Science lesson plan · 7th grade (Year 8 in England) · 60 minutes
Conservation of mass in reactions
Chemical reactions, acids and bases
CCSS NGSS.MS-PS1-5England: KS3 Chemistry: Chemical reactionsEngland: GCSE Combined Science: Quantitative chemistry
Objective and success criteria
Students will: Conservation of mass in reactions.
- I can answer questions on today's skill on my own, without help.
- I can explain each step and say why it works.
- I can check my answer and spot a mistake.
Before and after
Needs first: Conservation of mass; Spot the evidence that a chemical reaction happened
Leads to: Thermal decomposition of carbonates
Materials: Teacher demo or video, Indicator color chart, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).
Warm-up · 6 min
- 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.
- An ice cube of 120 g melts completely in a sealed bag. What is the mass of the water, in g? → 120 gPrompt: Does a change of state add or remove any material?
- What evidence of a chemical reaction is shown here: two clear solutions are mixed and a yellow solid appears? (a gas is given off / a colour change / a solid (precipitate) forms / an energy change (heat or light)) → a solid (precipitate) formsPrompt: Did you see bubbles, a new colour, a new solid or an energy change?
- In a sealed flask, 49 g of one chemical reacts with 31 g of another. What is the total mass of the products, in g? → 80 gPrompt: In a closed container, atoms are only rearranged.
Hook · 6 min
- 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.
I do: model it · 12 min
- 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.
We do: guided practice · 12 min
- Do these together. Students try each step on whiteboards before you reveal it.
- Use the hints as prompts rather than giving the answer.
- In a sealed flask, 5 g of substance A reacts completely with 8 g of substance B. 3 g of product D forms along with product C. What mass of C forms, in g? → 10 gPrompt: Total mass of reactants = total mass of products.
- 20 g of calcium carbonate is heated in an open tube until it all breaks down. 11.2 g of calcium oxide is left. What mass of carbon dioxide escaped, in g? → 8.8 gPrompt: Mass seems to be lost. Where did it go?
You do: independent practice · 17 min
- 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.
Exit ticket · 7 min
- Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
- Magnesium ribbon is burned in a crucible. The ash weighs more than the ribbon did. Why? (Burning creates new atoms / Heat has mass / The crucible got heavier / Oxygen atoms from the air have combined with the magnesium) → Oxygen atoms from the air have combined with the magnesiumPrompt: What is the product of burning magnesium?
- In a sealed flask, 14 g of substance A reacts completely with 25 g of substance B. 3 g of product D forms along with product C. What mass of C forms, in g? → 36 gPrompt: Total mass of reactants = total mass of products.
Watch out for
- Everyday meanings of words getting in the way of the science meaning: define key words explicitly.
- Remembering facts without the reason: ask "why?" and "what is the evidence?"
Support and stretch
Support
- Review first: Conservation of mass.
- Use concrete materials: teacher demo or video.
- Give the first step of each question, or a partly worked example to finish.
- Pair with a confident partner for the guided practice.
Stretch
- Move on to: Thermal decomposition of carbonates.
- Explain a common mistake in writing and how to fix it.
- Write a word problem that uses today's skill, then solve a partner's.
Homework and more practice
Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-7.
Free lesson plan from eurekami.com/lesson-plan-builder