Classifying Pure Substances, Solutions and Mechanical Mixtures
About this lesson
Students work in pairs to sort twelve everyday materials into pure substances, solutions and mechanical mixtures, then explain their thinking about the tricky ones. They practise using particle ideas to classify matter. It takes about 12 minutes and needs only a list written on the board and scrap paper.
- Grade
- 7
- Subject
- Science
- Time
- 12 min
- Prep
- 3 min
Objective
Students will classify everyday materials as pure substances, solutions or mechanical mixtures and justify each choice using what they can see and what the material is made of.
Prep
3 min: write the 12 materials and three headings on the board
Materials
- Whiteboard and markers
- Scrap paper
- Pencils
Steps
- Write three headings on the board and define them: a pure substance has only one kind of particle, a solution is a mixture that looks like one substance throughout, and a mechanical mixture has parts you can see or separate by hand.
- List these 12 materials: aluminium foil, table salt, distilled water, sugar, oxygen gas, air, salt water, clear apple juice, soil, trail mix, oil-and-vinegar dressing, steel.
- Have pairs fold scrap paper into three columns and sort all 12 materials in 5 minutes, writing one reason beside any material they disagree on.
- Read each material aloud and have pairs hold up one, two or three fingers for their column, then ask a pair to explain any split vote, focusing on air, steel and distilled water (air and steel are solutions, distilled water is pure).
- Finish by asking each pair to name one object in the classroom and say which category it belongs to and why.
Quick Check
Ask each student to write on a scrap of paper why salt water is a solution but table salt is a pure substance, then collect them at the door.
Differentiation
Make it easier: Remove air, steel and apple juice, and give one clear example for each heading before pairs start sorting.
Make it harder: Have pairs also mark each pure substance as an element or a compound, and suggest a method to separate each mixture.
Good For
- Review
- Supply teacher
- Before lunch
- End of day
