Law of Conservation of Mass: Sixth Grade Science
The Law of Conservation of Mass is a foundational principle of physical science stating that matter cannot be created or destroyed in any chemical reaction or physical change.
The Principle of Mass Conservation
Formulated in 1789 by the French chemist Antoine Lavoisier, the Law of Conservation of Mass revolutionized natural science. Before Lavoisier's precise quantitative experiments, early natural philosophers believed that when wood burned into a small pile of ashes, matter simply disappeared into nothingness, or that when a shiny nail rusted into a heavier mass, matter was created out of thin air. Lavoisier proved that in any closed system, the total mass of the reactants before a transformation must exactly equal the total mass of the products after the transformation.
+-----------------------------------------------------------------------------+
| THE LAW OF CONSERVATION OF MASS |
| |
| TOTAL MASS OF REACTANTS = TOTAL MASS OF PRODUCTS |
| |
| [ Reactant Atom A ] + [ Reactant Atom B ] |
| | |
| v (Chemical Bond Rearrangement) |
| [ Product Compound AB ] |
| |
| No atoms are created! No atoms are destroyed! Only rearranged! |
+-----------------------------------------------------------------------------+
The Atomic Basis of Mass Conservation
To understand why mass is conserved, we must look at matter at the atomic level. All matter is made of indestructible, indivisible building blocks called atoms (in ordinary chemical reactions). During a chemical reaction:
- Chemical bonds holding reactant molecules together are broken.
- The atoms separate, collide, and rearrange into new geometric bonds.
- The exact same number and types of atoms present at the start of the reaction are present at the end of the reaction.
Because every individual atom retains its specific atomic mass throughout the rearrangement, the total mass cannot change. A balanced chemical equation reflects this atomic conservation: the number of atoms of each element on the left side (reactants) must perfectly balance the number of atoms on the right side (products).
+-----------------------------------------------------------------------------+
| BALANCED CHEMICAL EQUATION MODEL |
| |
| Reaction: Methane Combustion |
| Equation: CH4 + 2 O2 --------> CO2 + 2 H2O |
| |
| Left Side (Reactants): Right Side (Products): |
| - Carbon atoms: 1 - Carbon atoms: 1 |
| - Hydrogen atoms: 4 - Hydrogen atoms: 4 (2 x 2) |
| - Oxygen atoms: 4 (2 x 2) - Oxygen atoms: 4 (2 in CO2 + 2 in H2O)
| |
| Atomic Balance: 1 C, 4 H, 4 O = Atomic Balance: 1 C, 4 H, 4 O |
| Reactant Mass = 80.0 grams = Product Mass = 80.0 grams |
+-----------------------------------------------------------------------------+
Open Versus Closed Systems in Laboratory Investigations
When testing the Law of Conservation of Mass in a school laboratory, experimental results depend critically on whether the investigation is conducted in an open system or a closed system.
+-----------------------------------------------------------------------------+
| OPEN VERSUS CLOSED SYSTEMS |
+-------------------+---------------------------------------------------------+
| System Type | Characteristics and Mass Measurement Outcome |
+-------------------+---------------------------------------------------------+
| Open System | Matter can freely enter or escape into the surrounding |
| | environment. Gases produced by reactions escape into |
| | the room, causing an apparent, false "loss" of mass. |
+-------------------+---------------------------------------------------------+
| Closed System | An impermeable physical barrier seals the experiment. |
| | No matter can enter or exit. The balance reading stays |
| | perfectly identical before and after the reaction. |
+-------------------+---------------------------------------------------------+
The Classic Alka-Seltzer Experiment
Consider an investigation testing the reaction between an effervescent antacid tablet and water:
- In an Open Flask: A student places 100.0 grams of water and a 3.0-gram antacid tablet into an open glass beaker sitting on a digital scale (initial total mass = 103.0 grams). When the tablet is dropped into the water, vigorous fizzing occurs. After bubbling stops, the scale displays 101.8 grams. A careless observer might infer that 1.2 grams of matter was destroyed. In reality, the missing 1.2 grams was carbon dioxide gas that escaped from the open mouth of the beaker into the surrounding room.
- In a Closed Flask: The student repeats the experiment, but places the water and tablet inside an airtight Erlenmeyer flask sealed with a rubber stopper. The scale reads 103.0 grams. As the tablet fizzes, the flask fills with pressurized carbon dioxide gas. When the reaction completes, the scale still reads exactly 103.0 grams. Because the gas was trapped, the conservation of mass is definitively verified.
Exercises and Solutions
Exercise 1: Calculating Unknown Product Mass in a Closed Reaction. A student conducts a chemical reaction inside a sealed glass reaction vessel. The student mixes 12.0 grams of solid iron powder with 7.0 grams of yellow sulfur powder. The mixture is ignited with an internal electric spark. The two elements react completely to form a single solid compound called iron(II) sulfide (FeS). (a) State the total mass of the iron(II) sulfide product formed; (b) Explain how the Law of Conservation of Mass justifies your calculation; (c) If the seal on the vessel broke during heating and some gas escaped, what would happen to the measured mass?
Solution 1: (a) The total mass of the iron(II) sulfide product formed is exactly 19.0 grams. (b) According to the Law of Conservation of Mass, matter cannot be created or destroyed in a chemical reaction. In a sealed closed system, the total mass of the products must equal the sum of the masses of the reactants: 12.0 grams of iron plus 7.0 grams of sulfur equals 19.0 grams of product. Every iron and sulfur atom present at the beginning is now chemically bonded inside the iron(II) sulfide crystal lattice. (c) If the seal broke and airborne gases or smoke particles escaped into the room, the measured mass on the scale would decrease, creating an apparent loss of mass due to matter escaping the system.
Exercise 2: Analyzing Apparent Mass Gains During Oxidation. A student places a 50.0-gram wad of fine steel wool on a digital balance pan. Using a battery, the student ignites the steel wool in the open air. The iron filaments glow bright orange, reacting with air to form iron oxide. When the glow fades, the student is shocked to see that the digital balance reading has increased to 64.5 grams! The student wonders whether mass was created out of nothing. Provide a scientific explanation for where the extra 14.5 grams of mass came from and explain how this preserves the Law of Conservation of Mass.
Solution 2: Mass was not created out of nothing. The experiment took place in an open system where atmospheric gases could freely participate in the reaction. During combustion, iron atoms (Fe) in the steel wool reacted chemically with invisible oxygen gas molecules (O2) from the surrounding room air, forming solid iron(III) oxide (rust/ash) that accumulated directly on the balance pan. The extra 14.5 grams of measured mass represents the exact mass of atmospheric oxygen gas that was drawn into the reaction and chemically bonded to the iron atoms. If the student had conducted the combustion inside an airtight, sealed glass chamber containing both the steel wool and the air, the total mass of the chamber and its contents would have remained exactly identical before and after burning.