C10 Potable Water (page 164) Flashcards
What is Potable Water?
It is water that has been treated or is natrally safe for humans to drink - it is essential for life.
We all need safe drinkng water. the way that water’s made safe depends on local conditions.
What do Pure Water contain?
Pure water only contains H2O molecules.
Where as Potable water can contain lots of other dissolved substances, so chemists wouldn’t call it pure.
Why is it important that the levels of dissolved salts aren’t too high in the water?
it is important that the water has s a pH between 6.5 and 8.5 and also that there aren’t any nasties (like bacteria or other microbes) swimming around in it.
Rain water is a type of what type of water?
fresh water.
Fresh water is water that doesn’t have much dissolved in it.
When it rains, where can the water be collected?
it can either be collected as surface water (in lakes, rivers and reservoirs) or as groundwater (in rocks called aquifers that trap water underground)
In the UK, the source of fresh water used depends on the location, explain in more detail?
Surface water tends to dry up first, so in warm areas, eg. the south-east, most of the domestic water supply comes from ground water.
Even though rain water has low levels of dissolved substances, water from these fresh water sources still need to be treated to make it safe before it can be used. What is the process?
Filtration & Sterilisation
Explain the process of Filtration?
a wire mesh screensout large twigs etc, then the gravel and sand beds filter out ay other solid bits.
(see diagram on page 164)
Explain the process of Sterilisation?
the water is sterlised to kill any harmful bacteria or microbes. This can be done by bubbling chlorine gas through it or by using ozone or ultraviolet light.
What other chemicals can be added to the water supply after filtration and Sterilisation?
Fluoride can be addded to the water supply, which is good for teeth. This is controversial because people aren’t given any choice over whether they consume them or not.
In some very dry countries e.g. kuwait, there’s not enough surface or groundwater, so what water is used?
seawater must be treated by desalination to provide potable water.
doing an experiment in the lab, Distillation can be used to desalinate seawater, by what process?
First, test the pH of the water using a pH meter. If the pH is too high or too low, you’ll need to neutralise it. You do this by adding some acid (if the sample’s alkaline) or some alkali (if the sample’s acidic) until the pH is 7.
Set up the equipment as shown on page (164).
As the water in the flask heats up, it’ll evaporate (become a gas) and will enter the condenser as steam.
The drop in temperature inside the codenser, due to the cold water around it, will cause the steam to consense back into liquid water.
Collect the water running out of the condenser in a beaker.
Retest the pH of the water with a pH meter to check it’s neutral.
You can tell whether there were salts in your initial sample by looking to see whether there are any crystals in the round bottomed flask once the water’s been distilled.
Seawater can also be treated by a processes that use membranes - like reverse osmosis, what is this?
The salty water is passed through a membrane that only allows water molecules to pass through. Ions and larger molecules are trapped by the membrane so separated from the water.
Is Distillation and reverse osmosis practical for producing large quantities of fresh water, and why?
No it is not practicle as it needs loads of energy so they’re really expensive and not practical for producing large quantities of fresh water.
Describe the steps used to treat fresh water to make it portable?
E.g. the water is filtered using a wire mesh, and then using filter beds (1 mark)
The filtered water is then sterlised using chlorine/ozone/ultraviolet light (1 mark)
