C3 2 Water Flashcards
Rainwater is soft. 🌧
How is hard water produced from rainwater? 💦
Calcium and/or magnesium ions are dissolved from rocks which cause water hardness.
How is hard water softened by ion exchange?
1) Hard water is passed through an ion-exchange column which contains a resin packed with sodium ions*.
2) The sodium ions are exchanged for the calcium and magnesium ions in the hard water.
3) The resin can be recharged with sodium ions after they have been exchanged for calcium and magnesium ions. The resin is washed with salt (sodium chloride) solution. This puts the sodium ions back in. This is why water softeners must be topped up with salt.
* some columns work by swapping hydrogen ions.
How is hard water softened by adding sodium carbonate (washing soda)?
When we add sodium carbonate to hard water, a reaction takes place. Its soluble carbonate ions precipitate out calcium and magnesium ions. The dissolved metal ions form insoluble carbonates:
Ca2+ (‘hardness’) + Co3 2- (from sodium carbonate ➡️ CaCO3
What are advantages and disadvantages of ion exchange?
Advantages:
• It is an easy and quick method to use
• It is a continuous process
Disadvantages:
• The resin needs periodic replenishment with sodium/hydrogen ions or sodium chloride
• It increases sodium content of water (if sodium ions used)
What are advantages and disadvantages of adding sodium carbonate (washing soda)?
Advantages:
• It is an easy and quick method to use
• Relatively cheap
• It can happen where and when we want it to happen, without having to waste energy
Disadvantages:
• It is a batch process
• It leaves a residue of precipitated carbonate in the water
• It increases sodium content of water
Water in Britain is taken from reservoirs to use as drinking water.
What are the two main steps used to treat water from reservoirs?
1) Filter to remove solids or insoluble particles
2) Add chlorine to reduce the number of microbes
Water filters remove hardness from hard water.
What is in water filters that removed hardness from water?
An ion exchange resin
Why do water filters used in the home contain particles of silver?
To prevent growth of microbes.
Pure water can be produced by distillation.
Why is distillation not usually an economic method of treating water for drinking?
It uses a lot of energy and is therefore expensive
What are advantages and disadvantages of drinking hard water?
Advantages:
• It helps to develop/maintain bones
• It helps to develop/maintain teeth
• It reduces heart disease
Disadvantages:
• Limescale reduces the efficiency of appliances
• It forms scum
• It is expensive because you need to use more soap
Acids are used to to remove scale.
Give 2 examples of a carbonate in scale.
1) calcium carbonate
2) magnesium carbonate
What gas is produced when acids react with scale?
Carbon dioxide
Why does hydrochloride acid react faster than ethanoic acid?
Because hydrochloride acid is a stronger acid which ionises more/has more hydrogen ions.
Why shouldn’t hydrochloric acid be used to dissolve scale in kettles?
Because it is corrosive
A student titration 25cm of hydrochloric acid with sodium hydroxide solution of concentration 0.2 moles per dm:
HCl + NaOH ➡️ NaCl + H2O
The student added 28.6 cm of sodium hydroxide solution to neutralise the hydrochloric acid.
Calculate the concentration of the hydrochloric acid.
1) 28.6 x 0.2 = 5.72
2) 5.72/25 = 0.2288
