• Quick retrieval from earlier learning (Alkanes and the fractions of crude oil). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. Alkanes have the general formula CₙH₂ₙ₊₂. How many hydrogen atoms are in an alkane with 12 carbon atoms? → 26Prompt: Substitute n into 2n + 2.
2. Why is crude oil called a finite (non-renewable) resource? (It formed over millions of years and is used much faster than it forms / It is made in factories / It never runs out / It is made from sunlight each day) → It formed over millions of years and is used much faster than it formsPrompt: Crude oil came from ancient plankton buried in mud.
3. What is the molecular formula of butane, the alkane with 4 carbon atoms? (C₄H₉ / C₅H₁₀ / C₄H₈ / C₄H₁₀) → C₄H₁₀Prompt: Use CₙH₂ₙ₊₂.

• Drop an antacid tablet in water (or show a clip) and ask whether a new substance is being made.
• Share the objective: Cracking, alkenes and the bromine water test. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: Cracking: C₁₁H₂₄ → C₅H₁₂ + an alkene. How many hydrogen atoms are in one molecule of the alkene?
• Step 1: C: 11 − 5 = 6
• Step 2: H: 24 − 12 = 12
• Step 3: The alkene is C₆H₁₂.
• Step 4: So the answer is: 12
• Answer: 12
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. Cracking: C₁₀H₂₂ → C₈H₁₈ + an alkene. How many carbon atoms are in one molecule of the alkene? → 2Prompt: Atoms are conserved: subtract the atoms in the smaller alkane.
2. Which conditions are used for catalytic cracking? (freezing the oil / adding acid at room temperature / vaporised hydrocarbons passed over a hot catalyst / dissolving the oil in water) → vaporised hydrocarbons passed over a hot catalystPrompt: Cracking is a thermal decomposition.

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 6 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. Bromine water is shaken with a hydrocarbon and turns from orange to colourless. What does this show? (The hydrocarbon contains oxygen / The bromine has evaporated / The hydrocarbon is an alkene (it has a C=C double bond) / The hydrocarbon is an alkane) → The hydrocarbon is an alkene (it has a C=C double bond)Prompt: Alkenes are unsaturated.
2. Alkenes and cracking: which statement is NOT true? (Alkenes are more reactive than alkanes. / Ethene can be used to make the polymer poly(ethene). / Alkanes decolourise bromine water. / Alkenes contain a carbon–carbon double bond.) → Alkanes decolourise bromine water.Prompt: Which hydrocarbons are saturated?

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-10.