• 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. Which gas makes up most of the air we breathe? (hydrogen / oxygen / nitrogen / carbon dioxide) → nitrogenPrompt: It is about four-fifths of the air.
2. Where did most of the oxygen in today's atmosphere come from? (the Sun / photosynthesis by algae and plants over billions of years / the oceans boiling / volcanoes) → photosynthesis by algae and plants over billions of yearsPrompt: Early Earth had almost no oxygen.
3. What happened to much of the carbon dioxide in the early atmosphere? (It escaped to the Moon / It dissolved in oceans and was locked up in rocks and fossil fuels / It turned into nitrogen / 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.

• 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. 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. Why did the amount of carbon dioxide in the early atmosphere decrease? (It dissolved in oceans, was used in photosynthesis and was locked in rocks and fossil fuels / It escaped into space / It turned into nitrogen / Volcanoes absorbed it) → It dissolved in oceans, was used in photosynthesis and was locked in rocks and fossil fuelsPrompt: More than one process removed it.

• 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. Why did the amount of carbon dioxide in the early atmosphere decrease? (It dissolved in oceans, was used in photosynthesis and was locked in rocks and fossil fuels / It escaped into space / Volcanoes absorbed it / It turned into nitrogen) → It dissolved in oceans, was used in photosynthesis and was locked in rocks and fossil fuelsPrompt: More than one process removed it.
2. Dry air is about 21 % oxygen. In 400 cm³ of air, roughly what volume is oxygen, in cm³? → 84 cm³Prompt: To find a percentage of an amount, multiply and divide by 100.

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