• 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, 17 g of substance A reacts completely with 37 g of substance B. 11 g of product D forms along with product C. What mass of C forms, in g? → 43 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. Why is mass conserved in a chemical reaction? (Reactions happen too fast to change mass / Products are always the same substances as reactants / Energy is conserved / Atoms are only rearranged; none are made or destroyed) → Atoms are only rearranged; none are made or destroyedPrompt: Count the atoms on both sides of an equation.

• 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 + carbon dioxide / calcium hydroxide + water / calcium oxide + carbon dioxide / calcium + oxygen + carbon) → calcium oxide + carbon dioxidePrompt: A carbonate contains a metal, carbon and oxygen.
2. Making cement is responsible for a large share of global CO₂ emissions. Why? (Cement is made from coal / Heating limestone releases CO₂, and burning fuel to heat kilns releases more / Cement absorbs oxygen / Concrete gives off CO₂ forever) → Heating limestone releases CO₂, and burning fuel to heat kilns releases morePrompt: Two sources of CO₂ are involved.

• 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. How can you show that carbon dioxide is given off when a carbonate is heated? (bubble the gas through limewater, which turns cloudy (milky) / hold a glowing splint in it, which relights / hold a lit splint in it, which gives a squeaky pop / smell it) → bubble the gas through limewater, which turns cloudy (milky)Prompt: This test is specific to carbon dioxide.
2. Magnesium carbonate is heated strongly and breaks down. What are the products? (magnesium oxide + oxygen / magnesium hydroxide + water / magnesium + carbon dioxide / magnesium oxide + carbon dioxide) → magnesium oxide + carbon dioxidePrompt: A carbonate contains a metal, carbon and oxygen.

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