Separating Mixtures, Grade 7

Grade 7 Practical task 2 asks a class to separate beads, sand, iron filings and salt out of water, and to get every one of them back.

It is formally assessed, it is worth 30 marks, and every Grade 7 class in the country has to do it. There is one step where it goes wrong in almost every classroom, and it is not the step anyone expects.

This page is the whole task, in order, with the fix for the step that fails. It also carries the free worksheet and the marking memorandum.

Where this fits in the curriculum

Subject Natural Sciences
Grade 7
Term 2
Strand Matter and Materials
Topic Separating mixtures
Status Practical task 2, formally assessed, 30 marks
Marks 30 for the task. The topic revision is 30. Our worksheet is 40

The step that fails: lifting wet iron filings out of wet sand

This is the one to prepare for.

Dry iron filings jump onto a magnet and the demonstration looks like magic. Wet iron filings do not. They cling to wet sand, and the few that do come up then cling to the magnet and will not let go, so they end up back in the beaker or on the bench.

The fix takes ten seconds: wrap the magnet in a plastic bag or a layer of cling film. Sweep it over the mixture, lift, hold it over a dish, and peel the bag off. Every filing drops cleanly into the dish.

And take the filings out FIRST, while the mixture is still dry, before any water goes in. The order of the steps is the difference between a task that works and a task that does not.

The order that works

# Step What it separates Why here
1 Pick out the beads by hand Beads They are big and they are in the way of everything else
2 Sweep a bagged magnet over the dry mixture Iron filings Do this while it is dry. Once water is added it gets much harder
3 Add water and stir Dissolves the salt Sand does not dissolve, so this splits the two
4 Filter Sand stays in the paper Salt water runs through as the filtrate
5 Evaporate the filtrate Salt The water leaves, the salt stays behind

Four techniques, four properties. Hand sorting uses size, the magnet uses magnetism, filtering uses particle size, and evaporating uses boiling point. That is the answer to "which property did you use?", which is where most of the marks sit.

Apparatus, and what each piece is actually for

Item Price Used in
Beaker, borosilicate, low form R19 Two of them: one to mix in, one to catch the filtrate
Filter funnel, borosilicate glass R30 Step 4
Qualitative filter paper, 100 circles R29 Step 4. This is the only thing you use up
Alnico bar magnets with keeper R500 Step 2, wrapped in a bag
Tripod stand, stainless steel R205 Step 5
Gauze, wire with ceramic centre R250 Step 5, so the beaker is not in the flame
Spirit burner, stainless steel R260 Step 5
Safety goggles R30 Step 5, on every learner

The cheapest way to run this for a whole class is to share the heating station. One tripod, one gauze and one burner at the front, and a beaker, funnel and filter paper per group. The consumable is filter paper and nothing else, at well under a rand a circle.

Evaporate gently, or you will lose the salt

Do not boil the filtrate hard. A rolling boil spits salt water out of the beaker, and what lands on the bench is exactly the mass a learner is supposed to weigh at the end.

Heat it gently and take the flame away while there is still a little liquid left. The last of it dries off on its own. Leaving a wet beaker in a warm windowsill overnight works just as well and cannot spit at all.

Pure substance or mixture? The question that trips learners up

A pure substance is made of one kind of particle all the way through. A mixture is two or more substances stirred together, each keeping its own properties, and that is why a mixture can be separated by physical means at all.

You will meet a picture of someone blowing up a balloon, labelled as carbon dioxide and offered as an example of a pure substance. It is a mixture, and it is a good one to argue about.

Breathed-out air is roughly 78 % nitrogen, 16 % oxygen and only 4 to 5 % carbon dioxide, plus water vapour. It has more oxygen in it than carbon dioxide.

Limewater will go milky with it, which proves carbon dioxide is present. It does not prove the balloon is pure. That distinction is worth a whole lesson, and it is the difference between detecting something and measuring it.

Only iron, nickel and cobalt? Not quite

You will read that only iron, nickel and cobalt are magnetic. Check it against the magnets in your own classroom.

Gadolinium is magnetic too. And far more usefully, the strong magnets everyone actually owns are alloys: the silver button magnets are neodymium, iron and boron, and the red horseshoe magnets are alnico, which is aluminium, nickel, cobalt and iron.

The honest version is easy to say: the only common magnetic ELEMENTS are iron, nickel and cobalt, and a great many alloys are magnetic as well. The sentence is only wrong because of the word "only".

Distillation: why "only the alcohol evaporates" is wrong

Distillation separates two liquids that have different boiling points. Ethanol boils at 78 °C and water at 100 °C, so heating the mixture to 78 °C drives off mostly ethanol, which is then cooled back to a liquid and collected.

But "only the ethanol will evaporate, leaving the water behind" is not true, and this is worth knowing.

The vapour is always a mixture. Ethanol and water cling to each other in a way that means simple distillation can never get past about 95,6 % ethanol, no matter how carefully you do it.

That is why spirits are what they are, and why industry has to use fractional distillation, which is the same idea repeated over and over up a tall column. A Grade 7 does not need the column. They do need to know that one pass gives you "mostly", not "only".

Chromatography: the numbers as printed will ruin it

THIS IS THE SECOND THING THAT GOES WRONG, AND IT IS PURE ARITHMETIC.

The usual instructions say: draw your pencil line 2 cm from the bottom, then fill the beaker to 2 cm with solvent.

Put those two together and the ink spot is sitting IN the solvent. It dissolves straight off the paper into the beaker and the class watches a jar of grey water for twenty minutes.

The rule is one line long: the solvent must stay BELOW the pencil line. Fill to about 1 cm, keep the line at 2 cm, and the solvent creeps up the paper and carries the colours with it.

Use a pencil, never a pen, because pen ink runs and ruins the result. And the word comes from the Greek chroma, meaning colour, which is worth saying because it tells a learner exactly what the technique was invented to do.

Where the salt on your table actually comes from

South Africa makes most of its table salt by evaporation, not by mining it, and the two words get mixed up.

Saltworks or salt pan Salt mine
Sea water is let into shallow ponds and the sun evaporates it Solid rock salt is dug out from underground
Coega near Gqeberha, and Velddrif on the West Coast Not how the big South African operations work
Exactly step 5 of the practical task, at the size of a suburb A different process entirely

So the last step of the task is a real industry, and a learner can drive past it. That is a better answer to "where is this used in real life?" than anything invented.

If it does not work

Problem Cause Fix
The magnet will not pick up the filings The mixture is already wet Bag the magnet, and next time take the filings out before adding water
The filings will not come off the magnet Bare magnet Wrap it in a bag and peel the bag off over a dish
The filter runs painfully slowly Paper folded flat against the funnel Fold into quarters, open one side into a cone, and wet it first so it sticks
The filtrate is still cloudy Paper torn, or poured over the rim Refilter through fresh paper and keep the level below the top of the cone
Salt spits out during evaporation Heated too hard Gentle heat, and stop while a little liquid is left
Chromatography gives a grey blob Spot sitting in the solvent Solvent below the pencil line. Fill to 1 cm, not 2
The colours do not move at all Pen ink, or the wrong solvent Pencil for the line, koki for the spot, and try water and then surgical spirits

A warning about the version with sweets in it

You will see this task set with sweets in place of the beads. Do not use sweets.

They dissolve. The sugar shell goes into the water, the colouring stains everything, and the learner is then asked to "collect all the materials", which is now impossible. The beads exist in the original for a reason: they do nothing.

Glass beads, plastic beads or even small pebbles all work. Anything that will not dissolve and is too big to pass a filter paper.

How the 40 marks are made up

Part Marks
Pure substances and mixtures 8
The separation sequence, and the property used at each step 12
Filtration and evaporation in detail 8
Distillation and chromatography 7
Recycling and sorting materials 5

The formally assessed task itself is 30 marks, and the topic revision is also 30. Our worksheet is 40 and the split above is ours.

Free worksheet and marking memo

Both free, no sign up, straight to the PDF.

  • Learner worksheet, 40 marks, including the separation sequence, the balloon question and the chromatography numbers
  • Marking memorandum, with what to accept for each answer and the five answers that look right and score nothing

Related practicals

What you need to run it

A group set is a beaker, a funnel and a filter paper. That is R78, and the filter paper is the only part you replace.

The heating station is shared: tripod, gauze and spirit burner at the front of the room, with goggles at R30 on every learner who goes near it.

One thing we do not stock: iron filings. Ask your hardware shop for the sweepings from a bench grinder, or use a fresh pot scourer cut up small. We would rather say so than sell you something else.

Glassware is in laboratory glassware, the magnets are in magnetism, and the filter and chromatography papers are in pH and litmus test papers.