A Secondary 1 student sees a container of water left in a warm place. The water level falls. I ask where the missing water went, and the first answer is often, “It disappeared.” In ordinary conversation, we know what that means. In Science, it is the beginning of a much better question. The water did not cease to exist merely because some entered the air as vapour. When students learn to track where matter goes, they begin to understand one of the central habits of Chemistry: accounting for substances before and after a change.
If you are looking for a Secondary 1 Chemistry tutor in Hougang, conservation of mass is a useful bridge between everyday observation, the particle model and later chemical equations. The principle is that, in ordinary physical changes and chemical reactions, total mass is conserved when we properly account for matter in a closed system. An open system can appear to gain or lose mass because matter crosses its boundary. Understanding the boundary prevents many confident but mistaken answers.
Most Secondary 1 students in Singapore study Chemistry-related ideas within integrated lower-secondary Science, not a separate national Pure Chemistry examination. Some of the examples below may appear later or in different depth according to the school’s G1, G2 or G3 sequence. I will begin with safe thought experiments and numerical tables, mark the more advanced extensions and explain what the student should actually do with the idea. The goal is to learn how to reason, not to run improvised reactions at home.
The question I ask before every conservation calculation
What exactly are we weighing? It sounds like an unexciting question. Yet it can decide whether a pupil understands the whole problem. A beaker of water on an open table, a sealed bottle containing water and vapour, a metal object exposed to air and a closed reaction vessel all have different accounting boundaries.
Conservation of mass is not a promise that every object keeps the same mass while it exchanges matter with its surroundings. It is a statement about the total matter accounted for in the defined system under appropriate conditions. If vapour leaves an open beaker, the beaker-and-remaining-liquid measurement can fall even though that matter entered the surrounding air.
I ask students to draw a box around the chosen system before they calculate. Anything crossing the box matters. This simple drawing clarifies more than an abstract definition repeated ten times.
1. Mass is not the same thing as visible amount
A change in appearance is not necessarily a change in total mass. A liquid can become vapour, a solid can dissolve, or a mixture can produce gas during a reaction. In each case, some material may no longer be visible where it started. That does not establish that its atoms vanished.
Imagine a sealed transparent container containing a small amount of water and a space above it. Some water may evaporate into the gas space. If the entire sealed container remains closed and nothing leaks, the total mass measured for the container and contents should remain essentially constant in an ordinary school experiment even though the liquid amount changes.
A child who says “the water level falls, so its mass is destroyed” has confused the liquid phase with the whole system. The first repair is to distinguish how much liquid remains from how much matter is present across all phases.
2. What counts as a closed system?
A closed system, in the school sense relevant here, does not exchange matter with its surroundings. Energy may still pass across the boundary—for example as heat—without matter entering or leaving. The exact physical definition depends on context, but the important classroom idea is to track all matter inside the boundary.
A tightly sealed appropriate vessel containing a liquid and its vapour can illustrate the principle conceptually. If a reaction occurs inside a suitable genuinely closed apparatus, gases produced remain inside and are included in the total mass. The arrangement must be appropriate for safety; closed vessels must never be improvised for reactions that create pressure or heat.
Use a diagram rather than a dangerous demonstration. Draw the container, cap, liquid, gas space and arrows showing what would cross the boundary if it were opened. Ask the student whether any particles are able to leave in the specified setup.
3. What counts as an open system?
An open system can exchange matter with its surroundings. An uncovered cup of water can lose water molecules to the air by evaporation. A material exposed to air can take in oxygen during a chemical reaction. A reaction mixture can release a gas that leaves the container.
A balance measuring only the remaining contents can therefore show a mass change even when conservation of mass is not violated. We need to account for the substances that entered or left. The direction of the change depends on the process: an open system can lose mass or gain mass.
I like asking the learner to point out the boundary on an ordinary drawing of a beaker. “Is the air included in what is weighed?” is more illuminating than repeatedly saying, “Matter cannot be created or destroyed.”
4. Evaporation explains an apparent mass loss
Suppose a fictional open beaker contains 100 g of water, ignoring the beaker’s constant mass. After a period, 92 g of liquid water remains. The remaining liquid has lost 8 g because some water entered the air as vapour. The mass of the liquid portion decreased. Total mass accounting requires considering that 8 g of vapour in the surroundings.
That does not mean exactly 8 g of vapour will remain near the beaker; it can move through a room, be carried by airflow and later condense elsewhere. The relevant point is that the material has changed location and phase rather than ceased to exist.
In a sealed container, the liquid may reach a different level as evaporation and condensation occur, yet the total mass of the sealed container and all its contents remains the same to the resolution of a suitable school measurement, assuming no leak or loss of material.
5. Condensation reverses a change of location and phase
Water vapour can condense into liquid under suitable conditions. When it does, the water becomes visible again as droplets. A learner may think those droplets were created from nothing because the surface looked dry earlier. The particle model explains that water molecules were already present in the gas phase.
Consider condensation on the outside of a cold container. The droplets can come from water vapour in the surrounding air, not necessarily from liquid leaking through the container wall. The system boundary matters: if we weigh only the container, droplets arriving from outside may add mass to what is on its surface.
This example shows that apparent mass gain can also reflect transfer of matter. The principle is not limited to losses from evaporation. We account for where the substance came from and where it went.
6. Dissolving does not make the solute vanish
When an appropriate amount of table salt dissolves in water, the solid crystals are no longer visible in their original form, but the dissolved components remain present in the solution. If no material enters or leaves the system, the masses of the starting ingredients contribute to the final solution’s total mass.
Consider a school-style question: 100 g of water is combined with 10 g of salt, and all the salt dissolves without loss of material. The total mass of the resulting solution is 110 g. The volume is not necessarily the simple sum of any separately measured volumes; mass conservation and volume additivity are different issues.
A student who answers 100 g because “the salt dissolved away” is missing the meaning of dissolving. The correction should include the idea that invisibility does not imply disappearance. You can practise this on paper without preparing solutions at home.
7. Why mass and volume must not be confused
A gas can expand to fill a container or be compressed into a smaller volume under appropriate conditions, while its mass remains constant if no gas escapes. The occupied volume and total mass are different quantities. Their values do not have to increase or decrease together.
This is a useful link with the particle model. Gas particles in a simple representation can move closer together or farther apart as conditions change. We do not claim that the particles themselves lose their identity because the gas occupies a smaller volume.
When a child sees a balloon get larger, they may assume mass was created. But the relevant questions are whether material was added, whether the balloon contains more or less gas, and what the full system includes. A bigger shape does not by itself prove a larger mass or a chemical reaction.
8. Mass is not lost just because gas bubbles appear
Imagine a chemical reaction in an appropriate laboratory setup producing a gas. If that gas leaves an open beaker, the measured mass of the beaker and remaining contents may decrease. In a safely designed closed system that retains the gas, the total mass remains conserved under ordinary reaction conditions.
The frequent wrong explanation is “the reaction destroyed some mass.” The correct interpretation is that some material changed into gas and crossed the boundary of what was being weighed. This is why conservation questions often depict a before-and-after balance reading with different apparatus arrangements.
Any real gas-producing reaction needs supervised school apparatus and safety controls. Do not seal a bottle or jar to imitate such a demonstration; pressure buildup can make improvised closed containers hazardous. A diagram with labelled reactants, products and arrows is the right learning tool at home.
9. A fictional gas-forming reaction table
A teacher-provided thought experiment starts with a reaction mixture containing 20.0 g of material, excluding the unchanged container. After the reaction in an open setup, the measured remaining material is 17.5 g. Suppose the only unaccounted matter is gas that escaped. The inferred mass of gas released is 2.5 g.
The calculation is 20.0 − 17.5 = 2.5 g. But the arithmetic is meaningful only because the question specifies the assumptions. If liquid splashed out, material adhered to an apparatus not weighed, or something entered from the air, the same difference would not necessarily equal gas alone.
I ask students to write the balance equation in words before numbers: initial matter = matter remaining + matter that left, provided that describes the defined system. The equation helps them recognise when a calculation is justified.
10. The same reaction in a genuinely closed vessel
Now imagine the same conceptual reaction carried out in safe purpose-designed apparatus with all material retained, including any gas formed. If the initial total mass of the sealed system is 250.0 g, including the vessel, it should remain approximately 250.0 g afterward at the precision of the school instrument, assuming the apparatus remains closed and appropriate conditions apply.
The mass of a particular reactant may decrease as it is consumed, and the mass of a particular product may increase as it forms. Those are changes in component amounts. The total mass of the defined closed system is conserved.
A student who insists every individual substance has unchanged mass has misread the principle. Conservation concerns the combined accounting, not each reactant separately. This is exactly why it supports chemical reactions rather than forbidding them.
11. What happens when a metal rusts in air?
An iron object can gain mass as it reacts with oxygen and forms corrosion products under suitable conditions. If we weigh the reacting metal and rust without accounting for oxygen taken from the air, an observed mass increase may seem puzzling. The extra oxygen was supplied by the surroundings.
That is not matter appearing from nowhere. The boundary around the original iron object was open to incoming reactants. To apply conservation properly, we must include the contributing oxygen and all relevant products in the mass accounting. The amount and nature of rust depend on real conditions; do not assume a single simple formula for every rusty object.
This example is useful because it balances evaporation and gas escape. An open system can appear to gain mass as well as lose it. The deciding question is what entered or left.
12. Burning can produce an apparent mass loss
When a candle burns, material may react and gaseous products can leave the immediate candle-and-holder system. The mass of the remaining visible candle can decrease. But a correct full chemical account also includes oxygen from the air and products dispersed into the surroundings.
We should not teach “the candle is lighter, therefore matter was destroyed.” Nor should we teach that the mass of the candle alone must stay the same because mass is conserved. The candle alone is not a closed accounting system during burning.
A school video or diagram is sufficient for this reasoning. Do not ignite candles or attempt to capture combustion gases in improvised vessels for a home assignment. Chemistry understanding comes from correctly defining inputs and outputs, not from creating a risky experiment.
13. Balancing a chemical equation is an atom-counting idea
At a later appropriate level, balanced chemical equations express conservation of each element’s atoms in an ordinary chemical reaction. The number of atoms of each element represented on the reactant side equals that on the product side. Atoms are rearranged into different chemical species rather than simply disappearing.
For an extension, consider 2H₂ + O₂ → 2H₂O. The left has four hydrogen atoms and two oxygen atoms; the right also has four hydrogen and two oxygen atoms. That is why the equation is balanced. Students do not need to perform this symbolic step before their school course introduces formulae and equations.
The beginner’s conceptual version is enough: the particles that make up the original substances are accounted for in the products and any remaining substances. A strong foundation makes later equation balancing less like a game of moving numbers.
14. Why we cannot change subscripts just to balance an equation
When learners first meet chemical formulae, they may try to balance an equation by changing the small numbers inside formulae. That changes which chemical substances the formulae represent. In balancing, coefficients in front of the correct formulae change the represented amounts, while the chemical identities of those substances are retained.
Using the water example, H₂O is water. Replacing it with H₂O₂ would represent hydrogen peroxide, a different compound, not another way of writing the same amount of water. That is a valuable preview for learners who have reached symbolic Chemistry.
For a Secondary 1 student not yet studying equations, do not introduce this as a required assessment skill. The purpose of the extension is to show the long connection between tracking matter now and balancing equations later.
15. Counting atoms is different from counting molecules
Two water molecules contain four hydrogen atoms and two oxygen atoms in total. If a diagram shows two H₂O groups, students should not say there are only two atoms because they see two groups. They need to distinguish the number of represented molecules from the number of constituent atoms.
Similarly, a reaction can have a different number of molecules before and after while conserving the atoms of each element. The number of molecular groups is not generally what conservation of atoms requires to stay unchanged.
This is a subtle model-reading task. Use it only when the student is ready for formulae or particle groups. At the beginning, simply tracking that material remains present when it changes form is a sufficient and meaningful goal.
16. What if we weigh the entire apparatus?
Sometimes the container contributes most of the mass reading. Suppose an empty sealed vessel has mass 150 g and the contents initially have mass 25 g. The balance reads 175 g. If the reaction proceeds without exchange of matter and the vessel remains intact, the total should be about 175 g afterward.
A student may subtract the container’s mass at the wrong step or compare 25 g to 175 g as though they represented equivalent systems. This is primarily a reading and accounting error. Label what is included in each measurement before calculating.
Ask the child to underline words such as “including the container,” “excluding the stopper” and “gas escaped.” Small phrases change which totals may be compared. This habit supports both Science and mathematical literacy.
17. What can cause an unexpected experimental reading?
In an actual supervised experiment, imperfect seals, spills, evaporation, temperature effects on a balance, recording errors or other practical factors may change the recorded value. A measured difference is not automatically evidence that conservation failed. First inspect whether the system was truly closed and what the apparatus measured.
A student should not be taught to erase inconvenient readings and replace them with the expected constant total. Unexpected data are a prompt to investigate. The correct scientific method is to record observations honestly and consider possible sources of error.
The question may specifically ask for an explanation of a small measured loss. A defensible answer identifies a plausible exchange of material or measurement issue grounded in the described setup, not a vague claim that “Science sometimes does not work.”
18. Conservation is a statement about the whole chosen system
Draw a box around a beaker. A gas escapes; measured mass inside falls. Now draw a larger box that includes the gas captured in an appropriate part of a closed apparatus. The total mass within the larger closed boundary can remain constant. The principle has not changed; the accounting boundary has.
This is why the word “system” matters. Without it, a student may try to memorise contradictory slogans: boiling water makes mass decrease, but mass is conserved. Both can be understood once we specify whether we are measuring only the remaining liquid or all matter in a closed system.
In later sciences, defining the system boundary becomes increasingly important in energy, environmental studies, engineering and chemical processes. The habit can begin with a simple Secondary 1 drawing.
19. What a balanced mass table should look like
A clear table labels initial reactants, any material added, products remaining and matter leaving or entering. It also states which rows belong to the measured system. In a simplified question with no other exchanges, initial mass equals final retained mass plus mass of escaped material.
Suppose a hypothetical reaction starts with 30 g of substances and releases 4 g of gas to the surroundings. If no other material enters or leaves, the remaining material has mass 26 g. The calculation is simple; the conceptual challenge is explaining why 30 g can still be conserved across the full accounting.
A good learner can reconstruct the total: 26 g remaining + 4 g escaped = 30 g originally present. It is useful to check the units and the meaning of each number before celebrating the correct subtraction.
20. Worked question: the uncovered cup
An uncovered cup has 80.0 g of water. Later it contains 74.0 g of liquid water, with no spill or other change in the cup. What is the apparent mass loss from the measured liquid? The answer is 6.0 g. What happened to that mass? Under the stated scenario, some water left as vapour.
Now ask whether total mass of matter in a suitably larger system, including that vapour, has vanished. No. The liquid portion lost water molecules; those molecules entered the surroundings. If a question says some water also splashed out, the same measurement difference could not be attributed solely to evaporation without more information.
This combination of calculation and caveat teaches the child to respect the conditions supplied.
21. Worked question: salt dissolved in a closed container
A closed container initially holds 120 g of water and 15 g of salt, with no material lost and ignoring constant container mass. After all salt dissolves, what is the total mass of the contents? It remains 135 g. This is the simplest useful example of conservation during a physical process.
Why might a pupil answer 120 g? They may think dissolved salt has disappeared. Ask them to reason about whether the dissolved substance remains in the solution. Do not merely tell them to add the two numbers again.
Then replace salt with another soluble substance in a written question. If the same assumptions hold and no material enters or leaves, the accounting principle still applies. A new context is the best check of understanding.
22. Worked question: gas collected in a closed system
A fictional safe purpose-designed sealed apparatus has an initial total mass of 300.0 g. A reaction occurs inside and visible gas appears. There is no leak or material exchange. What is the expected final total mass? Approximately 300.0 g at the balance’s measurement resolution, even though the internal chemical composition changed.
A wrong answer might be “less than 300 g because gas weighs nothing.” Gases have mass. Another wrong answer is “more than 300 g because the gas was created.” Products form by rearrangement of existing atoms; the total mass accounting includes them.
The teaching point is not to encourage constructing sealed reaction apparatus at home. It is to interpret a diagram of a properly designed scientific setup.
23. Worked question: an iron sample gaining mass
A piece of iron gains mass after exposure to conditions that produce rust. A student says this disproves conservation. What is missing? The iron has reacted with material from outside the original iron-only boundary, including oxygen. The mass of the resulting material can therefore be larger than the mass of the original iron sample.
The correct explanation refers to mass entering the chosen system as reactant atoms become incorporated in corrosion products. If the task asks for a precise calculation, it needs the relevant chemical composition and measured quantities. Do not invent numbers or assume all corrosion is one pure substance.
This is a stronger conceptual test than the closed flask because it forces the learner to recognise a mass increase instead of always looking for a loss.
24. Worked question: why a fizzing mixture becomes lighter
A teacher-provided question describes two substances reacting in an open container. The balance reading for the container and its remaining contents falls by 1.8 g. The description states that a gas was produced and no other material moved. What explains the decrease? Under those stated assumptions, 1.8 g of material left as gas.
The response should not say “the gas has no mass.” In fact, the measured loss is evidence that mass associated with the escaping material is no longer on the balance. The conservation principle remains intact when all products and losses are counted.
If the question does not rule out splashing or evaporation, those are possible complications. A good student identifies how much the stated evidence warrants rather than supplying an unsupported exact cause.
25. Why visible smoke is not all the product mass
When materials burn, some products may be visible as smoke or residue, while other gaseous products may be invisible. A student who counts only ash or visible smoke misses part of the matter accounting. Oxygen from the surrounding air may also participate.
The school-level message is that appearances do not reveal the total mass of all substances present. It may be necessary to know which reactants entered and which products left to interpret the measurement.
Use a classroom diagram and safety-approved explanations. Attempting to contain smoke, gases or flames in ordinary household jars is not an appropriate way for a Secondary 1 student to practise this principle.
26. G1 learning pathway: track where the material went
For a G1 student, begin with something familiar and simple: an uncovered puddle becoming dry or a solid dissolving. Ask whether the material has disappeared from existence or moved, dispersed or changed state. Encourage a short explanation grounded in the child’s current school content.
A drawing can show water in a cup and water vapour represented above it. The student labels what remains and what has moved out of the originally measured region. If formal equations are not part of the current topic, do not require them. The underlying conservation idea can be taught without advanced symbolism.
The MOE G1 Lower Secondary Science syllabus and the school’s lesson sequence should guide expectations. Scientific confidence grows when the learner can justify a simple model independently.
27. G2 learning pathway: calculate with an explicit boundary
For a G2 learner ready for quantitative questions, provide a mass table and ask what is included in each total. A basic task might give the initial mass, the remaining mass and a statement that only gas escaped. The student calculates the gas mass and explains what crossed the boundary.
Next change the setup to a closed container. The same internal process may occur, but no gas leaves the overall system. Does the expected total change? No. The learner should reason from the changed conditions rather than repeat the earlier subtraction automatically.
The MOE G2/G3 Lower Secondary Science syllabus provides curriculum context. Follow actual class content before deciding whether to include chemical equations or more advanced mass calculations.
28. G3 learning pathway: count atoms and critique the model
A confident G3 learner who has studied chemical formulae can examine a balanced equation and count atoms of each element on both sides. Then ask why products may have a different number of molecules but still obey atom conservation. This distinction separates the number of chemical groups from the constituent atoms.
A second extension asks the learner to explain an apparent mass gain when a substance absorbs oxygen from air, or a mass loss when gaseous products escape. The child should identify the system boundary explicitly.
The advanced answer is not simply longer. It is more precise about substances, conditions and what was actually measured. A tutor should adjust the challenge to the school’s course, not use senior equations as a badge of sophistication.
29. Common misconception: “Gas has no mass”
Gases can be invisible, fill available space and be difficult to weigh directly without suitable apparatus. None of that means they have no mass. The very fact that a gas escapes and a measured system becomes lighter under appropriate conditions can provide evidence that the escaping matter had mass.
Ask the child to revisit a familiar example: air occupies space and has mass, as explored in the existing Secondary 1 guide to proving air has mass. The current topic takes that knowledge one step further by asking how gas movement affects a mass balance.
This is a useful correction because the misconception appears in evaporation, boiling, fizzing and combustion contexts. Repairing one central idea can improve several chapters.
30. Common misconception: “The final object must weigh the same”
A reacting metal can gain mass when material from air becomes part of the products. A candle can lose visible mass when products depart. A wet towel loses water while drying. None of these isolated object measurements is an automatic test of conservation for the full system.
The correct principle does not require each individual object to retain the same mass as it exchanges matter. It requires a complete accounting across the chosen closed boundary. Teach the student to draw that boundary whenever a question feels contradictory.
After the explanation, present three scenarios and ask the child whether the measured object is closed to matter exchange. If they can answer that before calculating, they have learned a general problem-solving step rather than a collection of exceptions.
31. Common misconception: “Conservation means substances cannot change”
Chemical reactions produce new substances. Conservation does not prevent chemical identity from changing; it constrains the accounting of matter and, in ordinary reactions, the atoms of each element. A reaction can form a gas, solid or dissolved product without creating atoms from nothing.
Consider the distinction from the companion Physical vs Chemical Changes guide. A physical change may keep the same substance in another form. A chemical reaction changes the chemical substances present. Both can obey conservation when the system is correctly defined.
This is where Science becomes more coherent: topics that once seemed unrelated begin to support the same explanation from different angles.
32. A ten-minute mass-accounting revision session
In the first two minutes, ask the learner to explain why evaporated water is not destroyed. In the next two, draw an open container and a closed container. In the next three, solve one small mass table and label the boundary. Spend two minutes correcting an error about escaping gas or incoming oxygen. In the final minute, write a new question to answer later.
Do not make every session a calculation test. Sometimes a diagram or a clear spoken explanation is the better task. The point is to connect the physical story to the arithmetic. When the child understands what enters, leaves and remains, subtraction becomes easier to choose correctly.
For ongoing learning habits, use the Secondary 1 twenty-minute Science revision plan, adapting it to schoolwork and rest needs.
33. What parents should ask a prospective tutor
I would bring one wrong school question and ask the tutor to make the child explain the boundary. If the student’s answer says “mass disappeared because the substance evaporated,” ask what specific misunderstanding the lesson will address. The tutor should not simply supply a formula and another nearly identical problem.
In a small group, every student should draw their own accounting diagram, state which matter may cross the boundary and solve one changed case. A classmate’s explanation is useful but does not establish another student’s independent control. The strongest evidence is a correctly reasoned new question after hints have been removed.
For Hougang families, confirm the actual teaching location, travel time, level, class size, school topic scope and availability before engaging tuition. A local educational article does not itself establish an in-Hougang classroom or guarantee a score. Good support is observable in better reasoning.
34. Why conservation is bigger than school examinations
Mass accounting is part of how we understand water cycles, pollution control, manufacturing, recycling and the movement of substances through living systems. The same thought process appears again and again: define a system, identify what enters, identify what leaves and check whether the claimed changes are physically plausible.
A student who learns to do that is building a worldview, not just collecting marks. They are less likely to believe that waste disappears when it is thrown away or that a substance has ceased to exist because it can no longer be seen. The material has gone somewhere, changed form or participated in a process that deserves explanation.
I want the learner to carry that curiosity forward. A scientifically educated person asks where the matter came from and where it went before accepting a simple story.
Frequently asked questions
If mass is conserved, why does water in an open cup get lighter?
Water evaporates and enters the air. The remaining liquid can have less mass, but the material has crossed the open system’s boundary rather than vanished.
Can a chemical reaction have an apparent mass loss?
Yes. If gaseous products escape from the measured container, the recorded mass may decrease. The full mass accounting must include the escaped material.
Why can rusting metal gain mass?
Material from the surroundings, including oxygen, becomes incorporated in corrosion products. A mass gain of the measured metal-and-rust sample can therefore occur without violating conservation.
Does gas have mass?
Yes. Gases consist of matter and have mass even when invisible. This is a foundational reason why gas exchange must be included in the calculation.
Must my child know balanced equations in Secondary 1?
Not necessarily. Use the school’s actual G1, G2 or G3 Science sequence. A qualitative understanding of matter accounting can be valuable before formal equation balancing is taught.
Is it safe to make a home sealed-bottle reaction to prove conservation?
Do not improvise sealed reactions or pressure-generating experiments at home. Gas production and heating can make containers hazardous. Use diagrams, worked data and supervised school demonstrations instead.
Continue the learning route
Conservation becomes easier when three earlier distinctions are secure. First, Elements, Compounds and Mixtures explains what the substances are. Second, Physical vs Chemical Changes explains whether chemical identities change. Third, Diffusion in Liquids and Gases explains why substances can spread without disappearing. For accurate calculations and units, consult Measurement, Accuracy and Precision.
For curriculum alignment, review the MOE G1 Lower Secondary Science syllabus, the G2/G3 Science syllabus and the Hougang Chemistry learning library. Match examples and equations to the student’s current course.
If the student can look at a beaker that has become lighter and ask, “What left the system, and where did it go?”, they have discovered the most important skill in this chapter. They are beginning to account for the physical world rather than merely watch it.