• Quick retrieval from earlier learning (Choose the right technique to separate a mixture). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. Which technique is best to separate the fractions in crude oil? (fractional distillation / filtration / simple distillation / evaporation or crystallisation) → fractional distillationPrompt: Is the solid dissolved or not? Do you want to keep the liquid?
2. A mixture contains iron filings and sulfur powder. What is the quickest way to separate them? (filter them / use a magnet / distil them / use chromatography) → use a magnetPrompt: Which one is magnetic?
3. Which technique is best to separate food colourings in a sweet coating? (fractional distillation / chromatography / simple distillation / evaporation or crystallisation) → chromatographyPrompt: Is the solid dissolved or not? Do you want to keep the liquid?

• Drop an antacid tablet in water (or show a clip) and ask whether a new substance is being made.
• Share the objective: Alkanes and the fractions of crude oil. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: Alkanes have the general formula CₙH₂ₙ₊₂. How many hydrogen atoms are in an alkane with 11 carbon atoms?
• Step 1: 2 × 11 + 2 = 24
• Step 2: So the answer is: 24
• Answer: 24
• 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. Alkanes have the general formula CₙH₂ₙ₊₂. How many hydrogen atoms are in an alkane with 1 carbon atom? → 4Prompt: Substitute n into 2n + 2.
2. How does fractional distillation separate crude oil? (The oil is frozen in layers / Each fraction is filtered out / A magnet pulls out each fraction / Hydrocarbons condense at different heights of a column depending on their boiling points) → Hydrocarbons condense at different heights of a column depending on their boiling pointsPrompt: The column is hot at the bottom and cooler at the top.

• 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. What is the molecular formula of hexane, the alkane with 6 carbon atoms? (C₆H₈ / C₆H₁₂ / C₆H₁₄ / C₇H₁₄) → C₆H₁₄Prompt: Use CₙH₂ₙ₊₂.
2. As alkane chains get longer, how does viscosity change? (it disappears / it increases (they flow less easily) / it stays the same / it decreases) → it increases (they flow less easily)Prompt: Compare petrol with bitumen.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-9.