Video: GCSE Chemistry Higher Tier Pack 1 • Paper 2 • Question 2

GCSE Chemistry Higher Tier Pack 1 • Paper 2 • Question 2

06:18

Video Transcript

Water from the sea can be used to reduce drinking water. During this process, bacteria in the water may be killed by irradiation or chemical treatment. Name two other processes used to remove impurities from seawater. Give a reason for each process.

Let’s have a look at what’s in seawater. We have fish, salt, particulates, and various types of microorganism. Now, the fish should have a good sense not to swim into the purification factory and the various microbes, they’re killed by radiation or chemical treatment, leaving us with salt — various dissolved solids — and particulates — stuff that’s not dissolved, but is at least suspended in water.

Let’s remove the particulates first. The easiest way to remove solid particles from a liquid is to filter it. This leaves us with only the dissolved solids, things like sodium chloride. There are many ways of removing salt from water, things like membrane distillation, evaporation, or reverse osmosis. But these are all types of desalination.

So the two processes used to remove impurities from seawater are filtration to remove solid particles and desalination to remove dissolved salt.

Drinking water can also be produced from waste water. Explain how and why waste water is treated differently from seawater and why less energy is needed to produce drinking water from waste water than from seawater.

The first item that distinguishes waste water from seawater is that waste water generally contains more organic matter and more microbes, while seawater generally contains more dissolved salt. So waste water requires more filtration or purification steps, while seawater requires more desalination. In comparison to filtration, desalination requires a lot more energy.

We now have the framework around to build our answer. So the answer is conversion of waste water into drinking water often involves more purification and sterilization steps than for seawater because waste water contains more microbes and organic matter. However, waste water has a much lower salt content than seawater. So it requires less desalination.

Desalination is energy demanding. So the purification of waste water consumes less energy than for seawater.

Drinking water can be made from river water by removing solid particles. Complete Figure one to show how the funnel may be used to remove solid particles from the river water. Include labels for the river water, drinking water, and any additional apparatus used.

So first off, if we put our river water directly through the funnel, it’ll just splash straight onto the floor. So we need a barrier of some sort that will let water through, but not the solid particles. The best answer for this is filter paper. When made into a cone or into a fluted structure, filter paper will capture solid particles and water will drip straight through.

Now, we need a vessel to contain the drinking water. I’m going to use a conical flask, but you could have used a beaker. But you would have just had to clamp to the funnel in place. And there we have it: a setup that will filter river water, remove solid particles, and leave nice fresh drinking water in the conical flask.

How could a chemical test be used to ensure that drinking water contains no dissolved chloride ions? Give the expected result of the test if chloride ions are present.

Let’s say we have a solution with some chloride ions swimming around. What we need is some kind of visual change that will show us that those chloride ions are there. Any change in this list will do: light, colour, a precipitate, sound, heat, bubbling.

Now, you should remember that there’s a test for chloride ions that involves a precipitate: adding a little bit of silver nitrate. Silver nitrate dissolves to produce a mixture of silver ions and nitrate ions. The silver ions will migrate around until they find a chloride ion. And then, silver chloride which is a white precipitate will come crashing out of solution.

So aqueous silver nitrate in dilute nitric acid, where the dilute nitric acid is there to help prevent false positives, is added to the drinking water. If chloride ions are present, a white precipitate of silver chloride is formed. So this is an effective chemical test to ensure that drinking water contains no dissolved chloride ions. Because if there is chloride ions, you would see a white precipitate; otherwise, not.

Why is it useful to determine the boiling point of waste water after it has been treated to produce drinking water?

So first off, if we did have pure drinking water, what would we expect the boiling point to be? That’s right! 100 degree C. However, remember that if there’s something dissolved in water, it’s boiling point increases. So, by measuring the boiling point of your drinking water, you can determine if there’s some kind of contaminant.

So it’s useful to determine the boiling point of waste water after it has been treated to produce drinking water because the boiling point is measured to ensure the treatment is successful. Impurities raise the boiling point of water above 100 degree C.

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