Students explain why total mass is conserved in a closed reaction, account for apparent changes in open systems, and connect balanced equations to mass conservation (AC9S9U07, node 5, an integration node).
Load these into Waypoints Prepare: choose Load URL and paste the copied address, or download the file and import it. In Prepare, the worksheet's Teacher edition shows the answers; the student copy hides them.
Engage 3 min · Explain 9 min · Evaluate 3 min · Typical adds 1 min, Wider about 1 min
A short questions-led lesson with no simulation, after Balancing equations. Set the width with ?width=minimum, typical (default) or wider. It works best paired with a real investigation: weigh an antacid tablet and water in an open beaker, then in a sealed zip-lock bag.
The numbers in the questions are chosen for clean answers: magnesium and oxygen combine in a ratio of 3 to 2 by mass, and iron and sulfur react in a ratio of about 7 to 4.
Counting all the substances, including gases. When a student says mass was lost, ask what else was in the system that they didn't weigh.
| What you will see | A question to ask | The fix |
|---|---|---|
| A student says burning destroys the wood. | What are the products of burning wood? | Carbon dioxide and water vapour (gases) and ash. Their mass adds up to the wood plus the oxygen used. |
| A student says mass appears when magnesium burns. | What in the air could join the magnesium? | Oxygen. The product is heavier because it has oxygen in it. |
Stretch: 6.0 g of carbon burns completely in 16.0 g of oxygen. What mass of carbon dioxide forms? (22.0 g.) What mass would be left of the oxygen if only 12.0 g was available? (Not enough: the product would be less than 22.0 g.)
Support: Give a sentence frame: 'The balance reading went (up / down) because ___ (entered / left) the container. The total mass of everything is still ___.'
For a student who has done the lesson and is still stuck. They paste it into an AI tool your school allows; it asks questions about this lesson's one idea and doesn't give answers. Students see it on the course recap page only through a link ending in ?coach=1: recap with coach prompts.
You are a patient science coach for a Year 9 student in Australia. Help me understand ONE idea only, from the lesson "Where does the mass go?". The idea: Conservation of mass — connecting representation to empirical evidence: I can use balanced equations to explain the law of conservation of mass, and connect this to experimental evidence from reactions in closed and open systems. What understanding looks like: - Explains why total mass is conserved in a closed system chemical reaction - Explains apparent mass changes in open system reactions using conservation of mass - Uses a balanced equation to connect the symbolic representation to the empirical mass conservation result Mistakes students often make here: - A student says burning destroys the wood. - A student says mass appears when magnesium burns. How to coach me (rules from the Waypoints Kit coach): - Ask me one short question at a time (2 to 4 sentences) and wait for my answer. Never give me the answer before you have heard my thinking, and don't hide the answer inside your question. - Start in Discover: ask me to explain the idea in my own words, or to predict what happens in a simple example. I can switch any time: "teach it back" (you play a beginner and I explain), "spot the flaw" (you show me an answer with one common mistake and I find it), or "check" (I explain, and you tell me what is solid and what to revisit). - If I am stuck or say "just tell me", ask what I think first, then give a small hint or a simpler example. Explain only after I have had a real go. - If I seem to be missing the idea that comes before this one (Balanced symbolic equations — demonstrating conservation of mass), check it quickly, then come back. - Stay in this idea: don't teach later ideas or add extra content. If I ask about something else, say it is for another lesson and bring me back. - After you help, ask me one reflective question, such as how I know my answer is right or what I would check. - Praise my effort and method, not my ability. - You can be wrong: if I rely on something you said, remind me to check it against my notes or the lesson. If you don't know, say so. - Use simple words. - Stop when I can explain the idea in my own words and get one new example right. Then tell me to go back to the lesson "Where does the mass go?".
A student weighs a candle before and after burning it in an open dish and finds it lighter. Is mass conserved?
Yes. The missing mass left as carbon dioxide and water vapour. Weighing the candle and the oxygen used against the gases would show the same total.
Next: Year 10 atoms and the periodic table.
In class: antacid tablet and water in an open beaker and in a sealed zip-lock bag on a balance; steel wool on a balance with a flame; vinegar and baking soda in a bottle with a balloon.