In a chemical reaction atoms are rearranged, not made or destroyed, so the total mass of the reactants equals the total mass of the products. In a closed system the mass reading stays the same. In an open system it can seem to change because a gas escapes (mass falls) or a gas from the air joins in (mass rises); counting the gas shows the mass is still conserved.
Burn a pile of steel wool and it gets heavier. Burn a log and it gets lighter. Is mass appearing and vanishing?
A short lesson with no simulation. You can balance equations. Now check what a balanced equation says about mass.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: the fizzing flask
ExplainUse the particle model to explain what you saw.
Explain: counting everything
In your book: write a sentence explaining why the balance reading drops when an antacid tablet fizzes in an open beaker, and what you would change to show that mass is conserved.
In a sealed flask, 12.0 g of magnesium reacts with 8.0 g of oxygen. What mass of magnesium oxide forms (g)?
In an open dish, 2.4 g of magnesium burns and the white ash weighs 4.0 g. What mass of oxygen from the air joined in (g)?
Elaborate TypicalUse the same idea in a new context.
Elaborate: predict the products
Iron (5.6 g) reacts completely with sulfur (3.2 g) in a sealed tube to make iron sulfide. What mass of iron sulfide forms (g)?
Elaborate WiderUse the same idea in a new context.
Elaborate: waste
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
In a chemical reaction atoms are rearranged, not made or destroyed, so the total mass is conserved.
In a closed system the mass stays the same.
In an open system it can seem to change because a gas escapes or joins in from the air; counting the gas shows the mass is still conserved.
A balanced equation has the same atoms on both sides.
For teachers
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). About 15 minutes.
I can explain why mass is conserved in a closed system.
I can explain why mass seems to change in an open system.
I can say what a balanced equation shows about mass.
Before class
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.
Running the lesson
Engage: tally the predictions. Ask the 'destroyed' group where the carbon dioxide went.
Explain: the closed-system sum and the open-system case are the success criteria.
In your own words: why the reading dropped and what to change.
The hard part (on purpose)
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.
When students get stuck
A student says burning destroys the wood. Ask: What are the products of burning wood? Fix: 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. Ask: What in the air could join the magnesium? Fix: Oxygen. The product is heavier because it has oxygen in it.
Stretch and support
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 ___.'
Where it leads
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.
What each step is for
Engage: the fizzing flask: Students see a falling mass reading in an open vessel as mass being destroyed. The mass is in the gas that escaped.
Explain: counting everything: Closed-system results, open-system corrections and the link to the balanced equation meet the three success criteria.
Elaborate: predict the products: Using conservation to predict a mass of product from the mass of reactants shows the principle as a tool.
Elaborate: waste: Mass is always conserved, but not all of it is the product you want; this is what green chemistry tries to improve.
Now you can
Explain why mass is conserved in a closed system.
Explain why mass seems to change in an open system.