Law of Conservation of Mass — Simulation Description
A conical flask sits on a digital lab balance. Inside the flask are marble chips (calcium carbonate, CaCO₃) in hydrochloric acid (HCl) — a classic GCSE reaction that produces carbon dioxide gas as a visible product.
Students choose whether the flask is open or closed before starting. When the reaction runs, CO₂ bubbles rise through the liquid. In a closed flask the gas stays trapped and the balance holds at 25.0 g throughout. In an open flask the bubbles escape and the balance drops to 22.0 g — the 3.0 g difference being the CO₂ that left the system. The info bar updates live with the before/after masses and a plain-language verdict. The atom count on the balance display confirms the same number of each atom exists on both sides regardless of container type.
Suggested Class Activity
Predict → Observe → Explain — works well as a whole-class discussion or paired task.
Before the simulation (5 min) Ask students to write down their prediction: “If a gas is produced and escapes, does the total mass go up, down, or stay the same — and why?” Take a quick show of hands. Most will predict mass decreases; some will say it stays the same. Don’t correct either — let the simulation do it.
Running the simulation (10 min) Run it twice as a class on the board:
- First with the closed flask — students observe the balance stays at 25.0 g and discuss why.
- Then with the open flask — students observe the balance drops to 22.0 g and discuss what changed.
Key discussion questions:
- “Did any atoms disappear in the open flask?”
- “Where did the 3.0 g go?”
- “Which reading is the ‘true’ test of conservation of mass — open or closed?”
After the simulation (5–10 min) Students write a two-sentence explanation: one for the closed result and one for the open result. A strong answer will use the word system — distinguishing between mass within the flask and mass across the whole room.
Extension Challenge students to calculate: if 1.0 g of CaCO₃ produces 0.44 g of CO₂, how much CO₂ would escape from a 5.0 g sample? This bridges the simulation directly into quantitative chemistry.