Conservation of mass: evaluate the evidence
Use repeated measurements to compare open and closed systems, then improve an investigation plan.
Before you begin
- Conservation of mass and system boundaries
- Subtracting decimals and calculating a mean
Compare the same system before and after
A fair comparison weighs the same apparatus and all the materials included within its chosen boundary. Adding a lid after the first weighing, spilling liquid or removing a stirring rod changes what is measured. Start with separated reactants already inside the apparatus when considering a closed-system test, so mixing does not add unmeasured matter. These are planning principles; use the supplied data here rather than performing a reaction at home.
Look at changes and repeats
For each trial calculate final mass minus initial mass. A negative change means less mass remains on the balance; a positive change means more. Compare individual trials as well as their mean, because opposite changes can cancel. Readings rounded to 0.1 g cannot establish perfect equality at every smaller scale. Resolution is the smallest displayed step; it is not a guarantee of accuracy or the whole uncertainty.
Separate an explanation from a proof
Repeated losses in an open system and small changes around zero in a closed system are consistent with gas leaving only the open system. The pattern supports conservation when all matter is included, but mass readings alone do not identify a gas or prove that no leak occurred. Check for spills, evaporation, leaks and balance drift. A stronger investigation keeps starting quantities, equipment and measurement timing comparable, includes a non-reacting check, and repeats both conditions.
Improve the plan before collecting data
Write a prediction and specify exactly what will be weighed. Name the changed condition, such as whether gas can leave, then the measured mass change and the controls. Ask a teacher to choose suitable apparatus and quantities, assess pressure and chemical risks, and supervise any practical work. Never improvise a sealed gas-producing reaction in a rigid container. Explaining a safe plan here does not demonstrate supervised practical competence.
Worked example
Three illustrative closed-system trials have mass changes 0.0 g, −0.1 g and +0.1 g. Find the mean change and state a careful conclusion.
Show the worked solution
- Add the signed changes: 0.0 − 0.1 + 0.1 = 0.0 g.
- Divide by three: the mean change is 0.0 g. Keep the individual changes, which span −0.1 to +0.1 g.
- The readings are consistent with no net mass change at this measurement scale. They do not establish that every trial had exactly zero change or that the apparatus could not leak.
Mean change 0.0 g; small differences remain in the individual readings.
Try it yourself
A student weighs reactants in a cup, adds a 5 g lid, then weighs the sealed apparatus after the reaction. What must change in the plan?
Common mistakes
- A zero mean does not make every individual change zero.
- Do not call display resolution the full measurement uncertainty.
- Keep simulated practice data separate from real experimental observations.
Read the measurement table
Original illustrative data for practice, not measured experimental results. Each row represents a fresh trial; both setups start at 120.0 g. Readings are displayed to 0.1 g.
| Trial | Open before | Open after | Closed before | Closed after |
|---|---|---|---|---|
| 1 | 120.0 | 118.8 | 120.0 | 120.0 |
| 2 | 120.0 | 118.7 | 120.0 | 119.9 |
| 3 | 120.0 | 118.9 | 120.0 | 120.1 |
Compare and evaluate
Calculate each open-system mass change and its mean. Compare it with the closed-system mean, explain a possible mechanism, and name one control that would strengthen the investigation.
Write your reasoning before opening the guidance. Four suggested self-check points; these are not official marks or an automatically assessed score.
Compare my reasoning
- Final minus initial gives −1.2 g, −1.3 g and −1.1 g; mean −1.2 g.
- The closed-system mean is 0.0 g, with individual changes −0.1 to +0.1 g. The open mean is 1.2 g lower.
- Gas escaping the weighed open system could explain the loss. The pattern supports this explanation but does not identify the gas or rule out other losses.
- Keep the same apparatus and starting quantities in each before/after comparison, check for spills or leaks, and repeat with a stable balance and a non-reacting comparison.
Revisit: atoms and the system boundary
Curriculum reference: NGSS MS-PS1-5 · middle-school particle models and conservation. Grade 8 is StudyForA’s suggested placement; these two lessons do not complete the science curriculum.
What can you explain now?
A sealed apparatus reads 86.5 g before and 86.4 g after a reaction on a 0.1 g display. Does this single pair show that atoms were destroyed? Give two checks before drawing a conclusion.
Compare with the explanation
No. Check for leakage or loss of material, and check balance stability using repeats or a non-reacting comparison.
The observed change is −0.1 g, but one pair cannot distinguish measurement variation from matter leaving. A justified explanation must consider the system boundary and the quality of the measurements.
After trying it yourself, choose your next review. This is your self-assessment.
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