydrogen atoms are and a negative charge on the other side, where the oxygen atom is. Since opposite electrical charges attract, water molecules tend to attract each other, making water kind of "sticky." As the right-side diagram shows, the side with the hydrogen atoms (positive charge) attracts the oxygen side (negative charge) of a different water molecule. (If the water molecule here looks familiar, remember that everyone's favorite mouse is mostly water, too).


Water's Chemical Properties



All these water molecules attracting each other mean they tend to clump together. This is why water drops are, in fact, drops! If it wasn't for some of Earth's forces, such as gravity, a drop of water would be ball shaped -- a perfect sphere. Even if it doesn't form a perfect sphere on Earth, we should be happy water is sticky.


 Water is called the "universal solvent" because it dissolves more substances than any other liquid. This means that wherever water goes, either through the ground or through our bodies, it takes along valuable chemicals, minerals, and nutrients.

 Pure water has a neutral pH of 7, which is neither acidic nor basic.
 Diagram about pH





  • Water is unique in that it is the only natural substance that is found in all three states -- liquid, solid (ice), and gas (steam) -- at the temperatures normally found on Earth. Earth's water is constantly interacting, changing, and in movement.


  • Water freezes at 32o Fahrenheit (F) and boils at 212o F (at sea level, but 186.4 at 14,000 feet). In fact, water's freezing and boiling points are the baseline with which temperature is measured: 0o on the Celsius scale is water's freezing point, and 100o is water's boiling point. Water is unusual in that the solid form, ice, is less dense than the liquid form, which is why ice floats.


  • Water has a high specific heat index. This means that water can absorb a lot of heat before it begins to get hot. This is why water is valuable to industries and in your car's radiator as a coolant. The high specific heat index of water also helps regulate the rate at which air changes temperature, which is why the temperature change between seasons is gradual rather than sudden, especially near the oceans.


  • Water has a very high surface tension. In other words, water is sticky and elastic, and tends to clump together in drops rather than spread out in a thin film. Surface tension is responsible for capillary action, which allows water (and its dissolved substances) to move through the roots of plants and through the tiny blood vessels in our bodies.


  • Here's a quick rundown of some of water's properties:

    • Weight: 62.416 pounds per cubic foot at 32F

    • Weight: 61.998 pounds per cubic foot at 100F

    • Weight: 8.33 pounds/gallon, 0.036 pounds/cubic inch

    • Density: 1 gram per cubic centimeter (cc) at 39.2F, 0.95865 gram per cc at 212F



  1 gallon = 4 quarts = 8 pints = 128 ounces = 231 cubic inches
  1 liter = 0.2642 gallons = 1.0568 quart = 61.02 cubic inches
  1 million gallons = 3.069 acre-feet = 133,685.64 cubic feet




The Water (or Hydrological) Cycle



The hydrological cycle is the continuous movement of water between the earth and the atmosphere.  Water evaporates from water and land surfaces and transpires from living cells.  This vapour circulates through the atmosphere, condensing to form clouds and precipitating as rain or snow.  When water hits the earth's surface it either runs into streams and ends up in oceans or lakes, or seeps into the soil.  The water that seeps into the soil is then either absorbed by the roots of vegetation, or it sinks into the groundwater reservoir.


Although the balance of water on Earth remains fairly constant over time, individual water molecules can come and go in a hurry. The water in the apple you ate yesterday may have fallen as rain half-way around the world last year or could have been used 100 million years ago by Mama Dinosaur to give her baby a bath.





Water is made up of hydrogen and oxygen.  Two hydrogen atoms are linked by a single chemical bond to one oxygen atom.  Its chemical formula is H2O.

The water molecule is angular in shape, forming negative and positive charges on opposite sides.  This means that the H2O molecule is highly polar.  Because of this high polarity H20 molecules form hydrogen bonds which are very strong.  This is when hydrogen atoms in one water molecule are attracted to the non-bonding electron pairs of the oxygen atom in another H20 molecule.  These strong hydrogen bonds mean that water has a very high boiling point because it takes a lot more energy to overcome them and release H2O molecules from the liquid into the gaseous phase.


The H2O molecules in ice are highly ordered, although loosely structured.  When ice melts this orderly arrangement breaks and so H2O molecules can be packed closer together.  Therefore the liquid is denser than the solid and this explains why ice floats on water.  


One of the most important properties of water is that it can dissolve many other substances to form aqueous solutions.  This happens because of the H2O molecule's high polarity.  If, for example, an ionic compound such as sodium chloride (NaCl) is added to water, the positively charged Na atoms will be attracted to the negative end of the H2O molecules, and the negatively charged Cl atoms will be attracted to the positive end of the H20 molecules.  Therefore the Na and Cl ions will be pulled apart and hydrated, meaning they will be surrounded by H2O molecules.  This keeps the Na+ and Cl- ions from recombining.


Water can act as an acid or a base because it can dissociate to some extent into H+ (hydrogen) ions which are acidic, and OH- (hydroxyl) ions which are alkaline (basic).


Most hydrogen atoms consist of only 1 proton, but the isotopes deuterium and tritium have one and two neutrons in their nuclei respectively.  Deuterium oxide (D2O) is called heavy water because it has a greater molecular weight than H2O due to the extra neutron in the deuterium nucleus.  D2O can be produced through electrolysis and fractional distillation of water.  It is used as a moderator of neutrons in nuclear power plants and in biological research as an isotopic tracer.



Ice occurs when water vapour or liquid water freeze.  At temperatures below 0C (32F) water vapour becomes frost at ground level and snowflakes (each one a single ice crystal) in clouds, while liquid water becomes solid ice in the form of river ice, sea ice, hail etc.  Each H2O molecule forms hydrogen bonds with four neighbouring molecules, creating a tetrahedral shape.  An ice crystal is made up of rings of these tetrahedrons forming at various angles to each other.


At first sight ice seems brittle and shatters like glass when it is struck.  However it flows under low stresses over long periods of time or under high stresses where pressure stops the ice from splintering.  This happens because the layers of ice crystal can glide over and past each other without the hydrogen bonds being broken.  Flow is very slow because the different crystals glide in different directions and tend to interfere with one another.  Glaciers are an example of this flow.  


Ice is used as a refrigerating agent because it takes more energy to melt it than most other substances, due to strong hydrogen bonds.  Melting ice remains at a constant 0C (32F).


Because ice is less dense than water at 0C (32F) a mass of ice occupies 9% more volume than an equal mass of water.  This is why when water in pipes freezes it can cause the pipes to burst.  When water enters tiny cracks in rocks and freezes, the expansion creates great pressure that will split the rocks, causing erosion.


Another important point about ice being less dense than water is that it floats.  In rivers, lakes and oceans this means that the ice traps the warmer water below, allowing for fish and other creatures to survive the freezing temperatures of the surface. 



Steam is vapourised water and is an odourless, invisible gas.  It often looks white and cloudy because there are tiny water droplets present.  Steam is created in nature from volcanic processes heating underground water and is released through hot springs and geysers, for example.  The temperature at which water will boil depends on its pressure.  If pressure is reduced, the boiling point is also reduced.  If pressure is increased, more energy is required to allow the liquid molecules to escape into the gaseous phase, and therefore the temperature at which water boils also increases.


Modern industrial society makes extensive use of steam power.  Virtually all the world's electricity is created through steam power.  Power plants heat water into steam which, under pressure, drives turbines that produce electrical current.  Steam is also used in the manufacture of steel, aluminium, copper and nickel, and the production of chemicals, the refinement of petroleum and for cooking and heating in the home.



Most of the earth's water is undrinkable.  If a large bucket of water were to represent the sea water on the planet, an egg cup full would represent the amount of water locked in ice caps and glaciers and a teaspoonful would be all that was available as drinking water.  Human use of natural waters, especially freshwater resources such as rivers and lakes, has steadily increased over the centuries.  With population growth and increasing use of water for agriculture, industry and recreation, water is becoming an incredibly valuable resource.  


It is not only the scarcity of water that is becoming an issue, but also quality.  Mineral fertilizers, pesticides and herbicides have seeped into surface and subsurface waters contaminating them beyond human consumption and disrupting delicate ecosystems.  Dumping of sewage and industrial wastes and toxins pollute rivers and lakes and threaten the world's most important resource.  Will their be enough water to accommodate the needs of future generations?





As you know, the Earth is a watery place. Estimates vary, but somewhere between 70 and 75 percent of the Earth's surface is water-covered. But water also exists in the air as water vapor and in the ground as soil moisture and in aquifers. Thanks to the water cycle (view a graphic of the water cycle) our planet's water supply is constantly moving from one place to another and from one form to another. Things would get pretty stale without the water cycle!


When you take a look at the water around you, you see water in streams, rivers, and lakes. You see water sitting on the surface of the earth. Naturally, this water is known as "surface water." Your view of the water cycle might be that rainfall fills up the rivers and lakes. But, how would you account for the flow in rivers after weeks without rain? In fact, how would you account for the water flowing down this driveway  on a day when it didn't rain? The answer is that there is more to our water supply than just surface water, there is also plenty of water beneath our feet.


Even though you may only notice water on the Earth's surface, there is much more water stored in the ground than there is on the surface. In fact, some of the water you see flowing in rivers comes from seepage of ground water into river beds. Water from precipitation continually seeps into the ground to recharge the aquifers, while at the same time water from underground aquifers continually recharges rivers through seepage.


Humans are happy this happens because people make use of both kinds of water. In the United States in 2000, we used about 323 billion gallons per day of surface water and about 85 billion gallons per day of ground water. In a way, that underestimates the importance of ground water, since not only does ground water help keep our rivers and lakes full, it also provides water for people in places where visible water is scarce, such as in the desert towns of the Western United States. Without ground water, people would be sand-surfing in Palm Springs, Ca. instead of playing golf!


Just how much water is there on (and in) the Earth? Here are some numbers you can think about:

  1.  The total water supply of the world is 326 million cubic miles (a cubic mile is an imaginary cube (a square box) measuring one mile on each side). A cubic mile of water equals more than one trillion gallons.

  2.  About 3,100 cubic miles of water, mostly in the form of water vapor, is in the atmosphere at any one time. If it all fell as precipitation at once, the Earth would be covered with only about 1 inch of water.

  3.  The 48 contiguous United States receives a total volume of about 4 cubic miles of precipitation each day.

  4.  Each day, 280 cubic miles of water evaporate or transpire into the atmosphere.

  5.  If all of the world's water was poured on the United States, it would cover the land to a depth of 90 miles.

Of the freshwater on Earth, much more is stored in the ground than is available in lakes and rivers. More than 2,000,000 cubic miles of fresh water is stored in the Earth, most within one-half mile of the surface. Contrast that with the 60,000 cubic miles of water stored as fresh water in lakes, inland seas, and rivers. But, if you really want to find fresh water, the most is stored in the 7,000,000 cubic miles of water found in glaciers and icecaps, mainly in the polar regions and in Greenland.








  • Water is odourless and tasteless. It has a bluish tint that can be seen in very deep layers.

  • At Standard Atmospheric Pressure (760mm of mercury) water has a freezing point of 0C (32F) and a boiling point of 100C (212F).  Water is at its maximum density at 4C (39F).

  • Water can remain a liquid even below its freezing point, up to -25C, if it is not disturbed and if the temperature does not drop further and no particle or ice crystal is added to it.

  • Water is used in the metric system to define the gram.

  • Water is the only substance that occurs at ordinary temperatures in all three phases: liquid, gas and solid.

  • 75% of the earth's surface is covered by water.

  • The oceans contain 97.5% of the earth's water, the land 2.4%, and the atmosphere holds less than .001%

  • Only 1% of the earth's water is available for drinking; 2% is frozen.

  • 50-90% of the weight of living organisms is made up of water.  Blood in animals and sap in plants is mostly water.

  • The adult human body is composed of approximately 55 to 60% water--the brain is composed of 70% water, as is skin, blood is 82% water, and the lungs are nearly 90% water.

  • The world average rainfall is 860 mm.

  • You can survive about a month without food, but only 5-7 days without water.

  • It is possible to drink water today that was here in the dinosaur age.

  • The average urban home of 4.6 people uses 640 litres of water per day.

  • A dripping tap can waste as much as 60 litres per day or 1 800 litres per month.

  • A leaking toilet can waste up to 100 000 litres of water per year, enough to take three full baths every day.

  • It takes about 2.5 litres of water to cook pasta and about 5 litres to clean the pot.

  • The average bath holds between 150 and 200 litres of water when filled to the brim.

  • A toilet is the biggest user of indoor water.  On average, it uses 11 litres of water when flushed.





The boundaries between countries are political constructs which often do not take into consideration the distribution of natural resources.  For example, rivers have been used as international boundaries for centuries and, although rivers may form logical barriers between people, they by definition fall in the centre of drainage basins.  


Political boundaries therefore divide water courses, river basins and groundwater aquifers.  Where ever this happens the water course, river basin or aquifer becomes international.  This complicates the management of water resources enormously.  A good example of this is in Southern Africa where all rivers of size are shared between at least two countries and every country has at least 1 international river, with Mozambique has 9.


International water resources management is a complex process.  Whilst it may lead to tension and stress between countries, it also provides opportunities for co-operation to maximize the mutual benefits of the resources.  International water resources management requires both political and technical process which usually needs a legal basis through which to function.  In many instances the institution which manages the process is a river or basin Commission.  The establishment of all of the necessary elements often requires many years of negotiation and planning and is subject to many disruptions and hurdles which often have nothing to do with water.


Countries regulate their relationships with regards to shared water courses through treaties, protocols, agreements and other legal instruments.  These usually address such issues as water quality, water utilisation and abstraction, the construction of hydraulic structures such as dams and weirs for irrigation, hydro-power generation and flood management, notification and conflict resolution.


Take a True/False quiz about water properties.  Some of the answers may surprise you!









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