C6f Hardness of Water

Lathers

When you wash your hands with soap the water makes bubbles. We say that

the water ______________.

__________ 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

_____________________________ Ca(HCO3)2.

Task

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 30Boiled tap water in Kent 11

Rainwater 1Tap water in Cornwall 8

1. Which water supply is the hardest?

2. Can you explain the result for rainwater?

HIGHER

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 HCO 3- )

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 laundry

HIGHER

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

ions.

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.

HIGHERConstruct a symbol equation for the action of an acid on limescale.