Water
Introduction


Life on Earth is believed to have started in the oceans. So, whenever scientists send a space probe to explore another planet or moon, they are keen to look for any signs that liquid water had once existed there. The discovery of such signs makes headlines because it increases the possibility of life existing beyond Earth. However, at present, the Earth is the only planet in our solar system at the right temperature for water to exist as a liquid on its surface.



In this unit you can find out more about one of the most important substances in our lives – water.

The water cycle
When astronauts view the Earth from space, they can see very clearly that water covers most of the Earth's surface.
Figure 1.   About 70 per cent of the surface of the Earth is covered in water.
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The water cycle explains how water on the Earth changes state and circulates from the oceans to the land and back again:

Figure 2.   The water cycle.
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Look at the water cycle above, then fill in the blanks:

  • The energy that evaporates water from the sea comes from the . As the water vapour rises, it cools down and , forming clouds. The clouds get over land and rise still higher. More water vapour and the droplets of water get bigger. Eventually they fall from the clouds as rain, hail, or . Then the water finds its way back to the through rivers.
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Water molecules
Most people know the chemical formula for water – H2O – if they know any formula at all. This small
molecule
A molecule is a group of two or more atoms bonded together.
molecule
is essential for living things to survive. It is a simple two-element
compound
A compound is a substance made up of more than one type of atom.
compound
, with each of its molecules made up of two hydrogen atoms and one oxygen
atom
An atom is the smallest particle of an element that can still be defined as that element.
atom
.

Figure 3.   A water molecule.
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The atoms are bonded to each other by covalent bonds (see the unit Covalent Bonding).

Figure 4.   A water molecule contains covalent bonds
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How many atoms are there in a water molecule?
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Choose the correct word to complete the following statement:
The hydrogen and oxygen atoms in a water molecule are attached to each other by . . . . covalent bonds.
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Decide whether the following statements are true or false.
In a water molecule …
  • all the atoms have achieved complete outer shells (highest energy levels) of electrons.
    the oxygen atom has a complete outer shell (highest energy level), but the hydrogen atoms do not.
    each hydrogen atom has 8 electrons in its outer shell (highest energy level).
    the oxygen atom has 4 electrons in its outer shell (highest energy level) that are not involved in covalent bonding.
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Properties of water
Water is a V-shaped molecule, with oxygen at its point. The electrons (which are negatively charged) in a water molecule are more concentrated around the oxygen end of the molecule. This means that the oxygen end carries a slight negative charge compared with the hydrogen end. Because the charge is not evenly distributed, it is called a polar molecule.

Figure 5.   Water, a polar molecule.
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This helps to explain why water is such a good
solvent
A solvent is a liquid that dissolves another substance.
solvent
for many ionic compounds, such as common salt (sodium chloride). The charged parts of water molecules are attracted to the charged ions in ionic compounds. You can see how water dissolves an
ionic compound
A ionic compound is a compound made up of oppositely charged ions bonded to each other in a giant lattice.
ionic compound
in Fig.6 below:

Figure 6.   Water acting as a solvent.
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Decide whether the following statements are true or false.
When an ionic solid dissolves …
  • the positive ions attract the oxygen end of the water molecules.
    the negative ions attract the oxygen end of the water molecules.
    the negative ions attract the hydrogen end of the water molecules.
    the negative ions attract the oxygen end of the water molecules.
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Because they are polar, water molecules also have stronger forces between each other than we would expect for molecules of their relatively small mass. (Generally, the larger a molecule, the higher its melting and boiling points.) The charged ends of the water molecules result in forces of attraction between neighbouring molecules. These forces are about a tenth as strong as a
covalent bond
A covalent bond is a bond between atoms of non-metal elements formed by sharing one (or more) pairs of electrons.
covalent bond
and are called hydrogen bonds. These relatively strong forces are fortunate for us because, without them, a molecule the size of water would exist as a gas at the average temperature on Earth.

The water molecules and the bonds between them get fixed in place when water freezes, forming a crystalline structure which is less dense than water. This is why ice floats on water.

Summary


Water (H2O) is one of the most important substances on Earth, and is essential for all living things.

Water is a polar molecule – that is, it has a large imbalance in the way charge is distributed within its molecules.

The water molecule's polar nature gives rise to relatively strong forces of attraction between the oppositely charged parts of neighbouring water molecules. These forces are called hydrogen bonds. Because of hydrogen bonding, water is a liquid rather than a gas at room temperature.

Water is a good solvent for many ionic compounds because of the attraction between the ions and the charged parts of the polar water molecules.

Exercises
1. How many elements make up water?
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2. What type of bonding do we find within a water molecule?
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3. Which sequence gives the correct order for part of the water cycle?
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4. Which statement is true?

The three atoms in a water molecule are bonded to each other …
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5. Which end of a water molecule carries a slight, negative charge compared with the other end?
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6. Which statement is true?
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7. Which part of a water molecule is attracted to a chloride
ion
An ion is a charged particle made when an atom (or group of atoms) loses or gains one or more electrons, e.g. Na+ and Cl.
ion
, Cl, when potassium chloride dissolves?
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8. Which statement about the forces between water molecules is correct?
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9. Which two statements are true?
  • Ice is less dense than water.
    Ice is more dense than water.
    Ice melts at 0 °C.
    Water boils at 0 °C.
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