Lesson overview
Elements, compounds and mixtures: Is it one substance or several?
Mix iron filings with sulfur and pull the iron out again with a magnet. Then heat the mixture, test the black solid that forms, and find out why iron and sulfur always combine in the same ratio.
Suggested sequence
- Lesson 1 · Mix it, then make it. Iron, sulfur, their mixture and iron sulfide. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · What the particles show. What particle diagrams show: elements, compounds and mixtures. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 2 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 2 questions, 15 min, lesson closed.
Learning goals
Part 1: Mix it, then make it
- Describe the differences between a mixture and a compound
- Use observations to decide whether substances have only mixed or have combined to form a compound
Part 2: What the particles show
- Identify elements, compounds and mixtures from particle diagrams
- Describe the differences between elements, compounds and mixtures in terms of their atoms and particles
Misconceptions to listen for
- “Silver sulfide contains silver, so tweezers could pick silver grains out of it.” — Its silver atoms are joined to sulfur atoms all through the black solid. There are no loose silver grains to pick out: only a chemical reaction gets the silver back.
- “Statement 2 describes the stirred powders, because they contain silver too.” — Stirred powders can be in any proportions, so their make-up changes from sample to sample. Only a compound has the same make-up in every sample.
- “Statement 3 could describe the stirred powders if they are stirred very finely.” — However fine the grains, stirred silver and sulfur keep their own colours. Black all through is a new property: a new substance has formed.
- “Row B.” — J's liquid and gas are new substances, made by strong heating and always in the same ratio by mass. That is a compound splitting up, not a mixture coming apart.
- “Row C.” — Reacting with other substances makes new compounds, but K itself has never been split into anything simpler. That makes K an element.
- “Row D.” — Water, a physical method, separates L's two kinds of crystal, so they are not joined to each other: L is a mixture.
- “All 18 g turns into zinc sulfide.” — A compound has a fixed ratio. 5.0 g of sulfur can combine with only 10 g of zinc; the rest of the zinc has no sulfur to combine with.
- “All 13 g of zinc reacts, so it takes 6.5 g of sulfur.” — There is only 5.0 g of sulfur, less than 6.5 g. The sulfur runs out first, so some zinc is left over.
- “The copper in 12 g of copper oxide is 12 g × 4 = 48 g.” — The copper can't have more mass than the copper oxide it is part of. Of every 5 g of copper oxide, 4 g is copper.
- “The solid has less mass than 7.5 g, so some copper oxide must have escaped.” — Copper oxide is a solid and stays in the tube. A smaller mass means less oxygen combined: some copper did not react, so the solid is a mixture of copper oxide and copper.
Scope
The differences between elements, compounds and mixtures (2.1 Core 1), with iron, sulfur and iron sulfide and with particle diagrams. Atomic structure, formulae and equations, separation methods and energy changes as such are left to their own modules.