Conservation of mass Knowledge-Based Study

Study notes

1. The Law of Conservation of Mass

   The Law of Conservation of Mass states that in any chemical reaction or physical change within an isolated or closed system, mass is neither created nor destroyed. The total mass of the substances present before the change must equal the total mass of the substances present after the change.

   Formulated definitively by Antoine Lavoisier in the late 18th century, this principle forms a cornerstone of modern quantitative chemistry:

   Total Mass of Reactants = Total Mass of Products

   Atomic Explanation of Mass Conservation

   At the microscopic level, chemical reactions involve the breaking and forming of chemical bonds, not the creation or destruction of elements. According to modern atomic theory:

   • Individual atoms cannot be created or destroyed during ordinary chemical transformations.
   • The identities and total counts of each element's atoms remain strictly conserved from reactants to products.
   • Because each atom has a fixed mass, the conservation of individual atoms guarantees the conservation of total mass.

   This principle provides the theoretical foundation for balancing chemical equations, ensuring identical atom counts on both sides of a reaction arrow.

   Open vs. Closed Systems

   In laboratory investigations, seemingly unexpected changes in mass typically occur because the system is not sealed:

   • Closed System: A physical boundary prevents matter from entering or leaving. A gas-forming reaction conducted in a sealed flask maintains a constant reading on an analytical balance.
   • Open System: Matter can exchange freely with the surroundings. If a reaction generates a gaseous product that escapes into the atmosphere (such as carbon dioxide released when baking soda reacts with acid), the measured mass of the container decreases. Conversely, when iron oxidizes in open air, oxygen gas from the atmosphere binds to the metal, causing the measured mass of the solid product to increase.

   In both open scenarios, mass is still conserved overall; the apparent difference is solely due to the exchange of gas with the environment.

   Applying the Mass Balance Equation

   Because mass must balance exactly across all chemical transformations, unknown masses of reactants or products can be solved algebraically using the relationship:

   $\sum m_{\text{reactants}} = \sum m_{\text{products}}$

   Representative Example:

   Consider the thermal decomposition of solid calcium carbonate ($\text{CaCO}_3$) into solid calcium oxide ($\text{CaO}$) and carbon dioxide gas ($\text{CO}_2$):

   $\text{CaCO}_3(s) \rightarrow \text{CaO}(s) + \text{CO}_2(g)$

   If 100.0 g of pure $\text{CaCO}_3$ is completely decomposed in a sealed vessel to yield 56.1 g of $\text{CaO}$, the mass of $\text{CO}_2$ produced is determined as:

   $m_{\text{CO}_2} = m_{\text{CaCO}_3} - m_{\text{CaO}} = 100.0\text{ g} - 56.1\text{ g} = 43.9\text{ g}$