4.10.1.2 - Potable water

Is water essential for life?

Yes

For humans, decide the state of drinking water that is appropriate for us to drink.

• sufficiently low levels of dissolved salts and microbes

• pH between. 6.5 and 8.5

Define potable water.

Water that is safe to drink (as it’s been treated / is naturally safe)

Are potable water and pure water the same?

Potable water is not pure water in the chemical sense because it contains lots of other dissolved substances.

Pure water - only contains H2O molecules

What does the method used to produce potable water depend on?

The methods used to produce potable water depend on:

• available supplies of water

• local conditions.

What is rainwater a type of?

Rainwater is a type of fresh water

What is fresh water?

Water that doesn’t have much substances dissolved in it

Where does water collect when it rains?

Either as surface water (in lakes, rivers and reservoirs) or as groundwater (in rocks called aquifers- that trap water underground)

Where does most of the domestic water supply come from in warm areas and why?

In warm areas e.g. the south east of England, surface water tends to dry up first so most of the domestic water supply comes from groundwater.

What do certain dry countries have to do to provide potable water?

In some very dry countries, e.g. Kuwait, there’s not enough surface or groundwater

So instead, sea water must be treated by desalination to provide potable water

Does rainwater need to be treated?

Even though rain water only has low levels of dissolved substances - water from these water sources still needs to be treated to make it safe before it can be used.

Describe what is involved in the process of treating rainwater.

• Filtration (a wire mesh screens out large twigs etc. then gravel and sand beds filter out any other solid bits)

• Sterilisation (water is sterilised to kill any harmful bacteria or microbes. This can be done by bubbling chlorine gas through it or by using ozone or ultraviolet light)

Name the sterilising agents that can be used for potable water.

• bubbling chlorine gas through it

• using ozone

• using ultraviolet light

How is most portable water produced in their UK?

In the United Kingdom (UK), rain provides water with low levels of dissolved substances (fresh water) that collects in the ground and in lakes and rivers:

• choosing an appropriate source of fresh water

• passing the water through filter beds

• sterilising

When is desalination of salty water or sea water required?

If supplies of fresh water are limited - usually in very dry countries

In what two ways can desalination be done?

• process that uses a membrane such as reverse osmosis

• distillation

Both processes require large amounts of energy

Write out how distillation can used to desalinate sea water.

1. First test the pH of the water using a pH meter.   If the pH is too high or too low you can neutralise it.    You can do this with a titration but use a pH meter rather than an indicator to test when the solutions neutral so that it won’t contaminate the water

2. You should also test the water for the presence of sodium chloride  (the main slat in seawater).  To test for sodium ions - do a flame test on a small sample (if it’s present the flame will turn yellow) To test for chloride ion - take a another sample of your water and add a few drops of dilute nitric acid, followed by a few drops of silver nitrate solution, if present a white precipitate will form.

3. To distil the water, pour the salty water into a distillation apparatus. Heat the flask from below. The water will boil and form steam - leaving any dissolved salts in the flask.  The steam will condense back to liquid water in the condenser and be collected as it runs out

4. Retest the distilled water for sodium chloride to check that it has been removed. Retest the pH with a pH meter to check if it’s neutral

Why is a pH meter used when checking the pH?

As an indicator could contaminate the water

What is the main salt in seawater?

Sodium chloride

Describe how seawater can be treated by processes that use membranes (like osmosis)

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

What are the cons for these processes

• Both require lots of energy

• So they’re really expensive

• Not practical for producing larger quantities of fresh water