Page 10 - Curriculum Visions Dynamic Book

Hydrogen in water
Water is the most common chemical compound on the surface of the Earth, covering over two- thirds of the Earth’s surface. People are also about two-thirds water.
Water is a compound of hydrogen and oxygen. Although we are so familiar with the way water behaves that we take it as normal, water, in
fact, behaves differently from most other compounds. This is due to the strong attraction that each water molecule has for others. Water shrinks as it is cooled (down to 4°C) and then expands as it is cooled further still (to 0°C). It is also the only common substance that swells as it freezes.
All these unusual properties are due to
the special way that the hydrogen atoms link water molecules together, the property called hydrogen bonding (see pages 8 and 9).
The atoms that make up water molecules
are bound together very much more firmly
than are the molecules to each other.
Enormous amounts of energy are needed
to break them apart. Heating, for example,
only changes the physical state of water, converting solid ice to liquid water and then
to steam. Only electrical energy will break bonds within the molecules.
The process of decomposing water (breaking it apart) is called electrolysis. The demonstration on these pages uses electrolysis to collect hydrogen and oxygen gases separately. The equipment is called Hoffman’s voltameter.
 This is Hoffman’s voltameter. It consists of three connected glass tubes with electrodes
in the bottom of the outer two. The electrodes are connected to a source of direct current (dc) electricity.
Also...
Water (H2O) is one of the most common substances to contain hydrogen, yet it has some unusual properties. Only a few water molecules (about one in 500 million) break up into ions at room temperature. Because it has so few ions, pure (sometimes called deionised) water is a poor conductor of electricity (but see the discussion of how electrolysis works on page 11).
Hydrogen ions can exist only
in the presence of water, which explains why an acid (see page 18) behaves as an acid only when in water and why it does not react as solid crystals. Thus baking powder (which contains tartaric acid) only produces bubbles of hydrogen when mixed with water.
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