Simulation description
Mass Changes in Reactions is an interactive 3D Three.js simulation for AQA GCSE Chemistry, covering conservation of mass.
A conical flask sits on a digital mass balance, tared to zero. Pupils choose one of two open system reactions.
Oxidation (2Mg + O₂ → 2MgO). Magnesium ribbon burns with an intense white flame and white MgO smoke escapes the open neck. The balance climbs from 2.4 g to 4.0 g.
Decomposition (CaCO₃ → CaO + CO₂). Marble chips are heated and crumble to quicklime while CO₂ rises out of the flask. The balance falls from 10.0 g to 5.6 g.
The LED panel shows the live reading alongside BEFORE, AFTER and CHANGE in red. Pupils can orbit and zoom the scene. The full Adapt accessibility module is built in, including Irlen overlays, CVD filters, reading ruler, dyslexia spacing, text scaling, reduce motion, dark theme toggle and aria-live announcements, with light and high contrast as the classroom default.
The core idea is that mass only appears to change because the flask is open. Nothing is created or destroyed. Gas simply enters or leaves.
Suggested class activity
“Where did the mass go?” is a prediction, observation and explanation cycle lasting roughly 25 to 30 minutes.
Predict (5 min). Before running anything, show both equations. Pupils commit in writing to whether the mass will go up, down or stay the same, and justify it in one sentence. This surfaces the common misconception that burning always loses mass.
Observe (10 min). Run oxidation first. Pupils record the before and after readings and the change, then repeat for decomposition. Ask them to note what they can see leaving or joining the flask.
Explain (10 min). Pose the key question: if mass is always conserved, why did the balance change? Guide pupils towards the open flask. Then ask the clincher: what would happen if we sealed it with a bung? Both reactions would show no change. This separates conservation of mass, which is always true, from an open system measurement, which is what the balance shows.
Extension. Give pupils the closed system mass and have them work backwards. The magnesium gained 1.6 g, so where did it come from and how much did the surrounding air lose?
Adaptation. Supported pupils use a fill in the blank results table with the reasoning sentence started for them. Independent pupils calculate the mass of O₂ absorbed and CO₂ released directly from the balanced equations.
Assessment link. This targets the exam skill of explaining apparent mass changes in non enclosed systems, which is a recurring AQA question type.