Eighth Grade Science: The Law of Conservation of Mass
One of the most foundational tenets of physical science is that matter cannot be summoned into existence out of nothing, nor can it be annihilated into nonexistence. First formulated rigorously by French chemist Antoine Lavoisier in 1789, the Law of Conservation of Mass dictates that during any physical change or chemical reaction occurring within a closed system, the total mass of the system remains strictly constant.
Antoine Lavoisier and the Closed System
Prior to the late eighteenth century, many natural philosophers believed that matter could be destroyed during combustion. When a heavy piece of wood burned, only a tiny pile of lightweight ash remained. It appeared obvious to early observers that mass had vanished. Antoine Lavoisier revolutionized scientific thinking by introducing rigorous quantitative mass measurements and conducting experiments within closed systems—sealed containers that prevent matter from entering or escaping.
+---------------------------------------------+
| Open vs. Closed Systems |
+----------------------+----------------------+
| Open System: Matter and energy can freely |
| exchange with the surroundings.|
| Gases escape; apparent mass |
| loss observed. |
| |
| Closed System: Energy may exchange, but no |
| matter can cross boundaries. |
| Total mass before = after! |
+---------------------------------------------+
Lavoisier's Closed Reaction Flask
+---------------------------+
| [ Sealed Stopper ] |
| || |
| /----------\ |
| / Gas Phase \ | <-- Trapped Gas Reactants
| | O2 + CO2 | | and Products!
| | | |
| | [ Wood Ash ]| |
| +-------------+ |
+---------------------------+
Total Mass Before Ignition = 250.0 g
Total Mass After Combustion = 250.0 g
The Combustion Breakthrough
Lavoisier burned phosphorus and sulfur inside sealed glass vessels containing measured volumes of air. He demonstrated that the mass of the sealed flask remained identical before and after combustion. Furthermore, when he unsealed the flask, air rushed inward, proving that burning did not destroy matter; instead, the burning fuel combined chemically with atmospheric oxygen gas to produce new oxide products. The ash left behind in an open campfire appears lighter only because huge volumes of carbon dioxide ($CO_2$) gas and water vapor ($H_2O$) escaped into the open atmosphere unmeasured.
The Conservation of Mass at the Atomic Level
The Law of Conservation of Mass is directly explained by Dalton's atomic theory. Chemical reactions do not create new atoms, nor do they destroy existing atoms. A chemical reaction is fundamentally a process of breaking chemical bonds and rearranging existing atoms into new combinations.
Reactants: [Atom A] - [Atom B] + [Atom C] - [Atom D]
\ /
(Bonds Break)
/ \
Products: [Atom A] - [Atom C] + [Atom B] - [Atom D]
Because every single atom present in the reactants must be accounted for among the products, the total mass of the products must equal the total mass of the reactants.
Total Mass of Reactants = Total Mass of Products
m(Reactant 1) + m(Reactant 2) = m(Product 1) + m(Product 2)
Conservation in Physical Phase Changes
The Law of Conservation of Mass applies equally to physical changes. If you place 100.0 grams of solid ice into a sealed jar and allow it to melt into liquid water, the mass remains 100.0 grams. If you boil that water into steam within a sealed pressure vessel, the gaseous steam still has a mass of 100.0 grams. The physical arrangement, volume, and density of the molecules change, but the total number of $H_2O$ molecules—and thus the total mass—is conserved.
+-----------------------+--------------------+-----------------------+
| Initial State | Physical Process | Final State & Mass |
+-----------------------+--------------------+-----------------------+
| 100.0 g Ice Cube | Melting | 100.0 g Liquid Water |
| 100.0 g Liquid Water | Vaporization | 100.0 g Water Vapor |
| 50.0 g Water + 5.0 g | Dissolving | 55.0 g Salt Solution |
| Salt crystals | | (Homogeneous mixture) |
+-----------------------+--------------------+-----------------------+
Tracking Mass in Open versus Closed Systems
When evaluating laboratory data, you must account for whether the reaction chamber was open or closed to the surrounding atmosphere. Apparent discrepancies in mass are almost always explained by unmeasured gases.
Apparent Mass Loss: CaCO3 (s) + 2 HCl (aq) -----> CaCl2 (aq) + H2O (l) + CO2 (g)
If container is open, CO2 gas escapes into the room.
Balance reads LESS mass, but matter was NOT destroyed!
Apparent Mass Gain: 2 Fe (s) + 3 O2 (g) -----> 2 Fe2O3 (s)
Open iron nail reacts with unseen atmospheric oxygen gas.
Balance reads MORE mass, because oxygen was ADDED from air!
Chapter Exercises
Exercise 1: An investigator measures 10.0 grams of baking soda and 50.0 grams of vinegar. The chemicals are combined in an open beaker on an electronic balance. The mixture fizzes vigorously. Once bubbling ceases, the final reading on the balance is 58.2 grams. Explain why the final reading is less than 60.0 grams and determine the mass of carbon dioxide gas that escaped into the room.
Exercise 2: In a laboratory investigation, 32.0 grams of solid sulfur react completely with an unknown mass of oxygen gas inside a sealed, airtight glass chamber to produce 64.0 grams of sulfur dioxide gas ($SO_2$). Calculate the exact mass of oxygen gas consumed and state the physical law that justifies your calculation.
Exercise 3: A student leaves an iron nail with a mass of 5.0 grams outside in damp air. Several weeks later, the nail is completely covered in red rust (iron oxide) and now has a mass of 6.8 grams. The student argues that matter was created out of nothing because the nail became heavier. Evaluate this argument using the Law of Conservation of Mass.
Solutions and Explanations
Solution 1: The final balance reading of 58.2 grams is less than the starting mass of 60.0 grams ($10.0\ g + 50.0\ g$) because the experiment was conducted in an open system. The chemical reaction between baking soda (sodium bicarbonate) and vinegar (acetic acid) produced carbon dioxide gas ($CO_2$), which bubbled up and escaped into the surrounding classroom air. Because the gas escaped unmeasured, the balance recorded an apparent mass loss. In accordance with the Law of Conservation of Mass, the total mass of the reactants must equal the total mass of the products: $60.0\ g - 58.2\ g = 1.8\ g$. Therefore, exactly 1.8 grams of carbon dioxide gas escaped into the atmosphere.
Solution 2: According to the Law of Conservation of Mass, the total mass of the reactants entering a chemical reaction in a closed system must exactly equal the total mass of the products formed. The word equation is Mass of Sulfur + Mass of Oxygen = Mass of Sulfur Dioxide. Substituting the known numerical values into the equation yields $32.0\ g + m(Oxygen) = 64.0\ g$. Subtracting 32.0 grams from both sides reveals that the mass of oxygen consumed is $64.0\ g - 32.0\ g = 32.0\ g$. Exactly 32.0 grams of oxygen gas participated in the reaction.
Solution 3: The student's argument is incorrect; no matter was created. The rusting of an iron nail is a chemical synthesis reaction in which solid iron atoms ($Fe$) react with gaseous oxygen molecules ($O_2$) from the surrounding air in the presence of water to form iron(III) oxide ($Fe_2O_3$). The nail was located in an open system where invisible atmospheric oxygen was continuously present. The additional 1.8 grams of mass ($6.8\ g - 5.0\ g$) represents the mass of the oxygen atoms that were drawn from the air and bonded chemically to the iron atoms. If the nail, the air, and moisture had been enclosed in a sealed container, the total mass of the system would have remained constant throughout the rusting process.