• 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 4 carbon atoms? → 10Prompt: Substitute n into 2n + 2.
2. As alkane chains get longer, how does viscosity change? (it disappears / it increases (they flow less easily) / it decreases / it stays the same) → it increases (they flow less easily)Prompt: Compare petrol with bitumen.
3. Alkanes have the general formula CₙH₂ₙ₊₂. How many hydrogen atoms are in an alkane with 9 carbon atoms? → 20Prompt: 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 → C₇H₁₆ + 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. Why do refineries crack long-chain hydrocarbons? (Long chains are too useful / There is more demand for shorter chains such as petrol, and alkenes make polymers / To make crude oil / To remove all the carbon) → There is more demand for shorter chains such as petrol, and alkenes make polymersPrompt: Compare supply and demand of fractions.

• 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. Cracking: C₁₀H₂₂ → C₄H₁₀ + an alkene. How many hydrogen atoms are in one molecule of the alkene? → 12Prompt: Atoms are conserved: subtract the atoms in the smaller alkane.
2. Alkenes and cracking: which statement is NOT true? (Cracking is a thermal decomposition reaction. / Alkenes contain a carbon–carbon double bond. / Ethene can be used to make the polymer poly(ethene). / Cracking makes longer molecules from shorter ones.) → Cracking makes longer molecules from shorter ones.Prompt: Which hydrocarbons are saturated?

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