C. Matter and Energy · Pure Substances and Mixtures

Homogeneous and Heterogeneous Mixtures

C2.3 C2.2 A1.2 A1.4About 40 minutes

Learning goals

I can explain the difference between homogeneous and heterogeneous mixtures.
I can identify solutions, mechanical mixtures and suspensions.
I can classify everyday mixtures and give evidence for my choice.

Words to know

homogeneous mixture
A mixture that looks the same all the way through, even under a magnifying glass. Also called a solution.

heterogeneous mixture
A mixture where you can see two or more different parts.

solution
A homogeneous mixture: the particles of the substances are evenly mixed. Examples: salt water, tea, air, vinegar.

mechanical mixture
A heterogeneous mixture where you can easily see the different parts, like trail mix or gravel.

suspension
A heterogeneous mixture where bits of one substance are spread through a liquid or gas, but settle or separate when left to stand. Example: muddy water.

alloy
A solution of two or more metals (or a metal with another element), mixed while melted. Examples: brass, bronze, steel.

Two kinds of mixtures

Look at a bowl of trail mix. You can see the nuts, the raisins and the chocolate. Now look at a glass of apple juice. It looks like one liquid, even though it's a mixture of water, sugars and other substances.

Scientists sort mixtures into two groups:

A homogeneous mixture looks the same throughout. Homo means same. You can't see the different parts, even with a magnifying glass. Homogeneous mixtures are called solutions.
A heterogeneous mixture has parts you can see. Hetero means different.

A bowl of trail mix with nuts, raisins and candies beside a glass of apple juice. — In trail mix you can see and pick out each part: a heterogeneous mixture. The apple juice looks the same throughout: a homogeneous mixture. AI-generated image

Left: a homogeneous mixture (solution). The yellow particles are spread evenly among the blue ones. Right: a heterogeneous mixture. The yellow particles are bunched together in one area, so the mixture is different in different places.

Kinds of heterogeneous mixtures

Mechanical mixture: you can easily see and pick out the different parts. Examples: trail mix, a salad, gravel, a chocolate chip cookie, granite (a rock with different-coloured crystals).
Suspension: small pieces of one substance are spread through a liquid (or gas) but settle or separate if you leave it alone. Examples: muddy water, orange juice with pulp, oil-and-vinegar salad dressing (shake it, then watch the oil float back up), a snow globe after you shake it.

Solutions are everywhere

Solutions can be liquids, gases or even solids.

Liquid: salt water, tea, vinegar, clear apple juice.
Gas: air (nitrogen, oxygen and other gases).
Solid: alloys, which are metals mixed while melted. Brass is copper and zinc. Bronze is copper and tin. Steel is mostly iron with a little carbon.

Three kinds of mixtures side by side. In the solution the solute is spread evenly; in the suspension the pieces sink to the bottom when left alone; in the mechanical mixture you can see every part.

Mixture	Homogeneous or heterogeneous?	Type	Evidence
Salt water	Homogeneous	Solution	Clear, same throughout, nothing settles
Trail mix	Heterogeneous	Mechanical mixture	You can see and pick out each part
Muddy water	Heterogeneous	Suspension	Cloudy; soil settles to the bottom when left still
Air (clean)	Homogeneous	Solution	Gases evenly mixed
Brass	Homogeneous	Solution (alloy)	Same metal all the way through
Oil-and-vinegar dressing	Heterogeneous	Suspension	Separates into layers when left to stand

Classifying mixtures with evidence.

Worked example

Let's classify three mixtures

Classify tea (no tea leaves), a snow globe after shaking and granite. Give evidence.

Tea: it is clear (you can see through it), the same colour throughout, and nothing settles out even after a day. → homogeneous, a solution.
Snow globe after shaking: you can see white flakes floating in the water. When you stop, they slowly sink to the bottom. → heterogeneous, a suspension.
Granite: you can see different-coloured crystals (often pink, white and black) side by side. → heterogeneous, a mechanical mixture.

Tea is homogeneous (a solution). The shaken snow globe is a heterogeneous suspension. Granite is a heterogeneous mechanical mixture.

Just for fun

Trail mix is a heterogeneous mixture, which is science's polite way of saying you can see exactly which bits your friend picked out first.

What about milk?

Some mixtures, like milk, mayonnaise and fog, look the same throughout, but they contain tiny droplets or clumps that are bigger than the particles in a solution. These are called colloids. They are cloudy, not clear, and they scatter a beam of light. Different textbooks sort colloids differently, so in this unit we'll stick to clearer examples. A good clue: a solution is transparent (you can see through it), even if it is coloured.

Hands-on activity

The flashlight test

You need: 3 clear glasses, water, salt, a few drops of milk, a spoonful of soil from the garden, a spoon, a flashlight and a dark room.

Glass 1: water + 1 teaspoon of salt, stirred until it disappears. Glass 2: water + about 5 drops of milk, stirred. Glass 3: water + 1 spoonful of soil, stirred.
Predict which ones are homogeneous and which are heterogeneous.
In a dark room, shine the flashlight through the side of each glass. Look from the side: can you see the beam inside the liquid?
Leave the glasses still for 15 minutes. Does anything settle?
Record your observations in a table and classify each glass.

What to look for: in the salt water the beam is almost invisible inside the liquid, and nothing settles: a solution (homogeneous). In the milky water, the beam shows up as a glowing path, because tiny fat droplets scatter the light: a colloid. In the soil water, the liquid is cloudy, the beam is scattered, and soil settles to the bottom: a suspension (heterogeneous).

Safety: never shine the flashlight into anyone's eyes. Wash your hands after touching soil. Don't drink any of the mixtures; pour them out at the end (the soil water can go in the garden).

Your turn

What does homogeneous mean?

What is another name for a homogeneous mixture?

Homogeneous or heterogeneous? Cereal with milk.

Homogeneous or heterogeneous? Vinegar.

What is a suspension? Give an example.

Is brass a pure substance? Explain.

Your friend says a glass of grape juice can't be a solution because it is purple. Is your friend right?

More practice: warm-up

Homogeneous or heterogeneous? Pizza.

Homogeneous or heterogeneous? Clean air.

Homogeneous or heterogeneous? Sand and pebbles.

Mechanical mixture or solution? Trail mix.

Name the two metals in bronze.

More practice: on your own

Orange juice with pulp is a suspension. What would you see if you left a glass of it in the fridge overnight?

Is a chocolate chip cookie a mechanical mixture or a suspension? Explain.

A clear, colourless liquid looks the same throughout. Can you be sure it's a solution? What else could it be?

Why does salad dressing made of oil and vinegar need to be shaken before you use it?

Steel is used for bridges and cars. What kind of mixture is it, and what is it made of?

Challenge

Classify each and give one piece of evidence: (a) soda water with the cap on, (b) soda water poured into a glass and fizzing.

Draw (or describe) particle diagrams of (a) salt water and (b) muddy water. Explain the difference.

A science fair project says, "Homogeneous mixtures are always liquids." Give two counter-examples from different states of matter.

Check your understanding

Quiz

A homogeneous mixture...

Which is a solution?

Muddy water is an example of a...

Brass is a solution of...

Which is a heterogeneous mixture?

Which clue best shows that a liquid is a solution, not a suspension?

For parents and teachers

Big idea: Ontario expectation C2.3 asks students to distinguish between homogeneous mixtures (solutions) and heterogeneous mixtures (mechanical mixtures and suspensions). The flashlight test is a simple investigation (A1.2) done safely (A1.4).

How to help: At meals, ask "Homogeneous or heterogeneous? How do you know?" Look for the clues: Can you see different parts? Is it see-through? Does anything settle?

Common difficulties:

Thinking coloured liquids can't be solutions. Coloured but transparent liquids (tea, grape juice) are solutions.
Thinking solutions must be liquids. Air and alloys are solutions too.
Milk: it's a colloid, which textbooks classify differently. That's why we avoid it in the questions.

Safety: no flashlight in the eyes; wash hands after handling soil; nobody drinks the test mixtures.