• 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 food colourings in a sweet coating? (chromatography / evaporation or crystallisation / simple distillation / fractional distillation) → chromatographyPrompt: Is the solid dissolved or not? Do you want to keep the liquid?
2. Countries such as Saudi Arabia get much of their drinking water from the sea. Which method is used on a large scale? (filtration through sand only / adding more salt / chromatography / desalination by distillation or reverse osmosis) → desalination by distillation or reverse osmosisPrompt: Dissolved salt cannot be removed by a simple filter.
3. Which technique is best to separate pure water from sea water? (filtration / simple distillation / evaporation or crystallisation / fractional distillation) → simple distillationPrompt: 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 9 carbon atoms? → 20Prompt: Substitute n into 2n + 2.
2. Which fraction is the most flammable? (bitumen / diesel oil / refinery gases (shortest chains) / heavy fuel oil) → refinery gases (shortest chains)Prompt: Short-chain hydrocarbons ignite most easily.

• 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. Alkanes have the general formula CₙH₂ₙ₊₂. How many hydrogen atoms are in an alkane with 8 carbon atoms? → 18Prompt: Substitute n into 2n + 2.
2. How does fractional distillation separate crude oil? (Hydrocarbons condense at different heights of a column depending on their boiling points / Each fraction is filtered out / The oil is frozen in layers / A magnet pulls out each fraction) → 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.

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