• Quick retrieval from earlier learning (The Earth's atmosphere; Earth's 4.6-billion-year history on a timeline). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. Where did most of the oxygen in today's atmosphere come from? (volcanoes / the oceans boiling / photosynthesis by algae and plants over billions of years / the Sun) → photosynthesis by algae and plants over billions of yearsPrompt: Early Earth had almost no oxygen.
2. What happened to much of the carbon dioxide in the early atmosphere? (It dissolved in oceans and was locked up in rocks and fossil fuels / It turned into nitrogen / It escaped to the Moon / It is all still in the air) → It dissolved in oceans and was locked up in rocks and fossil fuelsPrompt: Think of limestone, coal and oil.
3. Early Earth's atmosphere was mostly… (helium / oxygen / carbon dioxide, with water vapour, released by volcanoes / argon) → carbon dioxide, with water vapour, released by volcanoesPrompt: Volcanic activity was intense.

• Pass round different rocks and ask how each one might have formed.
• Share the objective: How Earth's atmosphere changed over time. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: Today's dry air is about 78 % nitrogen and 21 % oxygen. What percentage is made up of all the other gases together?
• Step 1: 100 − 78 − 21 = 1
• Step 2: So the answer is: 1 %
• Answer: 1 %
• 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. Dry air is about 78 % nitrogen. In 250 cm³ of air, roughly what volume is nitrogen, in cm³? → 195 cm³Prompt: To find a percentage of an amount, multiply and divide by 100.
2. Where did most of the oxygen in today's atmosphere come from? (melting ice / photosynthesis by algae and plants over billions of years / volcanic eruptions / meteorites) → photosynthesis by algae and plants over billions of yearsPrompt: Early volcanic gases contained little or no oxygen.

• 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 8 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. Today's dry air is about 78 % nitrogen and 21 % oxygen. What percentage is made up of all the other gases together? → 1 %Prompt: Percentages of all the gases add up to 100.
2. Scientists think the early atmosphere was mostly carbon dioxide, like Venus and Mars today. Why are they not certain? (There is limited direct evidence from billions of years ago / The atmosphere has never changed / Nobody has studied rocks / Venus has no atmosphere) → There is limited direct evidence from billions of years agoPrompt: Think about how old the evidence would have to be.

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