C6f Hardness of Water
• When you wash your hands with soap the water makes bubbles. We say that the
• __________ water does not lather well with soap
• __________ water lathers very well
• We can buy _________ detergents and these lather well with hard or soft water
• The ________ soap solution needed to lather, the harder the water must be.
What makes water hard?
• Carbon dioxide dissolves in water making it slightly _________
• This acidic water flows through rocks made from ____________________
(chalk, limestone and marble)
• The rocks react with water and more carbon dioxide to form soluble
Look at the table and use the information to answer the questions that follow.
Water Sample Number of drops of soap solution
required to lather
Tap water in Kent 30
Boiled tap water in Kent 11
Tap water in Cornwall 8
1. Which water supply is the hardest?
2. Can you explain the result for rainwater?
What causes temporary and permanent hardness?
• Hardness is caused by dissolved ________ and __________ ions in water
• Temporary hardness is caused by dissolved __________________________
• Permanent hardness is caused by dissolved ______________________.
Removing Temporary Hardness (caused by HCO3-)
• Boiling destroys ____________ hardness in water (but not permanent hardness)
• Calcium hydrocarbonate ________________ on boiling leaving calcium carbonate,
water and carbon dioxide
• Calcium carbonate is insoluble so the water looks cloudy after boiling.
Removing Permanent Hardness (caused by SO42-)
1. Adding ________________, Na2CO3
Washing soda reacts with ________________ , Ca2+, to form calcium carbonate
which is insoluble. The calcium carbonate precipitates out as a white solid so the
water looks cloudy.
Water softened with washing soda cannot be used for drinking but it is okay for
2. Ion-exchange columns (swapping Ca2+ for Na+ ions)
• The hard water goes in at the ________ of the column
• It passes through _____________________ which have ___________, Na+,
attached. The calcium ions, Ca2+ in the resin ___________ with the sodium
• The water that leaves has __________ the calcium ions that make it hard and
is full of sodium ions that do not cause hardness.
This water is SAFE to drink.
What is Limescale and how do we get rid of it?
Compare different commercial limescale removers.
State that limescale is calcium carbonate.
State that limescale removers are acids.
Explain how a weak acid can be used as a limescale remover.
Construct a symbol equation for the action of an acid on limescale.