• Quick retrieval from earlier learning (Conservation of mass; Spot the evidence that a chemical reaction happened). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. An ice cube of 340 g melts completely in a sealed bag. What is the mass of the water, in g? → 340 gPrompt: Does a change of state add or remove any material?
2. What evidence of a chemical reaction is shown here: fizzing when vinegar is poured on baking soda? (a gas is given off / a colour change / a solid (precipitate) forms / an energy change (heat or light)) → a gas is given offPrompt: Did you see bubbles, a new colour, a new solid or an energy change?
3. In a sealed flask, 71 g of one chemical reacts with 19 g of another. What is the total mass of the products, in g? → 90 gPrompt: In a closed container, atoms are only rearranged.

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

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: In a sealed flask, 24 g of substance A reacts completely with 20 g of substance B. 8 g of product D forms along with product C. What mass of C forms, in g?
• Step 1: 24 + 20 = 44 g
• Step 2: 44 − 8 = 36
• Step 3: So the answer is: 36 g
• Answer: 36 g
• 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. In a sealed flask, 27 g of substance A reacts completely with 34 g of substance B. 14 g of product D forms along with product C. What mass of C forms, in g? → 47 gPrompt: Total mass of reactants = total mass of products.
2. 5 g of calcium carbonate is heated in an open tube until it all breaks down. 2.8 g of calcium oxide is left. What mass of carbon dioxide escaped, in g? → 2.2 gPrompt: Mass seems to be lost. Where did it go?

• 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. Magnesium ribbon is burned in a crucible. The ash weighs more than the ribbon did. Why? (The crucible got heavier / Oxygen atoms from the air have combined with the magnesium / Burning creates new atoms / Heat has mass) → Oxygen atoms from the air have combined with the magnesiumPrompt: What is the product of burning magnesium?
2. In a sealed flask, 8 g of substance A reacts completely with 12 g of substance B. 6 g of product D forms along with product C. What mass of C forms, in g? → 14 gPrompt: Total mass of reactants = total mass of products.

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