• 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 ethanol from a mixture of ethanol and water? (fractional distillation / chromatography / evaporation or crystallisation / filtration) → fractional distillationPrompt: Is the solid dissolved or not? Do you want to keep the liquid?
2. Why is the filtrate clear after filtering muddy water? (Water evaporates / The mud dissolves / The filter paper adds chemicals / The solid particles are too big to pass through the filter paper) → The solid particles are too big to pass through the filter paperPrompt: Filter paper has tiny holes.
3. Which technique is best to separate pure water from sea water? (fractional distillation / filtration / evaporation or crystallisation / simple 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 7 carbon atoms? → 16Prompt: 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 from sunlight each day / It is made in factories / It never runs out) → It formed over millions of years and is used much faster than it formsPrompt: Crude oil came from ancient plankton buried in mud.

• 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. As alkane chains get longer, how does viscosity change? (it stays the same / it decreases / it disappears / it increases (they flow less easily)) → it increases (they flow less easily)Prompt: Compare petrol with bitumen.
2. 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.

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