Conservation of Mass

Activities to try

1. 1

   Mass detective: track the whole system

   Why it works: Dissolved material retains its mass. Chemical reactions rearrange atoms into products, including gases, without destroying the atoms. A closed system retains the material; an open weighed container can lose material as gas leaves. Balance readings must be interpreted with the measurement conditions.

   1. Place water and sugar, initially separate but both inside the same covered container, on a balance and record the whole-system reading.
   2. Without adding or removing material, mix until the sugar dissolves. Reweigh the covered container in the same position, allowing any movement of the balance to settle.
   3. Compare the readings within the balance’s resolution and explain why invisible dissolved sugar is still present. If readings differ, consider spills, evaporation, material left outside the weighed system or measurement variation before drawing a conclusion.
   4. Use a particle picture or optional overlay to show sugar particles dispersed through the water rather than destroyed. Then test a change of state: weigh an ice cube sealed in a bag or covered container, let it melt at room temperature, wipe off any condensation that forms on the outside, and reweigh. Explain why the water's mass stays the same within the balance's resolution even though its state changed.
   5. Use prepared experimental observations or a labelled model of an open vinegar-and-bicarbonate reaction. Predict what happens as gas leaves the weighed system, then inspect the data or model output.
2. 2

   Consolidate: where did the mass go?

   Why it works: Conservation requires accounting for material and system boundaries. Invisible dissolved material remains present, and gas has mass; apparent measurement changes need an explanation based on what was weighed and how it was measured.

   1. Study before/after measurement data or diagrams for dissolving and open-versus-closed gas-producing reactions. Identify whether each display represents real measurements, idealised data or a model.
   2. Reveal a 'where did the mass go?' explanation flap and check it against your own reasoning.
   3. For the open reaction, explain why escaping gas can lower the reading. State which material must be included in a closed-system comparison, and why a flexible expanding apparatus can complicate the balance reading.
   4. Account for every bit of matter in the change by naming where it ended up (in solution, as trapped gas, etc.).
3. 3

   Predict and justify the balance reading

   Why it works: A useful prediction distinguishes conserved total mass from an instrument reading and is supported by tracking material, including gas, rather than by reciting a slogan.

   1. Before revealing the data, predict the total mass for a closed dissolving system and an idealised closed gas-reaction model. Separately predict what a real balance might show, noting measurement precision and whether the apparatus expands.
   2. Choose, from options, the correct explanation for why an OPEN fizzing reaction appears to lose mass.
   3. Justify each prediction by naming the system boundary, the material present and any material that can leave. For a chemical reaction, refer to atoms rearranging into new products, including gas.