Yes, atoms are conserved during a chemical change, a principle that governs all chemical reactions. This concept may seem contradictory when observing a reaction, such as a clear liquid turning cloudy or a solid burning away into smoke, which suggests matter has disappeared or appeared. Despite the dramatic transformation into entirely new substances, the fundamental building blocks—the atoms—are neither created nor destroyed. The total amount of matter you start with remains precisely the total amount of matter you end with.
Understanding Chemical Change
A chemical change is a process where substances transform into new substances with different chemical properties. This is distinct from a physical change, like ice melting into water, where the substance changes form but its chemical identity remains the same.
In a chemical reaction, the starting materials are called reactants, and the new substances formed are the products. The atoms are reorganized into different groupings at the molecular level. This reorganization causes the products to look and behave very differently from the reactants, which leads to the perception that the original matter has vanished.
The Law of Conservation of Mass
The conservation of atoms during a chemical change is formally described by the Law of Conservation of Mass. This law states that in any ordinary chemical reaction, mass is neither gained nor lost. This means the total mass of the reactants must exactly equal the total mass of the products.
French chemist Antoine Lavoisier provided the quantitative evidence for this law in the late 18th century, establishing it as a foundation of modern chemistry. He demonstrated through careful experiments that if a reaction is performed in a closed system, the total weight remains unchanged even as a new substance forms.
How Atoms Rearrange
The mechanism by which new substances form involves the breaking and forming of chemical bonds. The atoms in the reactant molecules are initially linked together by these bonds. Energy from the environment is often needed to break these existing connections for the reaction to proceed.
Once the old bonds are broken, the separated atoms are free to form new connections with different partners. They shuffle and recombine to construct the product molecules. Atoms act like building bricks: the bricks themselves remain intact, but they are disassembled from one structure and immediately reassembled into an entirely new one.
Counting Atoms Before and After
The conservation principle is visually and mathematically confirmed by looking at a balanced chemical equation. A chemical equation uses symbols to represent the reactants and products, and balancing it ensures that the count of every atom type is the same on both sides of the reaction arrow.
For example, when hydrogen gas (\(\text{H}_2\)) reacts with oxygen gas (\(\text{O}_2\)) to form water (\(\text{H}_2\text{O}\)), the balanced equation is \(2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}\). On the reactant side, the coefficient “2” in front of \(\text{H}_2\) means there are four total hydrogen atoms (\(2 \times 2\)), and the single \(\text{O}_2\) molecule provides two oxygen atoms. On the product side, the “2” in front of \(\text{H}_2\text{O}\) means there are four hydrogen atoms (\(2 \times 2\)) and two oxygen atoms (\(2 \times 1\)). The total count of four hydrogen atoms and two oxygen atoms is identical on both sides, confirming that not a single atom was gained or lost, only rearranged.