The law of conservation states that certain fundamental quantities in nature, like mass, energy, and momentum, cannot be created or destroyed. They can change form, move between objects, or convert from one type to another, but the total amount in an isolated system always stays the same. There are several conservation laws in physics and chemistry, and each one applies to a different quantity. Together, they form the backbone of how scientists understand everything from chemical reactions to collisions between objects to the behavior of stars.
Conservation of Mass
The law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction. If you add up the mass of everything you start with, it will exactly equal the mass of everything you end up with. Antoine Lavoisier first established this principle in 1789, and it laid the foundation for modern chemistry.
This works because atoms are extremely stable under normal Earth conditions. During a chemical reaction, atoms rearrange into new combinations, but no atoms appear out of nowhere or vanish. If you burn a log, for example, the wood seems to disappear, but the mass of the ash, smoke, water vapor, and carbon dioxide released adds up to the original mass of the wood plus the oxygen it consumed. Every balanced chemical equation reflects this principle: the number and type of atoms on one side must match the other.
The law applies to any closed system, from a sealed flask in a lab to an entire ecosystem. Ecologists use it to track how elements like carbon and nitrogen cycle through environments. Every individual reaction in a forest or ocean obeys this constraint, so the ecosystem as a whole does too.
Conservation of Energy
The law of conservation of energy, also called the first law of thermodynamics, states that the total energy in an isolated system is constant. Energy can shift between forms (heat, motion, light, chemical bonds, electrical current) but the total never increases or decreases.
You see this every day without thinking about it. A car engine converts the chemical energy stored in gasoline into the mechanical energy of moving parts. Solar panels convert radiant energy from sunlight into electrical energy. A ball at the top of a hill has stored energy from its position; as it rolls down, that converts into the energy of motion. In every case, the energy doesn’t come from nothing. It transforms from one type to another, and if you could measure every form of energy involved, the total would stay the same.
The first law of thermodynamics formalizes this by saying that the change in a system’s internal energy equals the heat added to the system minus the work the system does on its surroundings. If no heat enters or leaves and no work is done, the internal energy doesn’t change.
Conservation of Momentum
Momentum is the product of an object’s mass and its velocity. The law of conservation of momentum states that in a closed system with no outside forces, the total momentum before an event equals the total momentum after it. This is most visible during collisions. When a moving billiard ball strikes a stationary one, the first ball slows down and the second speeds up, but the combined momentum of both balls stays the same.
There’s also a rotational version. Angular momentum, the rotational equivalent, is conserved whenever no outside twisting force (torque) acts on the system. This is why an ice skater spins faster when pulling their arms in. Their body’s mass moves closer to the axis of rotation, so the spin rate increases to keep angular momentum constant. The same principle governs how planets orbit stars and why galaxies maintain their rotation over billions of years.
How Mass and Energy Connect
In everyday chemistry, conservation of mass and conservation of energy work as two separate rules. But Einstein’s special theory of relativity revealed they are actually two sides of the same coin. Mass and energy are interchangeable, related by the famous equation E = mc², where c is the speed of light.
This means that when an object absorbs energy, its mass increases by a tiny amount, and when it radiates energy, its mass decreases. In Newtonian physics, this was unthinkable: mass was a fixed property of an object that couldn’t change just because the object emitted light. But in special relativity, if one body radiates energy and another absorbs it, mass effectively transfers between them. Einstein called this “the most important upshot of the special theory of relativity.”
For chemical reactions on Earth, the mass change from energy release is so vanishingly small that it’s undetectable, which is why the classical law of conservation of mass works perfectly fine in chemistry. But in nuclear reactions, the energy involved is enormous relative to the particles, and measurable amounts of mass convert into energy. This is what powers the sun and nuclear reactors. So at the deepest level, there is one unified conservation law: the total mass-energy of an isolated system is constant.
Why These Laws Exist
In 1915, mathematician Emmy Noether proved a remarkable theorem connecting conservation laws to symmetries in nature. Each conservation law exists because the universe behaves the same way under a certain type of transformation. Conservation of energy comes from the fact that the laws of physics don’t change over time. Conservation of momentum comes from the fact that the laws of physics are the same everywhere in space. Conservation of angular momentum comes from the fact that the laws of physics don’t change if you rotate your frame of reference.
Einstein praised this as a piece of “penetrating mathematical thinking,” and it remains one of the most powerful ideas in theoretical physics. It means conservation laws aren’t just useful accounting rules. They reflect deep structural properties of the universe itself.
Where Conservation Laws Apply
Conservation laws are exact for an isolated system, meaning a collection of matter that doesn’t interact with the rest of the universe at all. No energy flows in or out, no forces act on it from outside, nothing enters or leaves. In that idealized case, energy, momentum, and angular momentum are all perfectly conserved.
In practice, perfectly isolated systems don’t exist. But the laws still work in real situations as long as you account for everything involved. If a system appears to lose energy, that energy went somewhere: into heat from friction, sound waves, light, or some other form. Widen your boundaries enough to include those outputs, and the books balance again. That’s the key insight behind every conservation law. Nothing is lost. It just moves somewhere else or changes form.