The Hydrosphere: Water Quality and Purification, Grade 10
Ninety-seven per cent of the water on Earth is ocean. Another two per cent is frozen. Almost everything humanity drinks, irrigates with and washes in comes out of the remaining fraction of one per cent.
That is the hydrosphere, and it is why a Grade 10 project on water quality is not an abstract exercise.
This page covers the whole topic, the project as we would actually run it, and the one test in the standard version that needs concentrated sulfuric acid and can safely be dropped.
What the hydrosphere is
The hydrosphere is all of Earth's water, wherever it happens to be: oceans, rivers, lakes and dams, groundwater, the ice caps and glaciers, permafrost, and the water vapour in the air.
It is one of four global systems, alongside the atmosphere, the biosphere and the lithosphere, and the four constantly exchange water between them.
Where the water actually is
| Where | Share | Can we use it? |
|---|---|---|
| Oceans | 97 % | No. Too salty to drink or irrigate with |
| Frozen: ice caps and glaciers | 2 % | Not practically |
| Groundwater | 0,5 % | Yes, by borehole |
| Surface water: rivers, lakes, dams | 0,2 % | Yes, and this is most of what we live off |
Read the last row again. Two thousandths of the world's water is doing most of the work, and it is the part most exposed to whatever we put into it.
Water quality in South Africa, which is what makes this project real
This topic is usually written as though water quality is a general concern. Here it is a specific one, and every learner has a version of it at home.
| The idea | What it looks like here |
|---|---|
| Tap water is not automatically safe | Boil-water notices. Municipalities issue them, and they are worth talking about |
| Water can carry chemicals and heavy metals | Acid mine drainage on the Witwatersrand. Water decanting out of old workings at pH 2 to 3 |
| Hard water holds dissolved calcium and magnesium | Borehole water across the interior is hard. The scale in a kettle is the visible version |
| Rainwater is a usable source | Tank water. Rainwater harvesting is ordinary here, not exotic |
| Dams change rivers and move people | The Lesotho Highlands Water Project supplies much of Gauteng and displaced communities to do it |
One thing that usually goes unsaid. The World Commission on Dams reported in 2000 that between 40 and 80 million people worldwide had been displaced by dam building. That commission was chaired by Kader Asmal, who was South Africa's Minister of Water Affairs and Forestry. A South African chaired the global inquiry into what dams cost, and most South African classrooms never hear it.
The test we have removed, and why you do not need it
The standard version of this project includes the brown ring test for nitrates, which uses concentrated sulfuric acid.
The method has a learner run two or three drops of concentrated acid down the inside of a test tube. It requires a fume cupboard and close supervision, and the safety warning attached to it is longer than the method itself.
Leave it out. The nitrate result is already on the page.
The same project has the class dip a six-way test strip that reads nitrite, nitrate, pH, carbonate, chlorine and total hardness in one go. So nitrate gets measured twice: once by a paper strip that costs about a rand, and once by a method that needs a fume cupboard.
Nothing was substituted, because nothing needed substituting. The safe method was already there. Nothing in the curriculum asks a Grade 10 class to handle concentrated acid.
Where this fits in the curriculum
| Subject | Physical Sciences |
|---|---|
| Grade | 10 |
| Term | 4 |
| Topic | Chemical systems, the hydrosphere |
| Status | Set as a formal project |
| Marks | 60 on our project sheet. None is prescribed |
The project
It runs on water the class collects. That is the part that makes it a project rather than a practical.
Set the collecting task a week ahead
Five labelled samples, about 200 ml each, in clean bottles.
| Sample | Where from |
|---|---|
| Tap water | The school |
| Borehole or tank water | A learner's home. Common across the interior |
| Rainwater | A tank, or a clean bucket left outside |
| River, stream or dam water | Whatever is local |
| Distilled or bottled | The control. Do not skip it |
Nobody tastes anything at any point, including water they have filtered themselves.
Apparatus
| Item | Qty |
|---|---|
| Water quality test strips, 6-in-1 | 25 |
| pH test paper | 1 pk |
| Filter paper, 90 mm | 1 pk |
| Funnels and conical flasks | 2 each |
| Beakers, 100 ml | 6 |
| Test tubes and a rack | 10 |
| Microscope, slides and cover slips | 1 set |
| 2 litre bottle, sand, gravel, stones, cotton wool, activated charcoal | per group |
The sand and gravel are free and the test strips are the only thing that really matters. One strip, dipped once, gives six numbers.
Part 1: what you can see
- Describe every sample before touching it. Colour, clarity, anything floating or settled
- Filter each one through paper in a funnel. Pour slowly down a stirring rod
- Look at the paper, not the water. Note what was caught
- Fresh paper and a rinsed funnel between samples
Pouring too fast is what spoils this. Water goes over the rim of the paper, sediment gets through, and the filtrate comes out cloudy. It looks like the filtration failed when the pouring did.
Part 2: what you cannot see
One test strip per sample, dipped once, held level, read at the time the packet specifies.
Reading it late is the commonest error in the whole project. The pads keep developing, so a strip read at two minutes when the chart says thirty seconds gives a confident wrong answer.
Then check pH again with pH paper. Doing it twice is deliberate: two methods should agree, and it is worth discussing when they do not.
Part 3: what is living in it
A drop of each sample on a slide, cover slip lowered at an angle, lowest power first.
The river or dam sample is the one worth the lesson. Expect visible movement: protozoa, algae, fragments of plant matter. Tap and distilled water should show almost nothing, and that contrast is the point.
A bubble looks like an organism to a Grade 10. A bubble has a hard black rim and does not move. Say so before they start drawing.
Part 4: build a filter
Cut the base off a 2 litre bottle, invert it neck down over a beaker, cloth over the neck held with an elastic band. Then from the neck upwards:
| Layer | What it catches |
|---|---|
| Cotton wool | Fine particles |
| Fine sand | Smaller suspended solids. The main working layer |
| Activated charcoal | Tastes, odours and some dissolved organics, by adsorption |
| Gravel | Coarser material |
| Stones | The largest debris, and it stops the pour disturbing the bed |
Rinse the sand and gravel until the rinse water runs clear. Unrinsed sand is the single reason a group's filtrate comes out dirtier than what they poured in.
Pour the muddiest sample through slowly and collect it. Run it a second time and it gets clearer again.
Then do the thing that turns it into science
Test the filtered water with a fresh strip.
It looks clean. The nitrate reading has not moved. Neither has the hardness.
That is the finding of the whole project. Filtration removes what you can see and does nothing to what is dissolved. It also does not sterilise anything.
Why: filtration works by size. Sand and cotton wool trap particles too big to pass between the grains. Nitrate ions, and the calcium and magnesium that make water hard, exist as individual ions far smaller than any gap in the sand. They go straight through.
A learner who can say that has understood water treatment better than one who produced clear water and stopped.
What you should see
Real ranges for South African samples. Yours will differ. The pattern is what matters.
| Sample | pH | Nitrate | Hardness | On the paper | Under the microscope |
|---|---|---|---|---|---|
| Distilled | 5,5 to 7 | None | Zero | Nothing | Nothing |
| Tap | 7 to 8 | Low | Soft to moderate | Nothing or a trace | Little |
| Rainwater | 5,5 to 6,5 | Low | Very soft | Roof debris | Some |
| Borehole | 7 to 8 | Low to moderate | Often hard | Fine sediment | Little |
| River or dam | 6,5 to 8,5 | Often the highest | Variable | Visible sediment | The interesting one |
Three results worth stopping the lesson for.
Rainwater is acidic. Not pollution, just dissolved carbon dioxide forming carbonic acid. It surprises learners who expect rain to be pure.
Borehole water is hard. Dissolved calcium and magnesium picked up from the rock it sat in. That is what scales a kettle.
River water usually carries the most nitrate, and the reason is generally fertiliser running off farmland upstream.
If it does not work
| What you see | What caused it |
|---|---|
| Filtered water is dirtier than what went in | Unrinsed sand. Rinse until the rinse water runs clear |
| Filtrate cloudy after paper filtration | Poured too fast, or over the rim. Use a stirring rod |
| The column runs dry and stops | Sand packed too tightly or too fine. Loosen the top layer |
| A wild test strip reading | Read at the wrong time, or the strip was already damp |
| Two samples give identical results | Same tap, different bottles. Check where they came from |
| Nothing visible under the microscope | Wrong focal plane, or the sample really is clean. Try the river one |
| Circles that look like organisms | Air bubbles. Hard black rim, and they do not move |
| pH paper and the strip disagree | Normal within half a unit. More than that, one was misread |
| The filtered water still fails the strip test | Correct. That is the result |
The failure worth keeping
A group whose filter makes the water worse because they did not rinse the sand has learned more than a group whose filter worked.
They added a treatment step that made things worse. That happens in real treatment plants, and it is exactly why the output of a plant is tested rather than assumed.
How water is actually treated
| Method | Removes | Does not remove |
|---|---|---|
| Boiling | Micro-organisms | Anything dissolved. It concentrates it |
| Filtration | Suspended solids | Dissolved substances, micro-organisms |
| Chlorination | Micro-organisms | Dissolved chemicals |
| Activated charcoal | Tastes, odours, many organics | Dissolved salts |
| Reverse osmosis | Almost everything, including dissolved salts | Expensive, and needs high pressure |
| Aeration | Dissolved gases, some organisms | Solids and salts |
No single method does everything, which is why a treatment plant is a sequence rather than a machine.
A note on chlorination, because it is the one people improvise: household bleach works, but the water cannot be chlorinated in advance and stored, because the chlorine leaves and it is no longer sterile. And too much bleach is genuinely dangerous. This is not a good candidate for a classroom experiment.
How the 60 marks are made up
| Section | Marks |
|---|---|
| Your samples and why a control is needed | 5 |
| What you can see, and filtration | 8 |
| Test strip and pH results | 9 |
| Microscope work | 8 |
| Building and running the filter | 10 |
| What the filter did and did not do | 9 |
| The bigger picture: water distribution and dams | 8 |
| Conclusion | 3 |
No mark allocation is prescribed. Two activities and two experiments are set with no marks attached, so the project sheet and this split are ours.
The mark most often dropped is explaining why filtration misses dissolved substances without mentioning size. "Because it is dissolved" earns one of three. "Nitrate exists as individual ions, far smaller than the gaps between sand grains, so it passes straight through" earns all three.
Close behind: offering boiling as the treatment for a dissolved chemical. Boiling removes water, so it concentrates nitrate rather than removing it.
If you have time
The ion tests, if your school holds the reagents. Silver nitrate gives a white precipitate with chlorides; barium nitrate gives one with carbonates and sulfates; dilute nitric acid tells the two apart, because barium carbonate dissolves and fizzes while barium sulfate and silver chloride do not. All at 0,1 mol per cubic decimetre, and no concentrated acid at any point. The chemistry is on our precipitation reactions page.
Soften the hard sample. Add a little sodium carbonate solution to the borehole water and the calcium and magnesium precipitate out as carbonates. Filter it off and test the hardness again. It drops, and that is water softening.
Ask which of the five they would drink. The honest answer for the river sample is that filtering it is not enough, and getting a class to that answer is the whole project.
Free project sheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner project sheet, 60 marks, with the sample log, all the results tables, space for microscope drawings and the filter diagram
- Marking memorandum, with typical South African readings for all five sample types, guidance on marking results that vary between learners, and the six places marks are most often dropped
Related practicals
- Precipitation reactions and testing for ions, Grade 10. The chemistry behind the extension tests
- Separating mixtures, Grade 10. Filtration done properly, in Term 1
- Solubility and dissolving, Grade 10. Why anything is dissolved in the water in the first place
- The pH scale and indicators, Grade 9. Where pH was introduced
Equipment for this project
Most of it is glassware a school already owns. Two things are worth checking your store cupboard for.
pH test paper is R20 for 80 strips and it is in the pH and litmus test papers range. It does everything this project needs. A pH meter gives you a number instead of a colour and has to be rinsed and standardised between samples, so it is an upgrade rather than a requirement.
A microscope is the piece that makes Part 3 possible, and it is the part learners remember. They are in the microscope range, with slides and cover slips separately.
Filter paper is the only other consumable, and the sand, gravel and bottle cost nothing.
We are putting together a Water Quality Testing Kit for Grade 10 with the test strips, pH paper, filter paper, funnels, flasks, beakers, tubes and rack in one box, plus the printed project sheet and marking memo. Coming shortly.