• Quick retrieval from earlier learning (Conservation of mass in reactions). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. In a sealed flask, 29 g of substance A reacts completely with 23 g of substance B. 3 g of product D forms along with product C. What mass of C forms, in g? → 49 gPrompt: Total mass of reactants = total mass of products.
2. 10 g of calcium carbonate is heated in an open tube until it all breaks down. 5.6 g of calcium oxide is left. What mass of carbon dioxide escaped, in g? → 4.4 gPrompt: Mass seems to be lost. Where did it go?
3. Who is often credited with showing that mass is conserved by careful weighing in sealed vessels? (Dmitri Mendeleev / Marie Curie / Antoine Lavoisier / Isaac Newton) → Antoine LavoisierPrompt: He was an 18th-century French chemist.

• Drop an antacid tablet in water (or show a clip) and ask whether a new substance is being made.
• Share the objective: Thermal decomposition of carbonates. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: Calcium carbonate is heated strongly and breaks down. What are the products?
• Step 1: metal carbonate → metal oxide + carbon dioxide
• Step 2: So the answer is: calcium oxide + carbon dioxide
• Answer: calcium oxide + carbon dioxide
• 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. Calcium carbonate is heated strongly and breaks down. What are the products? (calcium + oxygen + carbon / calcium oxide + carbon dioxide / calcium oxide + oxygen / calcium hydroxide + water) → calcium oxide + carbon dioxidePrompt: A carbonate contains a metal, carbon and oxygen.
2. How can you show that carbon dioxide is given off when a carbonate is heated? (smell it / hold a lit splint in it, which gives a squeaky pop / hold a glowing splint in it, which relights / bubble the gas through limewater, which turns cloudy (milky)) → bubble the gas through limewater, which turns cloudy (milky)Prompt: This test is specific to carbon dioxide.

• 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. Green copper carbonate is heated and turns black. What is the black solid? (copper oxide / copper sulfate / copper / carbon) → copper oxidePrompt: Carbon dioxide has left the solid.
2. When 100 g of calcium carbonate is heated, it forms 56 g of calcium oxide and 44 g of carbon dioxide. What mass of carbon dioxide forms from 5 g of calcium carbonate, in g? → 2.2 gPrompt: The masses stay in the same proportion.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-7.