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C2.4 · Test conservation of mass during a chemical reaction

Learn to test conservation of mass during a chemical reaction through clear examples and targeted practice.

Ontario Grade 10 Science

Chemistry: Chemical Reactions

How to compare mass before and after a reaction

A reaction can make a mixture look very different. It may bubble, change colour, or form a solid. Does the total mass change too? In this lesson, you will test that question by comparing the mass of a reaction system before and after a chemical change. A fair test must include all the substances involved, including any gas that forms.

What you will learn

Explain what conservation of mass means during a chemical reaction.
Describe how a closed system helps test whether mass is conserved.
Compare measurements and use evidence to explain results from an open or closed system.

1. From Grade 9 ideas to a testable question

Matter is anything that has mass and takes up space. Mass tells how much matter is in a sample. In Grade 9, you learned that substances can change in physical or chemical ways. In a chemical reaction, starting substances change into different substances. The starting substances are called reactants. The new substances are called products.

During a reaction, atoms are rearranged into new combinations. They do not vanish just because a product is hard to see. For example, a gas may form and spread into the surrounding air. If you weigh only the liquid left in a container, you may miss the mass of that gas.

Conservation of mass means that the total mass stays the same during a chemical reaction when all the matter is included. To test this, compare the mass of the complete system before and after the reaction. A system is the object or group of objects chosen for the test.

m_{\text{before}} = m_{\text{after}}

Reactants are the substances present before a reaction.
Products are the substances formed by a reaction.
A fair mass comparison includes all matter in the chosen system.

2. Why the container matters

A closed system does not let matter enter or leave. A sealed container can help keep gas products inside, so the balance includes them. A balance measures the mass of whatever is placed on it. For a useful comparison, weigh the same container and its contents before and after the reaction.

An open system allows matter to enter or leave. If a reaction produces a gas in an open cup, some gas may escape. The balance then measures less matter in the cup and its contents. This does not show that matter was destroyed. The gas has moved outside the part being weighed.

The system boundary is the limit of what you count as part of the system. In a sealed-container test, the boundary includes the container and everything inside it. In an open-cup test, gas that has escaped is outside the boundary. Thinking about the boundary helps explain why two measurements may differ.

A simple model is to treat the container and its contents as one unit. Imagine bubbles forming inside a sealed bag. The bubbles may change what you see, but their matter remains inside the bag. The total mass of the bag and contents should therefore be the same before and after, within the limits of the measuring equipment.

m_{\text{reactants}} = m_{\text{products}}

A closed system keeps matter inside the measured system.
Gas escaping from an open container can make the measured mass decrease.
A changed reading is not, by itself, proof that mass was destroyed.

3. Plan the test and judge the evidence

A useful test changes the substances by allowing a reaction to happen, while keeping the mass comparison fair. First, choose a suitable container and decide exactly what will be included in the measurement. Record the mass of the container and reactants before they react. Let the reaction happen, then measure the same complete system again.

For a closed-system test, the container must be sealed in a way that keeps matter inside. Follow the teacher’s instructions and use only approved materials and equipment. Never seal a reaction in a rigid container unless a teacher has specifically selected and approved that setup. A gas can build pressure. Do not open or handle a pressurized container.

Record the units and the balance readings. Compare the two readings by finding their difference. If the readings are close, that supports conservation of mass for the test. They may not match exactly because balances have limited precision or because material may be spilled or escape. Precision means how finely an instrument can show a measurement. State what the readings show, and identify a possible source of difference rather than claiming more than the evidence supports.

A safe classroom investigation should be planned and supervised by a teacher. Do not try chemical reactions at home. In particular, do not mix household products or seal reacting mixtures in containers. The point is to collect evidence safely with approved materials, not to create a reaction without supervision.

\Delta m = m_{\text{after}} - m_{\text{before}}

Weigh the same complete system before and after the reaction.
Keep the setup and measurement method consistent.
Use the readings as evidence and consider whether matter could have escaped.

4. Use a model to explain what you observe

A reaction may produce a solid, a liquid, or a gas. You can look for signs such as bubbling or a new solid, but appearance alone does not tell you whether total mass stayed the same. The mass comparison is the evidence for that question.

Use a simple before-and-after model. Before the reaction, the measured system contains the container and reactants. After the reaction, it contains the container and products. In a closed system, the products and any gas remain within the measured boundary. The total mass should match, even though the substances and their appearance have changed.

If an open-system reading is lower, ask whether a gas or another material left the measured container. If a closed-system reading differs slightly, consider the balance’s precision and whether the setup stayed closed. These explanations are reasons to check the method and evidence; they are not permission to ignore results.

Visible change and mass change are different questions.
The system boundary helps explain what the balance did and did not measure.
A good conclusion connects the readings to the setup and its limits.

Worked example

A closed system with a gas product

Hypothetical classroom data: A teacher-approved sealed setup has a mass of 128.6 g before a reaction. Afterward, the same sealed setup has a mass of 128.5 g. What do the readings suggest?

Compare the readings
Subtract the before reading from the after reading. This shows the difference for the complete sealed setup.
$128.5\,\text{g} - 128.6\,\text{g} = -0.1\,\text{g}$
Interpret the result
The reading is 0.1 g lower after the reaction. The measurements are close, so they support conservation of mass, but they do not match exactly. The small difference could relate to the balance’s precision or a problem with keeping the setup closed. The data alone do not show which explanation is correct.

Answer: The readings are close and support conservation of mass for this test. The 0.1 g difference should be reported and considered, not hidden.

Check: The setup is described as sealed, so gas formed in the reaction should remain part of the measured system.

Worked example

Gas escapes from an open container

Hypothetical classroom data: An open container and its contents have a mass of 74.2 g before a reaction. After bubbling stops, the container and remaining contents have a mass of 71.9 g. Give a reasonable explanation for the lower reading.

Find the change in the measured mass
Compare the after reading with the before reading. A negative difference means the measured container and contents have less mass after the reaction.
$71.9\,\text{g} - 74.2\,\text{g} = -2.3\,\text{g}$
Connect the change to the system
Bubbling suggests that a gas may have formed. Because the container is open, gas could leave the measured system. The lower reading can be explained by matter moving outside the system boundary; it does not show that the matter was destroyed.

Answer: The measured mass decreased by 2.3 g. Gas escaping from the open container is a reasonable explanation, though the observation does not by itself identify the gas.

Check: The explanation accounts for why matter could be missing from the balance reading without claiming that mass vanished.

Worked example

Use a closed-system result to complete a record

Hypothetical data: A complete closed reaction system has a before mass of 96.8 g. The expected after mass is not yet written in the record. What mass should be expected if the test supports conservation of mass?

Apply the mass comparison
The system is closed and the same complete setup is measured both times. If mass is conserved, its total after mass should equal its total before mass.
$m_{\text{after}} = m_{\text{before}}$
Substitute the recorded value
Use the before reading as the expected after reading. This is a prediction based on the conservation model, not a measured result.
$m_{\text{after}} = 96.8\,\text{g}$

Answer: The expected after mass is 96.8 g. An actual reading should still be recorded and compared with that prediction.

Check: This value is an expected result from the model, not invented experimental evidence.

Common mistakes and how to avoid them

The mass went down, so matter was destroyed.

Correction: Check whether gas or another material left the measured system. A balance records only what is on it.

A sealed system and an open system should give the same reading.

Correction: An open system can lose matter to its surroundings. A sealed system is designed to keep matter inside the measured boundary.

A small difference between readings proves the conservation model is wrong.

Correction: Report the difference and consider measurement precision and possible leaks or spills. Use the full evidence to judge the test.

Lesson summary

Conservation of mass means total mass stays the same during a chemical reaction when all matter is included.
Compare the same complete system before and after the reaction.
A closed system helps keep gas products within the measured boundary.
A lower reading in an open system may mean matter escaped, not that matter was destroyed.
Report differences and use them, along with the setup, to judge the evidence.

Check your understanding

Question 1

A reaction produces bubbles in an open cup. The measured mass of the cup and its contents is lower afterward. Which explanation best fits the setup?

Some gas may have left the part being weighed.
The balance measures the mass of the gas after it leaves the room.
The reaction has changed mass into colour.
The reading proves that no reaction happened.

Show answer and explanation

Some gas may have left the part being weighed.

An open cup allows gas to leave the measured system, so the balance may show a lower mass.

Question 2

Which comparison is the best test of mass conservation in a closed reaction system?

Compare the mass of the reactants alone with the mass of the empty container.
Compare the complete sealed setup before and after the reaction.
Compare the colour before and after the reaction.
Weigh only the liquid after the reaction.

Show answer and explanation

Compare the complete sealed setup before and after the reaction.

The same complete sealed setup includes the container and all matter inside it at both times.

Question 3

A closed setup reads 52.4 g before and 52.4 g after a reaction. What does this result support?

The reaction did not form any products.
The total mass of the measured setup stayed the same in this test.
Every chemical reaction must have exactly these readings.
The balance measured only the container.

Show answer and explanation

The total mass of the measured setup stayed the same in this test.

Matching readings support conservation of mass for this measured setup. They do not show that no new substances formed.

Key terms

Mass: A measure of how much matter is in a sample.
Reactant: A starting substance in a chemical reaction.
Product: A substance formed in a chemical reaction.
Closed system: A system that does not allow matter to enter or leave.
System boundary: The limit of what is counted as part of the system.
Precision: How finely a measuring instrument can show a value.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 10 Science (SNC2D), expectation C2.4. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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