Water
Drinking water
• Water of the correct quality is essential for
life. For humans, drinking water should
have sufficiently low levels of dissolved
salts and microorganisms. Why?
• Where does the water that we drink come
from?
• How do salts get into the water we drink?
The water cycle
• Water in rivers, lakes and the oceans is
•
•
•
evaporated by the heat of the sun.
Plants also give off water via transpiration.
These processes form water vapour that rises in
the atmosphere and cools so that it condenses
to form water droplets and eventually clouds.
When it rains, water is precipitated and falls
back into the oceans or onto the land.
• Water can run off the land into rivers etc,
or can seep into the ground.
• As the water travels down through the
rock, substances can dissolve in the water.
• This water may find its way back to the
sea or to the water table.
What can dissolve in water?
• Many substances dissolve in water. Most
ionic compounds are soluble in water.
– E.g.
• Some molecular (simple covalent)
substances are soluble in water, but many
are not.
– E.g.
Gases
• Many gases are soluble in water.
• Fish and other aquatic life rely on the oxygen
•
•
dissolved in water to survive.
If the temperature of the water rises, the
amount of oxygen that can be dissolved gets
less. This is a problem near factory outlets, as
the water discharged into rivers is often warmer
than the natural water.
The solubility of a gas in water increases as the
temperature increases / decreases.
Fit to drink
• Sourcing
• Water purification (filtering and sterilising)
• Water filters (charcoal, silver, ion
exchange resins)
• Distillation