Conservation of mass: atoms and system boundaries
Track every type of atom and explain why an escaping gas changes a balance reading.
Before you begin
- Atoms and molecules
- Reading a mass in grams
Count each type of atom
A chemical reaction changes how atoms are joined, but does not turn one element into another or remove atoms. Use labelled counters to track each type before and after. The number of molecules may change because atoms can be regrouped. Counting only the total number of circles is not enough: a model must keep the count of every element. A drawing shows a small representative set, not all the particles in a real sample.
Choose the system you are weighing
A system is the set of objects and materials you choose to study. For a closed reaction apparatus, include the container, its contents and any trapped gas both before and after. If no matter crosses its boundary, its total mass is conserved during a chemical reaction. A change of state or a gas spreading out does not make its atoms massless. The same material may occupy a different volume without a change in its mass.
Explain an open container
An open container can exchange matter with the surroundings. If a reaction releases gas, some atoms may leave the part resting on the balance; the reading can then fall. Count the escaped gas as well and the mass is still accounted for. A rising reading is also possible when material enters, such as oxygen from the air joining a reacting solid. Always state the boundary before comparing masses.
Worked example
An apparatus and all its contents have mass 132.4 g before a reaction. After gas escapes, the remaining apparatus and contents have mass 131.1 g. Assume nothing else enters or leaves. Account for the mass.
Show the worked solution
- The weighed system excludes the gas once it has escaped.
- The mass leaving is 132.4 − 131.1 = 1.3 g.
- The remaining 131.1 g plus the escaped 1.3 g accounts for the original 132.4 g. Atoms have moved out of the weighed system, not disappeared.
1.3 g of gas left; 131.1 g + 1.3 g = 132.4 g.
Try it yourself
A sealed apparatus produces bubbles but its mass reading stays the same. Which explanation fits?
Common mistakes
- Do not confuse number of molecules with number of atoms.
- A balance reading for an open container need not stay constant.
- Gas has mass even when it is invisible.
Read the particle model
Draw a new model
Use six H counters and three O counters to draw groups with two H atoms and one O atom in each group. Label your drawing, count both atom types, and explain what this model can and cannot show.
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
- Draw three separate H–O–H groups.
- Label and count six H atoms and three O atoms after regrouping; none is added or lost.
- Explain that unchanged atom counts account for conserved mass when no matter enters or leaves.
- State a limitation: counting counters does not show reaction conditions, the actual number of particles or whether a proposed reaction occurs.
Next: test the explanation against measurements
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 solid has mass 7.6 g. It reacts with 2.4 g of oxygen from the air, forming only one solid product. Find the product mass and explain the gain.
Compare with the explanation
10.0 g; oxygen atoms entered from the air.
7.6 + 2.4 = 10.0 g. The solid alone is an open system: its mass rises because oxygen joins it. Include the oxygen in the initial system to account for the total.
After trying it yourself, choose your next review. This is your self-assessment.
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