Hard and Soft Water

Where does our drinking water come from?

This means the water we drink has run through and across rocks.

This is a map of the rock formations in UK- what does this mean for our water?

Depending where you live you will get different chemicals in your tap water..

We call the two main water types: Hard water Soft water

You need to know: The difference between hard and soft water Which ions cause hard water How they get there The pros and cons of each water type Treatments for water hardness

There are 2 metal ions we need to focus on that make water HARD Calcium – Ca2+ Magnesium – Mg2+

REMEMBER While other metal ions can be in water, only Ca2+ and Mg2+ cause hardness.

How do Ca2+ and Mg2+ get into the water? You need to know the details for calcium This is a ‘limestone pavement’. Limestone is calcium carbonate- it does not dissolve in pure water, BUT it does dissolve in acid rain.

Acid rain on limestone causes the following reaction: CaCO3 + H2CO3 Ca(HCO3)2 Carbonic Acid (acid rain) Calcium Hydrogen carbonate Calcium carbonate

So now you have calcium ions (and HCO3- ions) in the water.

We say this water is HARD Ca2+

These are crystals of Calcium sulphate Look at the tiny tiny man

Unlike calcium carbonate (limestone), Calcium sulphate in rocks (Gypsum) is very soluble.

Calcium sulphate readily dissolves and puts calcium ions (and sulphate ions) in the water.

We say this water is HARD Ca2+

Calcium Hydrogen carbonate from limestone Which rocks have put the calcium in your water has important consequences: OR CaSO4 Ca(HCO3)2 Calcium Sulphate from gypsum Calcium Hydrogen carbonate from limestone Ca2+ ?

Both make hard water (ie water with Calcium/ Magnesium ions in it) but…. One is permanent hardness One is temporary hardness Ca(HCO3)2 CaSO4 Ca2+

Why do we care about water hardness? 3 problems: Ca2+

1) SCUM

Calcium and magnesium ions react with soap to make calcium and magnesium stearate – or scum. It looks horrible It’s really hard to get off

2) SOAP Because the calcium and magnesium ions are reacting with the soap, the soap isn't doing its job ie it isn’t making bubbles.

Soft Hard

This matters in hard water areas because everyone ends up using significantly more soap.

Both permanent and temporary hardness causes those 2 problems: scum and poor bubbles.

This 3rd problem is an issue for areas with temporary hardness, those with limestone rocks and calcium and hydrogen carbonate ions in the water.

3) LIMESCALE

Whenever you want to heat up water which has temporary hardness, you get a problem. The hydrogen carbonate ions (HCO3-)from the limestone are soluble- you wont see them in the water BUT heat up these ions and they fall apart- making carbonate ions (CO3-) These then join up with the calcium ions to make calcium carbonate- which is NOT soluble in water (remember it only dissolved in the first place due to acid rain). This calcium carbonate precipitates out, coating the insides of boilers, kettles, pipes

Can you think of any reasons why this, is a problem?

LIMESCALE It reduces the efficiency of any heating element, using more energy It can block pipes and damage equipment

Are there any positives? Living in a hard water area clearly causes issues Some people still prefer it- can you think of any reasons why?

Pros of hard water: Taste- some people prefer it

Pros of hard water: It’s good for your teeth and bones (Calcium ions)

Pros of hard water: Heart- it can protect against heart problems

If these positives of living with hard water are not enough to outweigh the negatives, what can you do?

Well yes you could move house What else?

Where do the ions in our drinking water come from? Water runs over rocks and dissolves ions

Which 2 ions cause hard water? Calcium and Magnesium

Which rock gives you temporary hard water? Limestone

What is the chemical name for limestone? Calcium carbonate

Why does this normally insoluble chemical dissolve? Acid rain

What ions are produced when limestone reacts with acid rain? Ca2+ and HCO3-

What happens to HCO3- ions if you heat them? They break down to CO32- ions

They precipitate as CaCO3 If you have CO32- ions and Calcium ions in the same water, what happens? They precipitate as CaCO3

What problem arises in heating systems and temporary hard water? Limescale

Why do we care about limescale? Reduces efficiency/ blocks pipes

What rock gives you permanently hard water? Calcium Sulphate

What effect does boiling have on permanently hard water? None

What do calcium and magnesium ions form with soap? Scum

What does this mean for the amount of soap needed? More soap for the same lather

What are the positives to hard water Taste/ bones and teeth/ heart