Law of Conservation of Matter: Fifth Grade Science
Across every physical transformation and chemical reaction in the universe, matter can neither be created nor destroyed; the total mass of the system remains perfectly constant.
The Foundations of the Law
In the late 1700s, the French scientist Antoine Lavoisier performed meticulous experiments using sealed glass vessels and precision balances. His work led to one of the most fundamental principles in all of natural science: the Law of Conservation of Matter (also called the Law of Conservation of Mass). The law states that matter cannot be created out of nothing, nor can it vanish into nothingness. Whenever matter undergoes a change—whether it freezes, melts, dissolves, burns, or explodes—the total mass of the starting substances (reactants) must equal the total mass of the ending substances (products).
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| LAW OF CONSERVATION OF MASS EQUATION |
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| TOTAL MASS OF REACTANTS = TOTAL MASS OF PRODUCTS |
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| [ 100 grams ] = [ 100 grams ] |
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| Every single atom present at the start of a transformation |
| must be accounted for at the end. Atoms are merely rearranged! |
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Atoms Cannot Disappear
Think of atoms as indestructible building blocks, like plastic toy bricks. If you construct a toy castle using 50 red bricks and 30 blue bricks, the total mass of the castle equals the combined mass of those 80 bricks. If you tear the castle down and rebuild it into a spaceship, you still have exactly 50 red bricks and 30 blue bricks. The appearance, size, and shape of your creation have changed completely, but the mass of the building blocks remains identical. In chemical reactions, atoms rearrange their partnerships, but no atom is ever obliterated or spontaneously created.
Conservation of Matter in Physical Changes
The Law of Conservation of Matter is easily verified during physical changes such as phase transitions and dissolving.
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| CONSERVATION OF MASS IN DISSOLVING |
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| Flask with 200 g Water + Watch Glass with 20 g Sugar |
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| [ 200 g ] + [ 20 g ] |
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| Total Initial Mass on Balance = 220 grams |
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| (Sugar is dumped into water and stirred until completely dissolved)|
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| Total Final Mass on Balance = 220 grams |
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Where Does Dissolved Sugar Go?
When you stir sugar into water, the white crystals seem to vanish into thin air. A beginning investigator might suspect that the sugar was destroyed. However, placing the beaker on a balance proves otherwise: 200 grams of water plus 20 grams of sugar results in exactly 220 grams of sweet water solution. The sugar molecules did not disappear; they simply detached from their solid crystal grid and dispersed evenly between the liquid water molecules.
Conservation During Phase Changes
If you place 100 grams of solid ice cubes inside a tightly sealed plastic ziplock bag and record the mass on a balance, it reads 100 grams (excluding bag mass). After allowing the ice to melt completely into liquid water at room temperature, the balance still reads exactly 100 grams. If you were to vaporize that water inside a sealed pressure container, the trapped steam would still have a mass of exactly 100 grams.
Conservation in Chemical Reactions: Open vs. Closed Systems
When people observe a massive campfire burn down to a tiny pile of gray ash, they often intuitively think that matter was destroyed. The mass of the remaining ash is vastly less than the mass of the original heavy wooden logs. This apparent disappearance occurs because the campfire is an open system.
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| OPEN VERSUS CLOSED SYSTEMS |
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| OPEN SYSTEM (Campfire) CLOSED SYSTEM (Sealed Flask) |
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| ^^^ Smoke, CO₂, H₂O vapor +--------------------+ |
| ^^^ escape into atmosphere | No gases can | |
| +-----+ | escape or enter! | |
| | Ash | | All products | |
| +-----+ | remain trapped. | |
| +--------------------+ |
| Measured mass appears to Measured mass remains |
| DECREASE because gases fled! EXACTLY CONSTANT! |
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Open Systems Explained
An open system is one where matter can freely enter or escape into the surrounding environment. During a wood fire, wood cellulose combines chemically with atmospheric oxygen gas. The reaction produces solid mineral ash, but it also produces enormous volumes of invisible carbon dioxide gas (CO₂) and water vapor (H₂O) that billow up into the sky as hot gases and smoke. If you fail to capture and weigh those escaping gases, the mass appears to vanish.
Closed Systems Prove the Law
A closed system is a sealed experimental environment where no matter can enter and no matter can escape. If you perform that same combustion reaction inside a sealed, airtight glass chamber: Mass of Wood + Mass of Oxygen = Mass of Ash + Mass of Carbon Dioxide + Mass of Water Vapor. The balance reading before the flame is lit equals the balance reading after the flame goes out to the exact milligram.
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| CLOSED SYSTEM BAKING SODA & VINEGAR LAB |
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| Before Reaction (Unmixed inside sealed bottle): |
| - Bottle with 50.0 g Vinegar |
| - Balloon attached to neck containing 10.0 g Baking Soda |
| - Total Balance Reading = 60.0 grams |
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| During Reaction (Baking soda dumped into vinegar): |
| - Violent fizzing, foaming, and bubbling occur |
| - Carbon dioxide gas inflates the rubber balloon tightly |
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| After Reaction (Reaction complete, balloon fully inflated): |
| - Total Balance Reading = 60.0 grams! (Conservation verified) |
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Exercises and Solutions
Exercise 1: Calculating Mass in a Closed Chemical Reaction. An experimenter places 15.0 grams of vinegar into a glass flask and puts 5.0 grams of baking soda inside a deflated rubber balloon sealed over the mouth of the flask. The entire sealed apparatus has a mass of 145.0 grams on a digital scale (including the mass of the flask and balloon). The student tips the balloon, dumping the powder into the vinegar. Intense fizzing occurs, and the balloon expands with carbon dioxide gas. State the total mass of the apparatus after the bubbling completely stops, and explain your reasoning.
Solution 1: The total mass of the apparatus after the reaction stops will be exactly 145.0 grams. This is because the reaction took place inside a sealed, closed system. Although a chemical reaction occurred that broke molecular bonds and produced a new substance (carbon dioxide gas), the rubber balloon prevented the gas molecules from escaping into the room. According to the Law of Conservation of Matter, atoms can neither be created nor destroyed, meaning the total mass of all reactants and products must remain identical.
Exercise 2: Analyzing Apparent Mass Loss in an Open Steel Wool Reaction. A student records the mass of a dry steel wool pad as 20.0 grams on a balance. The student soaks the steel wool in vinegar to strip its protective coating, exposes it to open air, and lets it rust over three days. When the rusted pad is reweighed, its mass has increased to 24.5 grams. The student is bewildered, wondering how matter could have been created out of nowhere. Explain why the mass increased and show how the Law of Conservation of Matter was maintained.
Solution 2: Matter was not created out of nowhere; the experiment was conducted in an open system where matter from the surroundings entered the reaction. Rusting is a chemical reaction (oxidation) where iron atoms in the steel wool combine chemically with invisible oxygen gas molecules and water vapor from the surrounding air to form iron oxide (rust): Iron + Oxygen -> Iron Oxide. The additional 4.5 grams of mass represents the exact mass of atmospheric oxygen gas molecules that bonded chemically to the iron atoms. If the student had weighed the surrounding air before and after the reaction in a closed chamber, the air would have lost exactly 4.5 grams of oxygen, proving that total mass was perfectly conserved.