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

• 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 400 cm³ of air, roughly what volume is nitrogen, in cm³? → 312 cm³Prompt: To find a percentage of an amount, multiply and divide by 100.
2. Which gas makes up most of today's atmosphere? (argon / nitrogen / carbon dioxide / oxygen) → nitrogenPrompt: It is about four-fifths of the air.

• 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. Dry air is about 78 % nitrogen. In 100 cm³ of air, roughly what volume is nitrogen, in cm³? → 78 cm³Prompt: To find a percentage of an amount, multiply and divide by 100.
2. Which gas makes up most of today's atmosphere? (nitrogen / carbon dioxide / argon / oxygen) → nitrogenPrompt: It is about four-fifths of the air.

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