• 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. What is the molecular formula of methane, the alkane with 1 carbon atom? (C₂H₄ / CH₂ / CH₄ / CH₃) → CH₄Prompt: Use CₙH₂ₙ₊₂.
2. Which fraction is the most flammable? (refinery gases (shortest chains) / heavy fuel oil / bitumen / diesel oil) → refinery gases (shortest chains)Prompt: Short-chain hydrocarbons ignite most easily.
3. Alkanes have the general formula CₙH₂ₙ₊₂. How many hydrogen atoms are in an alkane with 2 carbon atoms? → 6Prompt: Substitute n into 2n + 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: C₉H₂₀ → C₅H₁₂ + an alkene. How many hydrogen atoms are in one molecule of the alkene? → 8Prompt: Atoms are conserved: subtract the atoms in the smaller alkane.
2. Bromine water is shaken with a hydrocarbon and turns from orange to colourless. What does this show? (The bromine has evaporated / The hydrocarbon is an alkane / The hydrocarbon is an alkene (it has a C=C double bond) / The hydrocarbon contains oxygen) → The hydrocarbon is an alkene (it has a C=C double bond)Prompt: Alkenes are unsaturated.

• 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. Why do refineries crack long-chain hydrocarbons? (To make crude oil / There is more demand for shorter chains such as petrol, and alkenes make polymers / To remove all the carbon / Long chains are too useful) → There is more demand for shorter chains such as petrol, and alkenes make polymersPrompt: Compare supply and demand of fractions.
2. Alkenes and cracking: which statement is NOT true? (Alkenes contain a carbon–carbon double bond. / Alkenes are more reactive than alkanes. / Ethene can be used to make the polymer poly(ethene). / Methene is the smallest alkene.) → Methene is the smallest alkene.Prompt: Which hydrocarbons are saturated?

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