How do chemical reactions follow the law of conservation of mass?

The law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction. This principle ensures that the mass of the reactants equals the mass of the products in a closed system. Here’s how chemical reactions adhere to this law:

1. Conservation at the Atomic Level

• During a chemical reaction, atoms are rearranged, not created or destroyed.
• Bonds between atoms in the reactants are broken, and new bonds are formed to create products.
• Since the number of each type of atom remains constant, the total mass before and after the reaction remains the same.

Example: Combustion of methane:

$C{H}_4+2O_2\to C{O}_2+2H_{2}O$

• Reactants: 1 carbon atom, 4 hydrogen atoms, and 4 oxygen atoms.
• Products: 1 carbon atom, 4 hydrogen atoms, and 4 oxygen atoms.
• The mass of atoms on both sides is equal.

2. Balanced Chemical Equations

• Chemical equations are written to reflect the conservation of mass.
• Coefficients are adjusted to balance the number of atoms of each element on both sides of the equation.

Example: Formation of water:

$2H_2+O_2\to 2H_{2}O$

• Two hydrogen molecules (4 H atoms) react with one oxygen molecule (2 O atoms) to form two water molecules (4 H atoms and 2 O atoms).

3. Closed System Requirement

• For the law of conservation of mass to hold visibly, the reaction must occur in a closed system, where no mass is lost to or gained from the surroundings.
• In an open system, gases or other products may escape, making it seem like mass is “lost,” though it has merely dispersed into the environment.

4. Real-Life Demonstrations

• Precipitation Reactions: When solutions are mixed, and a solid forms, the combined mass of the solutions and the precipitate remains constant.
• Burning of Candle Wax: In a closed system, the mass of the wax and oxygen consumed equals the mass of carbon dioxide, water vapor, and other products.

5. Modern Validation

• The law has been validated by careful measurements using advanced tools.
• Antoine Lavoisier, the “father of modern chemistry,” first demonstrated this law by performing experiments in sealed containers, showing that the total mass remained unchanged.

In chemical reactions, the rearrangement of atoms and strict adherence to balanced equations ensure that the law of conservation of mass is upheld. This principle is fundamental to understanding chemical processes and serves as the basis for stoichiometric calculations in chemistry.