The shape of the lesson
By the end of the lesson, learners can
- One type of atom: Look at Fig. 1 and ask two questions of any box: how many types of atom are there, and are the atoms joined to each other?
- Iron, sulfur and iron(II) sulfide: Iron filings and yellow sulfur powder can be stirred together or heated together.
- Everyday mixtures: Most materials around you are mixtures.
- Element: one type of atom only, even if the atoms are joined in molecules.
- Compound: different elements chemically combined in a fixed ratio.
- Mixture: not chemically joined; composition can vary.
- Separation: mixtures by physical methods, compounds only by reactions.
One picture every idea returns to
An element is a substance made of only one type of atom. A compound is a substance made of two or more elements chemically combined in a fixed ratio, and it has new properties of its own. A mixture contains two or more substances that are not chemically combined: its composition can vary and its parts can be separated by physical methods such as a magnet, filtering or distillation.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Joined, or just mixed?. Every substance you will meet in chemistry is one of three things: an element, a compound or a mixture. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: One type of atom | Look at Fig. 1 and ask two questions of any box: how many types of atom are there, and are the atoms joined to each other? | Whole class, then pairs |
| 13 to 21 min | Teach and check: Iron, sulfur and iron(II) sulfide | Iron filings and yellow sulfur powder can be stirred together or heated together. | Pairs |
| 21 to 30 min | Teach and check: Everyday mixtures | Most materials around you are mixtures. | Jigsaw |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 16 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Everyday mixtures, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| A molecule is not the same thing as a compound. | Oxygen, O 2, and nitrogen, N 2, are made of molecules but contain only one type of atom, so they are elements. A compound needs at least two different elements, as in carbon dioxide, CO 2. |
Elements, compounds and mixtures: worksheet
6 questions, 16 marks. Written in the style of the papers; no past paper question is reproduced.
Define the terms element, compound and mixture.
A mixture of iron filings and sulfur is heated until it glows. Describe how a magnet could be used to show that a new substance has formed, and explain the result.
Classify each substance as an element, a compound or a mixture: copper, carbon dioxide, air, sodium chloride, sea water, nitrogen.
A box contains four particles each made of two identical atoms joined together, and three particles each made of one atom of one element joined to one atom of a different element. Classify the contents of the box and explain your answer.
Iron and sulfur always combine in the mass ratio 56 : 32. A student heats 14 g of iron with 10 g of sulfur. Calculate the mass of iron(II) sulfide formed and the mass of the substance left over. (Ar: Fe 56, S 32)
A student says that oxygen gas, O2, is a compound because each molecule contains two atoms. Explain why the student is wrong.
Answer key and marking notes
- Element: a substance made of only one type of atom.
- Compound: a substance made of two or more elements chemically combined in a fixed ratio.
- Mixture: two or more substances that are not chemically combined.
- Before heating, a magnet attracts the iron and pulls it out of the mixture.
- After heating, the black product is not attracted to the magnet (assuming no iron is left over).
- The iron has chemically combined with the sulfur to form a compound, iron(II) sulfide, FeS, which has different properties from iron.
- Elements: copper and nitrogen.
- Compounds: carbon dioxide and sodium chloride.
- Mixtures: air and sea water.
- The identical-atom particles are molecules of an element; the others are molecules of a compound.
- The two substances are not joined to each other, so the box contains a mixture of an element and a compound.
- 14 g of iron needs 14 × 32 ÷ 56 = 8 g of sulfur.
- Mass of FeS formed = 14 + 8 = 22 g.
- Sulfur is in excess: 10 − 8 = 2 g of sulfur is left over.
- Both atoms in an O2 molecule are oxygen atoms, so there is only one type of atom.
- A compound must contain two or more different elements chemically combined, so O2 is an element.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which of these substances can be broken down into simpler substances by a chemical reaction?
- Acopper(II) oxide
- Bcopper
- Csulfur
- Dneon
The diagram shows the particles in a sample of gas. Each circle is an atom; circles of the same colour are atoms of the same element. How is the sample best described?
- Aa mixture of two elements
- Ba pure compound
- Ca mixture of an element and a compound
- Da single pure element
Brass is made by melting copper and zinc together. Different samples of brass contain anywhere from about 55% to 90% copper. How should brass be classified?
- Aa mixture, because the proportions of copper and zinc can vary
- Ba compound, because it is made from two elements
- Can element, because it is a metal
- Da compound, because the metals were heated together
Hydrogen burns in oxygen to form water. Which observation shows that water is a compound and not just a mixture of hydrogen and oxygen?
- AWater is a liquid at room temperature and does not burn, unlike hydrogen
- BWater contains both hydrogen atoms and oxygen atoms
- CWater can be made from any proportions of hydrogen and oxygen
- DWater is colourless, like both hydrogen and oxygen
Magnesium and oxygen always combine in the mass ratio 3 : 2 to form magnesium oxide. What mass of magnesium oxide forms when 6 g of magnesium burns completely in excess oxygen?
- A10 g
- B8 g
- C9 g
- D4 g
Which list contains only elements?
- Ahelium, sulfur, mercury
- Bhelium, water, mercury
- Csulfur, sulfur dioxide, carbon
- Dair, nitrogen, oxygen
Pure substance X melts sharply at 801 °C. When an electric current is passed through molten X, a silvery metal and a pale green gas are produced. What is X?
- Aa compound
- Ban element
- Ca mixture of a metal and a gas
- Da mixture of two compounds
Which of these changes produces a compound?
- Aburning magnesium ribbon in air
- Bdissolving salt in water
- Cmelting ice
- Dstirring sand into water
Quiz key
Copper(II) oxide is a compound of copper and oxygen, so a chemical reaction (for example heating with carbon) can split it into simpler substances. Copper, sulfur and neon are elements: each contains only one type of atom and cannot be broken down into anything simpler by chemical means.
The blue particles are molecules made of two identical atoms, so they are an element. The grey particles are single atoms of a second element. No particle contains two different atoms joined together, so there is no compound. Two different substances are present and not joined to each other, so the sample is a mixture of two elements. It is not a single element because there are two types of atom.
A compound always contains its elements in a fixed ratio, shown by one formula. Brass can have many different compositions, so the copper and zinc are not chemically combined: brass is a mixture (an alloy). Being made from two elements is true of mixtures as well as compounds, and heating does not by itself create a compound. Brass contains two types of atom, so it cannot be an element.
A compound has properties different from the elements in it. Hydrogen and oxygen are gases and hydrogen is flammable, but water is a liquid that does not burn, so a new substance has formed. A mixture of the two gases would also contain hydrogen and oxygen atoms, so that does not prove anything. Water always has a fixed ratio (H2O), not any proportions, and sharing a property (colourless) with the elements is what you would expect from a mixture, not evidence of a compound.
Mass of oxygen = 6 × 2 ÷ 3 = 4 g. The product contains all the magnesium and all the oxygen that combined, so mass of magnesium oxide = 6 + 4 = 10 g. 4 g is only the oxygen; 8 g adds 2 g instead of scaling the ratio; 9 g comes from multiplying 6 by 3/2, which gives a mass of magnesium, not of the product.
Helium (He), sulfur (S) and mercury (Hg) each contain one type of atom, so all three are elements. Water (H2O) and sulfur dioxide (SO2) are compounds of two elements. Air is a mixture of several gases, even though nitrogen and oxygen themselves are elements.
A sharp melting point shows X is a pure substance, not a mixture (mixtures melt over a range). The current splits X into two different substances, a metal and a green gas (chlorine), which is a chemical change, so X contains two elements chemically combined: a compound (it is sodium chloride). An element cannot be split into two different substances, and a mixture of a metal and a gas would not be a single solid with a sharp melting point.
When magnesium burns, it chemically combines with oxygen to form a new substance, magnesium oxide, MgO, which is a compound. Dissolving salt and stirring sand into water only make mixtures that can be separated by physical methods, and melting ice is a change of state: the water is still water.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.