Learning targets

Georgia S8P1a — Develop and use a model to compare and contrast pure substances (elements and compounds) and mixtures.
Target 1 · Pure substance vs mixture
  • I can tell whether a sample is a pure substance or a mixture.
  • I can use a particle picture to show the difference.
Target 2 · Element vs compound
  • I can classify a pure substance as an element or a compound using its formula or its particle picture.
  • I can explain why a compound's properties are different from the elements that made it.
Target 3 · Homogeneous vs heterogeneous
  • I can classify a mixture as homogeneous or heterogeneous.
  • I can name a physical way to separate a mixture and explain why a compound cannot be separated that way.

Everything is one of four things

Pick up anything: a penny, a glass of juice, the air. It is either a pure substance (one kind of particle all the way through) or a mixture (two or more kinds of particles just sharing space). Pure substances split into elements and compounds. Mixtures split into homogeneous and heterogeneous. Four boxes. That is the whole page.

Left to right: a metal (element), a crystal (compound), water (compound), trail mix (heterogeneous mixture). Which one is a homogeneous mixture? None of them, yet. Keep reading.
Element
pure substance A solid copper cube
One kind of atom. Copper is only copper atoms. Oxygen gas is only oxygen atoms (even though they travel in pairs, O2). It is on the periodic table.
Compound
pure substance A crystal made of two kinds of atoms locked together
Two or more kinds of atoms chemically bonded in a fixed ratio. Water is always H2O. Salt is always NaCl. Its properties are brand new, not a blend of its elements.
Homogeneous mixture
mixture A glass of orange drink, the same all the way through
Evenly mixed, looks the same throughout. You cannot see the parts. Sweet tea, salt water, air, brass. Also called a solution.
Heterogeneous mixture
mixture A bowl of trail mix with visible nuts, pretzels, and chocolate
Unevenly mixed, you can see the parts. Trail mix, cereal in milk, sand in water, a pizza. Every scoop is a little different.

Step 1: pure substance or mixture?

The test question: could you separate it without a chemical reaction? Pick the parts out, filter it, evaporate the water away, use a magnet. If yes, it is a mixture: the parts were only physically combined and each part keeps its own properties.

If the only way to break it down is a chemical change (electricity, burning, a reaction), it is a pure substance: one kind of particle, the same properties in every sample, all the way down.

Particle picture rule: a pure substance shows one kind of particle repeated. A mixture shows two or more different kinds of particles in the same box.
A bottle of water beside a bag of trail mix
Water in the bottle: every particle is H2O, a pure substance. Trail mix: nuts, raisins, chocolate, pretzels, each one still itself. A mixture you could sort by hand.

Step 2a: pure substance, so element or compound?

Formulas are the clue

Count the kinds of capital letters (each capital starts a new element symbol).

  • Fe, He, O2, N2, O3 → one kind of symbol → element
  • H2O, NaCl, CO2, C6H12O6 → two or more kinds → compound
The subscript trap: O2 has a 2, but it is still only oxygen atoms. A subscript tells you how many, not how many kinds. O2 is an element.
A compound is not a blend

Sodium is a soft metal that explodes in water. Chlorine is a poisonous green gas. Bond them and you get NaCl: the salt on your fries. A compound has its own new properties, nothing like the elements inside it.

Analogy: elements are letters, compounds are words. C-A-T is always those three letters in that order. Change a letter and it is a different word. Change the ratio in a compound and it is a different substance (H2O is water, H2O2 is hydrogen peroxide).

Step 2b: mixture, so homogeneous or heterogeneous?

Homo- means same. A homogeneous mixture looks the same everywhere: the particles are spread out evenly and too small to see. Every sip of sweet tea tastes the same. Air is a homogeneous mixture of nitrogen, oxygen, and a few other gases.

Hetero- means different. A heterogeneous mixture has parts you can see, and one scoop is not the same as the next. Cereal in milk, a salad, sand in water, granite.

Memory hook: homogenized milk is the same all the way through. And in Spanish they are homogéneo and heterogéneo, nearly the same words.
Misconception: when salt dissolves it does not disappear or turn into water. The particles are still there, just spread out too small to see. Evaporate the water and the salt comes back. That is why dissolving is a physical change.
A glass of water with tiny dissolved particles spread evenly through it
A solution: the dissolved particles (purple) are spread evenly among the water particles. Homogeneous, even though it is two substances.

How mixtures come apart

Every one of these is a physical method. Each part keeps its own properties, so you use those properties to pull it out.

Picking it out (sorting)
Parts big enough to see and grab.
Trail mix, a salad, rocks from soil.
Filtering
A solid that did not dissolve stays on the filter, the liquid goes through.
Sand and water, coffee grounds, pasta in a colander.
Evaporation
Boil or let the liquid go, the dissolved solid is left behind.
Salt from salt water, sugar from sweet tea.
Magnet
Pulls out anything magnetic.
Iron filings from sand, paper clips from rice.
Distillation
Boil off the liquid and catch the vapor, leaving other things behind.
Fresh water from sea water.
Settling and pouring off
Let heavy bits sink, pour the liquid away.
Muddy water, oil and vinegar dressing.
Why none of these work on a compound: the atoms in water are bonded, not just sitting next to each other. No filter is fine enough and no magnet is strong enough to pull hydrogen away from oxygen. Only a chemical change, like running electricity through water, can do that.

Reading particle pictures

On the test you will see boxes of circles. Here is how to read them. Each color is a different kind of atom. Circles touching are bonded into one particle.

Pure substance if...
every particle in the box is identical. All lone circles of one color = element. Bonded pairs of one color (like O2) = still an element. Identical groups with two or more colors = compound.
Mixture if...
there are two or more different kinds of particles in the box. Spread evenly = homogeneous. Clumped or layered = heterogeneous.

The flow chart

Click your way down. Then hit Quiz me and walk the chart with a real item.

One-minute video

Simple Science: pure substances versus mixtures, in about a minute. Needs wifi.

Show the video

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Nothing here is graded. Switch levels any time.

Printable worksheets

Student page first, answer key on the next page. Shuffle to pull a fresh set.

Standard and targets

Georgia S8P1a — Develop and use a model to compare and contrast pure substances (elements and compounds) and mixtures. The particle pictures are the model; the flow chart is the compare-and-contrast.

  • Target 1: pure substance vs mixture, with a particle picture.
  • Target 2: element vs compound from a formula or picture; why compound properties are new.
  • Target 3: homogeneous vs heterogeneous; a physical separation method; why a compound cannot be separated physically.

How the levels differ

Level 1 · Coach
Two-way questions only (pure or mixture, element or compound, homo or hetero). A coach line above each question asks the deciding question. Every answer, right or wrong, gets a one-sentence reason.
Level 2 · Guided
All four boxes on every item. Named items, formulas, and drawn particle pictures. Two hint layers. Wrong answers name the mistake (for example, "you picked compound but this formula has only one kind of symbol").
Level 3 · Challenge
The classic trap items, randomized particle pictures, and follow-ups on separation methods and reasoning. Tracks no-hint answers.

Misconceptions this page targets

  • "O2 is a compound because it has a subscript." The formula section and Level 2 hit this repeatedly.
  • "Dissolved salt disappears or becomes water." Solution particle picture plus the evaporation separation method.
  • "Air is a pure substance" and "brass is an element." Both are homogeneous mixtures; Level 3 items.
  • "Homogeneous means pure." A solution is two substances, evenly mixed. The four-box grid keeps the columns apart.

Pairs with the Unit 2 guided notes Part 1 and the FuseSchool "Element, Mixture or Compound?" clip. Nothing is collected: no names, no codes.