Potable Water Flashcards
What must drinking water have to be save to drink
Drinking water has to have sufficiently low levels of dissolved salts such as sodium chloride
Drinking water cannot have high levels of microbes such as bacteria
pH must be between 6.5 and 8.5
Define potable water
Potable water is water that is safe to drink
What is pure water
Pure water is water that only contains H2O molecules
Explain why potable water cannot be classified as pure water
Pure water in the chemical sense only contains no dissolved substances at all (pure water contains H2O molecules only). However potable water does contain dissolved substances, although in quite small amounts
What provides most of the potable water in the UK and why?
In the UK, rain water provides most of our potable water. This is because rain water contains low levels of dissolved substances
Where does rain water get collected
Rainwater collects in the ground in aquifers and in lakes, rivers and in reservoirs
State some good sources of fresh water
Aquifers
Lakes
Rivers
Reservoirs
Describe the steps used to produce potable water from fresh water
To produce potable water, first choose a good source of fresh water e.g. a river
Then, pass the water through filter beds. That is to remove solids such as leaves and suspended particles
The water is sterilised to kill microbes.
Sterilising agents used for potable water include chlorine, ozone or ultraviolet light.
(Add chlorine to kill microbes)
What does fresh water contain in terms of dissolved minerals
Fresh water contains very low levels of dissolved minerals
When might the desalination of salty water or sea water be required to produce potable water.
If supplies of fresh water are limited, potable water is produced by the desalination of salty water or sea water.
What does sea water contain in terms of dissolved minerals
Sea water has very high levels of dissolved minerals.
What does desalination do
Desalination reduces the levels of dissolved minerals down to an acceptable level for potable water.
State some of the ways to carry out desalination and produce potable water from sea water
Distillation
Another way is to pass the water through membranes. This is called reverse osmosis
Both ____ and _____ reduce the levels of dissolved minerals
Both distillation and reverse osmosis reduce the levels of dissolved minerals
State a disadvantage to using distillation and reverse osmosis to produce potable water
Both processes require very large amounts of energy which makes them very expensive
Describe how distillation is used to produce potable water
Sea water is boiled
Water molecules boil (impurities stay behind)
Steam is cooled and condensed to pure water
This process requires a lot of energy for heat. This makes the process very expensive
Describe how reverse osmosis is used to produce potable water
Sea water is pushed through a semi-permeable membrane under pressure
Dissolved substances cannot pass through
This process requires a lot of energy for pressure. This makes the process very expensive
Describe how to analyse a sample of water for purity
Check the pH of the water by placing a small amount of the water onto a piece of universal indicator. Pure water has a pH of 7. Universal indicator is green if the pH is 7.
If the pH is not 7 then the water sample contains dissolved acid or alkali and is therefore not pure.
If the pH is 7 then that does not mean that the water is pure - the water could still contain dissolved solids, so now we need to test for those.
To do this, use a balance to weigh an empty evaporating basin. Record the mass of the evaporating basin
Fill the evaporating basin with our water sample and place this on a tripod and gauze
Use a Bunsen Burner to gently heat the water until it has all evaporated.
At this point, all the evaporating basin to cool and then weigh it again.
If the water sample contained any dissolved solids then the mass of the empty evaporating basin will have increased. That is because the water has evaporated but the dissolved solids will have formed crystals on the surface of the evaporating basin.
If our water sample contained any dissolved solids, then the water is not pure.
However if the mass of the evaporating basin did not increase, then the water did not contain any dissolved solids and could be pure.
Describe how to purify a sample of water by distillation
Gently heat the water using a Bunsen Burner
At this point, the water will evaporate and form water vapour.
The water vapour travels up the conical flask and into the condenser The condenser cools the vapour and condenses it. The vapour turns back into a liquid , and collects in the receiving flask. This is distilled water
The salt has been left behind in the round bottomed flask
What does distilled water contain
Distilled water contains no dissolved solids and has a pH of 7
Distilled water is pure water
State the equipment used to purify a sample of water by distillation
Conical flask on a tripod and a gauze
Delivery tube
Test tube
Beaker containing ice and water
state the pH of pure water
7
state the boiling point for pure water
100°C
state boiling point of impure water/potable water
Impure/potable water boils slightly higher than 100°C
Name the substance removed from seawater by desalination
Sodium Chloride
Explain why it is more difficult to produce drinking water from waste water than from water in lakes
Water needs more processes because it contains more microbes and more toxic chemicals or detergents
State an advantage of adding fluoride to drinking water
Water needs more processes because it contains more microbes and toxic chemicals or detergents
How could the student check that all the water had evaporating after investigating how much solid was dissolved in sea water
Reheat the evaporating basin
Weigh the evaporating basin again. If all the water had evaporating there would be no change in mass
Types of water
Pure water
Sea water
Potable water
Waste water (from homes, industry, agriculture)
Ground water (underground streams & aquifiers)
Fresh water (lakes rivers, ice caps, glaciers, underground streams & aquifiers
Which types of water contain microbes - and how much
Sea water - contains microbes
Ground water - contains microbes
Fresh water - contains microbes (very low from some sources)
Waste water - contains microbes
Potable water - does not contain microbes (or very low amounts)
Pure water - does not contain microbes
Which types of water contains dissolved substances
Sea water - high
Waste water - high
Fresh water - low
Potable water - low
Pure water does not contain any dissolved substances
