lesson
Updated 6 days ago
When a giant log burns in a campfire, it leaves behind only a tiny pile of lightweight ash. It looks like most of the wood's matter simply vanished into thin air — but did it?
What actually happens to those atoms when chemical bonds break and reform?
The Law of Conservation of Mass
In 1789, French chemist Antoine Lavoisier carefully sealed chemical reactions inside glass vessels and weighed them before and after, discovering that the total mass never changed by even a fraction of a gram.
The law of conservation of mass states that no atoms are created or destroyed during a chemical reaction. Because every single starting atom still exists at the end, the total mass of reactants always equals the total mass of products.
📊An interactive-style visual showing the reaction 2H2 + O2 -> 2H2O. Left side: Reactants card containing 4 blue hydrogen atoms (in 2 pairs) and 2 red oxygen atoms (in 1 pair) on a balance scale pan labeled 'Reactants: 36g'. Right side: Products card containing 2 water molecules (each 1 red oxygen bonded to 2 blue hydrogens) on the opposite balance scale pan labeled 'Products: 36g'. The balance beam is perfectly level in the center with a green '=' checkmark. Clear color-coded legend: Blue circles = Hydrogen, Red circles = Oxygen. Clean modern card styling with #1e2945 text and light neutral background.
Chemical reactions are simply rearrangements of existing atoms — old bonds break and new bonds form, but the atomic building blocks remain identical.
So how do we use this rule to solve mass calculation problems on an exam?
Tracking Mass in Reactions
Because mass is conserved, you can find the mass of any missing substance by setting the sum of reactants equal to the sum of products: Massreactants=Massproducts.
For example, when 48 g of magnesium ( extMg) reacts completely with 32 g of oxygen gas (O2), they combine to form magnesium oxide (MgO):
48 g (Mg)+32 g (O2)=80 g (MgO)
📊A step-by-step math card showing conservation of mass arithmetic. Top row shows chemical equation: 2Mg + O2 -> 2MgO. Middle row shows a dual-pan balance with 48g Mg + 32g O2 on the left (total 80g) balancing 80g MgO on the right. Bottom row shows a worked practice problem: 'If 100g of calcium carbonate (CaCO3) thermally decomposes into 56g of calcium oxide (CaO) and carbon dioxide (CO2), find the mass of CO2.' Equation: 100g = 56g + mass(CO2) -> mass(CO2) = 100g - 56g = 44g. Highlighted in green callout box.