Law of Conservation of Mass

Introduction

During an ordinary chemical reaction, atoms rearrange into new substances; they are not created or destroyed by the chemical change. In a closed system, total reactant mass equals total product mass. An open beaker may appear to lose or gain mass because gases can escape or enter.

Core explanation

For 2H₂ + O₂ → 2H₂O, two moles H₂ have mass about 4 g and one mole O₂ about 32 g, giving 36 g reactants. Two moles water have mass about 36 g. Balanced atom counts make the mass equality possible. If an equation is unbalanced, using its coefficients as a quantitative prediction can violate mass conservation. The conservation law is therefore a check on the equation and calculations.

A reaction in an open vessel can seem to lose mass when a gaseous product escapes. Heating CaCO₃ to form CaO + CO₂ leaves less solid mass because CO₂ enters the air. The whole system including collected CO₂ still conserves mass. Likewise, rusting iron in open air increases the mass of the iron-containing solid because oxygen atoms from the air join it. The additional mass has an external source; it is not mass created by rust.

Define the boundary before doing a balance. A closed container that retains gas, liquid and solids has no material crossing its boundary, though energy may cross as heat. In an open system, include material inputs and outputs: initial contents + material entering = final contents + material leaving, with consistent mass units. A balance reading of only a solid residue measures one part of the system, not the full chemical mass account.

Conservation also guides limiting-reactant problems. If 10 g reactant is mixed with 20 g another reactant, the total final mass of all products plus unused reactants in a closed system is 30 g. It is not necessarily 30 g of the desired product. Excess material can remain, and multiple products may form.

For the precision of ordinary chemical measurements, mass is conserved. In a broader physical treatment, mass and energy are related, but the energy-associated mass change in routine chemistry is far below the scale of school laboratory balances. It does not justify ignoring a missing gas or an unbalanced equation.

Step-by-step reasoning

1. Draw a boundary around the system whose mass is being compared. 2. List every initial and final substance, including gases and excess reagents. 3. Use a balanced equation to match atom amounts. 4. Add all masses on each side of the boundary account. 5. Explain apparent changes by identified material entering or leaving.

Visual explanation

Draw a sealed flask before and after reaction, with the same total mass on a balance. Next draw an open flask with an escaping CO₂ arrow; the remaining flask mass falls while the gas mass is shown outside.

Real-world analogy

Rearranging pieces of a closed puzzle does not change the total weight of the pieces. If a piece is carried away, the table becomes lighter without the piece ceasing to exist. Chemical reactions rearrange atoms, while open systems exchange matter.

Real-world example

A metal burning in air may leave a heavier oxide than the starting metal. Oxygen from the surrounding air contributes the extra measured mass. A sealed apparatus accounting for oxygen would show overall mass conservation.

Why?

Why is a balanced equation necessary for mass predictions? It preserves the number of each type of atom across the reaction; each atom carries mass into the products.

Common misconception

“Gas leaving means mass was destroyed.” It has crossed the measurement boundary. Include or collect the gas to complete the balance.

Worked example

Heating 100.0 g pure CaCO₃ to complete decomposition produces CaO and CO₂. Using approximate molar masses 100, 56 and 44 g mol⁻¹, one mole yields 56.0 g CaO and 44.0 g CO₂. The solid residue weighs less, but 56.0 + 44.0 = 100.0 g total products in the closed chemical account.

Quick check

1. A residue becomes lighter because CO₂ escapes. Has total chemical mass vanished? Answer: No. The escaping CO₂ carries mass outside the weighed residue; the complete system still conserves mass.

Exam focus

State the system boundary and include gases and excess materials. A product mass alone need not equal total reactant mass.

Advanced insight

Mass-balance equations are used in reactors with continuous inflow and outflow. At steady state, input mass rate equals output mass rate when accumulation is zero, even while individual species are converted.

Summary

Ordinary chemical reactions conserve total mass when all matter is accounted for. Balanced equations preserve atoms; open-system measurements can change because material crosses the boundary. Identify every input, output and remaining phase.

Practice questions

1. Why can an iron sample gain mass when rusting in air? Answer: Oxygen from air joins the iron-containing solid, adding mass from outside the original sample. 2. A closed reaction begins with 8 g and 12 g reagents. What is total final mass, including excess? Answer: It is 20 g if no matter enters or leaves the closed system. 3. CaCO₃ gives 56 g CaO and 44 g CO₂ from 100 g input. What mass remains if only solid is weighed? Answer: The solid residue is 56 g; 44 g exists as CO₂ rather than being destroyed.