C. Matter and Energy · Pure Substances and Mixtures

Separating Mixtures

C2.7 A3.3 A1.2 A1.4About 50 minutes

Learning goals

I can choose a method to separate a mixture, based on the properties of its parts.
I can explain how filtration, evaporation, distillation, chromatography, magnetism, sieving, decanting and centrifuging work.
I can separate a solution as well as a mechanical mixture.
I can describe how First Nations knowledge of separation is used in making maple syrup and processing wild rice.

Words to know

filtration
Separating an insoluble solid from a liquid by pouring it through a filter. The liquid that passes through is the filtrate.

residue
The solid left behind in the filter (or in the dish after evaporation).

evaporation
Separating a dissolved solid from a solution by letting the liquid turn into a gas, leaving the solid behind.

distillation
Boiling a liquid and cooling its vapour to collect the liquid again, leaving dissolved substances behind.

chromatography
Separating the dissolved substances in a mixture (like the dyes in ink) by letting a solvent carry them along paper at different speeds.

decanting
Carefully pouring off a liquid and leaving a settled solid (or a heavier liquid) behind.

centrifuge
A machine that spins mixtures very fast so the heavier parts move to the bottom.

sieving
Separating solids of different sizes with a screen or strainer.

Undo the mix!

You knock a jar of salt into a box of beads. Or your little cousin mixes the sand from the sandbox with the iron nails from the garage. How do you get each part back?

Because the parts of a mixture are not chemically joined, they keep their own properties. We can use those properties to separate them.

A red horseshoe magnet lifting iron nails out of a tray of sand. — A magnet separates iron from sand because only the iron is attracted to it. Choosing a method depends on the properties of each part. AI-generated image

Method	Property used	Works for	Everyday or industry example
Hand sorting	Size, colour, shape	Large pieces	Sorting recycling; picking raisins out of trail mix
Sieving	Particle size	Solids of different sizes	Sifting flour; draining pasta in a colander
Magnetism	Attraction to magnets	Iron or steel mixed with other things	Pulling steel cans out of recycling
Decanting	Settling (density)	A solid that sinks, or two liquids in layers	Pouring off the clear liquid above settled sediment
Filtration	Particle size	Insoluble solid + liquid	Coffee filter; furnace filter; water treatment plants
Evaporation	Liquid becomes a gas, solid doesn't	Dissolved solid + liquid (keep the solid)	Sea salt from salt water; maple syrup
Distillation	Different boiling points	Solution (keep the liquid)	Distilled water; separating crude oil
Chromatography	How strongly each substance is carried by a solvent	Small amounts of dissolved substances	Separating ink dyes; testing food or blood samples
Centrifuging	Density (heavier parts move out)	Mixtures that settle slowly	Separating blood cells from plasma; cream from milk

Separation methods and the properties they use.

Separating solutions

Filtration can't separate a solution. The dissolved particles are so small they pass right through the filter with the water. To separate a solution, we need a change of state:

Evaporation lets the water escape into the air as water vapour. The solute is left behind. (You did this in Grade 5 with salt water.)
Distillation boils the solution and then cools the vapour so it condenses back into a liquid in another container. You get to keep both: the pure liquid (the distillate) and the solid left behind. Your Grade 5 sun-powered still was a slow distillation.
Chromatography separates dissolved substances from each other, like the different dyes in a black marker. Water travels up a strip of paper. Dyes that are more strongly carried by the water move farther; dyes that stick more to the paper stay lower.

Distillation of salt water (simplified). Heat turns the water into vapour, the cold condenser turns it back into liquid in a new container, and the salt is left behind in the flask.

Worked example

Let's separate a three-part mixture

You have a mixture of sand, salt and iron filings. Separate all three and keep each one.

Look at the properties. Iron is magnetic. Salt is soluble in water. Sand is insoluble in water.
Step 1 – Magnetism: put the magnet in a plastic bag (so filings don't stick to it) and move it over the mixture. The iron filings jump to the bag. Turn the bag inside out over a dish to collect them.
Step 2 – Dissolve: add water to the sand and salt and stir. The salt dissolves; the sand doesn't.
Step 3 – Filtration: pour through a coffee filter. The sand stays in the filter (residue). The salt water passes through (filtrate).
Step 4 – Evaporation: leave the salt water in a shallow dish in a sunny spot for a few days. The water evaporates and salt crystals are left.

Magnet → iron. Dissolve and filter → sand. Evaporate → salt. Each step uses a different property.

The three-part separation as a flowchart. Each step pulls out one substance using a different property: magnetism, then insolubility (filtering), then solubility (evaporating).

Just for fun

Why did the iron filings leave the party early?

They felt a strong attraction to someone else: the magnet.

Knowledge from First Nations communities

People have been separating mixtures for thousands of years.

Maple syrup. First Nations peoples in the Great Lakes region and eastern Canada developed ways to collect maple sap and evaporate most of its water, concentrating the sugars into syrup and sugar. Sap is mostly water, so it takes about 40 litres of sap to make about 1 litre of syrup. Many First Nations families and communities continue this practice every spring, and settlers learned it from them.
Wild rice (manoomin). Anishinaabe harvesters gather manoomin by canoe in late summer. After the rice is dried and parched, it is jigged or "danced" to loosen the hulls. Then it is winnowed: tossed in a shallow tray, often made of birchbark, so the wind carries away the light hulls and the heavier grains fall back. Winnowing separates solids by mass using moving air.

Hands-on activity

Separation stations

You need: a mixture of 2 tablespoons of dried beans, 2 tablespoons of uncooked rice, 1 tablespoon of salt and 5 steel paper clips; a magnet in a sandwich bag; a colander or strainer with holes bigger than rice grains; a coffee filter in a funnel (or a filter held over a jar with an elastic band); water; a shallow dish.

Part 1: separate everything and keep each part.

Plan your steps first. For each step, name the property you are using.
Try your plan. Record what worked and what didn't.

Part 2: chromatography. Cut a strip from a coffee filter (a grown-up can help with scissors). Draw a thick dot of washable black marker 2 cm from the bottom. Tape the top to a pencil and hang the strip in a glass with 1 cm of water. The water must touch the paper below the dot. Watch for 10 minutes.

What to look for: Part 1: magnet → paper clips; colander → beans stay, rice and salt fall through; add water to rice and salt → salt dissolves; pour through the coffee filter → rice stays (the colander holes are too big to catch rice); evaporate the salty water in a sunny window over several days → salt. Part 2: the black dot spreads into bands of different colours (often blue, purple, yellow or pink), because black ink is a mixture of dyes.

Safety: paper clips and beans are choking hazards for small children. Keep the magnet away from phones, bank cards and young children. Don't eat the rice or beans after the activity.

Your turn

Why can mixtures be separated by physical methods?

Which method separates sand from water?

Why can't you separate salt water with a filter?

How do you get salt back from salt water?

What is the difference between evaporation and distillation?

What does chromatography show about black marker ink?

What property does a centrifuge use?

More practice: warm-up

Which method uses a magnet?

Draining pasta in a colander is which method?

What is the solid left in a filter called?

What is the liquid that passes through a filter called?

About how many litres of sap make 1 L of maple syrup?

More practice: on your own

Suggest a way to separate a mixture of sawdust and iron nails. Explain.

Oil and water form two layers. How could you separate them?

A survival guide shows how to get drinking water from sea water by boiling it and collecting the steam on a cold surface. Which method is this, and why does it work?

Why does winnowing wild rice work?

In chromatography, why must the ink dot be above the water level?

Challenge

Plan how to separate a mixture of sugar, pepper and small pebbles and keep all three.

Why is chromatography useful to police scientists checking whether a note was written with a certain pen?

Maple sap is often only about 2% sugar. Explain why making syrup is a separation, not a chemical change, and why it takes so much sap.

Check your understanding

Quiz

Which method would best separate iron filings from sand?

Which method keeps the water from salt water?

Filtration works to separate...

Chromatography of black marker ink shows that the ink is...

Making maple syrup from sap mainly uses...

Winnowing wild rice separates the grains from the hulls using...

For parents and teachers

Big idea: Ontario expectation C2.7 asks students to explain processes used to separate mixtures, including solutions, and to identify applications. Each method uses a property: size, magnetism, solubility, density or boiling point. The First Nations practices (maple syrup and manoomin) connect to A3.3, contributions from various communities.

How to help: Point out separations at home: draining pasta (sieving), coffee filters (filtration), a salad spinner (spinning water off lettuce). Ask "What property makes that work?"

Common difficulties:

Trying to filter a solution. Dissolved particles pass through filters.
Mixing up evaporation and distillation. Distillation collects the liquid; evaporation doesn't.
Forgetting to dissolve before filtering in multi-step separations.

Safety: small objects (beans, paper clips) are choking hazards for younger children; keep magnets away from little ones and electronics. Any boiling or distillation is done only by an adult. Evaporation in a sunny window needs no heat.