What is hard water?
Water which doesn't easily form a lather with soap
Which ions cause hardness of water?
Calcium (Ca²⁺) and Magnesium (Mg²⁺)
How is scum formed?
When calcium/magnesium ions combine with stearate ions from soap to form insoluble calium/magnesium sterate which makes the water cloudy (this is known as scum)
How is scale formed?
When hard water is heated
HOW do pipes, kettles etc become effected by scale
- They can become blocked
- Kettles become less efficient because the scale causes the heating to take longer (scale is a thermal insulator)
Give advantages of hard water?
- Calcium is good for bones and teeth
- Calcium ions can cause less risk of developing heart disease
What are the two types of hardness?
Tempory and permamnent
What is temporary hardness? And what is it caused by?
This can be removed by boiling, and is caused by the hydrogencarbonate ion
What is permanent hardness? And what is it caused by?
It cannot be removed by boiling, it is caused dissolved calcium sulfate
How is temporary hardness removed? (what happens)
It is removed by boiling.
When heated, the calcium hydrogencarbonate decomposes to form calcium carbonate which is insoluble
This solid is 'limescale'
How can both types of hardness be softened?
By adding washing soda (sodium carbonate) to it
→ The added carbonate ions react with the hard ions to make an insoluble ppte
How can both types of hardness be removed?
By running the water through an ion exchange column
The hard water is passed through a column containing an ion-exchange resin
This resin takes the calcium/magnesium ions (which cause hardness) and replaces them with sodium ions (which don't cause hardness)
How can you compare the hardness of water samples?
Where does most of our drinking water come from?
Explain how water is treated in the treatment works
- Water passes through a mesh screen and remove large solids eg twiga
- Chemicals are added to make solids and microbes stick together and fall to the bottom
- The water is filtered through gravel beds to remove all the solids
- Water is chlorinated to kill of any microbes left
How else can people at home filter their water?
By buying filters that contain carbon/silver to remove substances from their tap water
How do carbon/silver filters work?
The carbon removed the chlorine taste and the silver kills bugs
How can totally pure water be produced? What is it?
The water is boiled to make steam and the steam is condensed
Why isn't distillation used for tap water? Where is it used?
It is too expensive to pay for the heat energy
Instead it is used in chemistry labs
Why is flouride added to water in some parts of the country?
It helps to reduce tooth decay
Why is chlorine added to water?
To prevent disease
Why is it controversial to use chlorine in water?
Some studies have linked adding chlorine to water with an increase in certain cancers
→ Chlorine can react with other natural substances in water to produce toxic by-products which some people think can cause cancer
Why is it controversial to add flourine to drinking water?
Because a high dose of flourine can cause cancer and bone problems in humans
→ There is also a concern about whether to 'mass medicate', as people can choose to use flouride toothpaste but not drink flouride water