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Tap water is safe to drink and nowhere near pure, and the gap between those two words is the whole of this topic. Every process here is somebody deciding what standard they actually need, and what they are willing to pay in energy to reach it.
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Safe is cheap. Pure is expensive
The word pure means something specific in chemistry: a single substance and nothing else. The water out of your tap is not pure and was never meant to be. It is potable, which means safe to drink, and it is full of dissolved substances that nobody has any reason to remove. That is not a vocabulary quibble. It is the whole of this topic. Getting water to safe is cheap. Getting it to pure costs a great deal of energy. So nobody pays for pure unless they have no choice, and every process in this node is somebody deciding which standard they actually need. The metals half of the topic is the same question wearing different clothes. Rock with too little metal in it for a furnace to be worth firing is still worth working, if you will accept a slower and cheaper route. So carry one question through: what standard does this have to reach, and what does reaching it cost? How to weigh a whole product across its lifetime is a separate question and a separate module on this topic.
Five words worth getting exactly right
Two of these describe how much of something there is, one describes a standard, and two are ways of getting metal out of rock that a furnace would reject.
Follow the water to the tap
A water company is supplying a town from a freshwater reservoir. Put the stages into the order they happen.
- A suitable source of fresh water is chosen, such as a reservoir or a river
- Large debris is screened out before anything else is attempted
- The water is passed through filter beds, which take out the solid particles
- It is sterilised, using chlorine, ozone or ultraviolet light, which deals with the microbes
- Samples are tested to confirm it now meets the standard for drinking
- It is piped out to the town as potable water
The analysis that looks like bad news
A sample of treated tap water is analysed and found to contain dissolved calcium, magnesium and chloride compounds. A student concludes that the treatment has failed. What is wrong with that conclusion?
- Treatment aims to make water safe to drink rather than chemically pure. Dissolved substances at these levels do no harm, and taking them out would cost far more energy for no benefit
- Nothing is wrong with it: water that is safe to drink should contain nothing dissolved at all
- Those substances prove the filter beds were skipped, since filtering removes dissolved compounds
- The analysis must be faulty, because sterilising with chlorine destroys dissolved compounds
The cheap route and the expensive one
Every process in this topic sits in one of these two columns, and which column you are in is decided entirely by what standard you need to reach.
Five places, five different answers
- A town sitting beside a large, clean freshwater lake
- A coastal town with no fresh surface water and no useful groundwater
- A laboratory needing water to make up solutions for accurate analysis
- A deposit of rock containing copper, but far too little for a furnace to be worth firing
- Sewage arriving at a treatment works
- There is already fresh water to start from, so taking out the solids and then dealing with the microbes is enough, and nothing has to be boiled
- There is no fresh water to treat at all, so salt has to come out of sea water, and the energy cost is accepted because there is no alternative
- Anything dissolved would change the result being measured, so this is one of the few places where the expensive standard really is the one required
- Living things can concentrate the metal slowly and with very little energy, which makes rock worth working that would otherwise simply be left where it is
- What arrives is in worse condition than any natural source, so the solids have to be separated off and the remaining liquid treated biologically before any of it can go anywhere
Work out how much rock
A low-grade copper ore contains 2 kg of copper in every 1000 kg of rock. A company wants to obtain 60 kg of copper from it. How many kilograms of rock must they process?
Two sentences to strike out
Five statements from a set of revision notes. Tap the TWO that are wrong.
- Potable water is safe to drink but is not chemically pure
- Filter beds remove solid particles but not dissolved substances
- Sterilising the water removes the dissolved salts from it
- Desalination is used where there is no fresh water to treat instead
- Phytomining and bioleaching are used because they are faster than heating ore in a furnace
The water that arrives in the worst condition
Sewage and agricultural waste water arrive carrying organic matter and harmful microbes, and industrial waste water may carry harmful chemicals as well. Far more has to be done to it than to water taken from a reservoir, and it is done in a set order. First screening and grit removal, which takes out the large solids and the grit. Then sedimentation, which separates what arrived into two things that are then treated quite differently: a settled sludge and a liquid effluent. The sludge is broken down by anaerobic digestion, without air, which also produces a useful gas. The effluent is treated aerobically, with air bubbled through it so that microbes can break down what is left. The pattern is the same one again. Nobody tries to turn sewage into pure water, because they do not need to. They treat it until it is fit for the standard it is going to, and then they stop. Whether a whole product is worth making at all, weighed across its lifetime, is the question the other module on this topic asks.
Name the process, five in a row
Each description points to one process. Three lives.
How clean does it actually have to be
A school caretaker is asked how clean something needs to be, and gives a different answer every time. A chopping board in the kitchen is scrubbed and then disinfected, because food is going on it. A glass slide for the science department is worked on far longer, until there is genuinely nothing left on it at all, because anything remaining would show up under the microscope and ruin the lesson. The playground fence is not cleaned whatever, because it does not have to reach any standard. Clean is not one thing. It is a target, and the effort climbs very steeply as the target rises. The same caretaker keeps a jar of bent screws in the workshop. Straightening them takes an hour and is not worth it while new ones are cheap. When the supplier puts prices up, that same hour becomes worth spending, and the jar stops being rubbish and starts being stock. Nothing about the screws changed. What changed was what it was worth doing to them.
Put the proper words back
Water that is safe to drink is called _____ water, which is not the same thing as chemically pure. In the United Kingdom fresh water is passed through _____ beds to take out the solid particles, and is then made safe by _____ it with chlorine, ozone or ultraviolet light. Where there is no fresh source at all, salty water must be treated by _____, the removal of the salt, which uses far more energy. Metal ores are _____, meaning the supply cannot be replaced once it has been used, which is why a slow process such as _____, in which bacteria produce a solution containing the metal compound, can be worth running on rock a furnace would reject.
What the slow routes actually buy you
Select every statement that is a fair description of phytomining and bioleaching.
- They make it worth working rock that holds too little metal for a furnace to be worth firing
- They use much less energy than heating ore in a furnace
- They are slower than conventional extraction
- They produce metal that is purer than metal from a furnace
- They give you the metal itself, with no further processing needed
One engineer, four requests
The same engineer is asked to advise on four different problems. Each stage sets out where things stand and hands you the decision.
- A town sits beside a large, clean freshwater lake and wants a drinking supply. Decide what the treatment has to do.
- The next request comes from a coastal town with no fresh surface water and no useful groundwater. Decide what has to change.
- A laboratory in that same coastal town asks whether the new town supply will do for making up solutions for accurate analysis. The supply is already safe to drink. Decide what to tell them.
- The last request is from a company that owns a large deposit of rock containing copper, in far too low a concentration for a furnace to be worth firing. Decide what they can do with it.
Build the answer an examiner wants
Assemble the sentence that gets the marks when a question asks why tap water is not pure, or why desalination is not used everywhere.
Explain what each supply actually needs
Explain how fresh water is made safe to drink, why that is not the same as making it pure, and why some places treat sea water instead despite the cost. Do not write about life cycle assessment or recycling: this answer is about reaching a standard.
- Explain what potable means, and say clearly how it differs from pure
- Describe the stages used to treat fresh water, saying what each stage removes or deals with
- Explain why filtering and sterilising leave dissolved substances behind, and why that is acceptable
- Explain why desalination is used in some places, and what it costs to use it
- Finish with one situation in which water really does have to be pure, and say why the cheap route will not do there