C6f Hardness of Water

Here are the key points you need to take away from this unit and are likely to come up in the exam.

What is hard water? - caused by calcium or magnesium ions

• Carbon dioxide dissolves in water to make it slightly acidic
• This acidic water then reacts with calcium carbonate in rocks
• A solution of calcium hydrogen carbonate is formed - this is temporary hardness

Removal of Temporary Hardness

• Temporary hardness can be removed by boiling the water - it decomposes it!
• This forms calcium carbonate - this is the limescale that forms on the filament of the kettle!
• Weak acids (so they dont damage metal) can then be used to remove limescale from the kettle

Permanent Hardness

• Caused by dissolved calcium sulfate -  this cannot be removed by heating!!

Removal of permanent Hardness

1) Adding washing soda (sodium carbonate). Works by calcium ions in hard water reacting with carbonate ions in the soda. This forms insoluble calcium carbonate which is then rinsed out with the dirty water.

2) Ion exchange column (picture below): Works by sodium ions in the column swap with calcium ion in hard water. Calcium ions stay in the column and sodium ions (do not cause hard water) come out.

How can we measure water hardness? (Very likely to come up in exam!!)

Water hardness stops soap from lathering (getting foam!!). To test water hardness in the lab we can:

• Get different water samples in different test tubes
• Add drops of soap from pipette and count the drops added until a lather (foam) forms
• The water that needs the most drops to form a lather is the hardest

NOTE: Remember if you have tap water and boiled tap water the boiling will remove some hardness (the temporary hardness) but the permanent hardness still remains!