The law of conservation is a set of fundamental principles in physics and chemistry stating that certain measurable quantities in an isolated system cannot be created or destroyed. They can change form, move between objects, or transform from one type to another, but the total amount stays the same. The most common versions apply to mass, energy, momentum, and electric charge.
Conservation of Mass
The law of conservation of mass states that during a chemical reaction, the total mass of the products must be equal to the total mass of the reactants. In plain terms: matter cannot appear out of nowhere or vanish into nothing. If you start a reaction with 50 grams of ingredients, you end with 50 grams of products.
This principle was formalized in the 1790s by Antoine Lavoisier, often called the father of modern chemistry. He emphasized careful, repeatable measurements of reacting substances and showed that mass always balanced on both sides of a reaction. A simple classroom demonstration illustrates this well: when you mix a magnesium sulfate solution with a sodium carbonate solution, a white solid forms in the liquid. It looks like something new appeared, but if you weigh everything before and after, the total mass is identical. All the individual atoms that made up the original substances are still present in the products, just rearranged into new combinations.
Conservation of Energy
The conservation of energy (also known as the first law of thermodynamics) says that within a defined system, the total amount of energy remains constant. Energy is neither created nor destroyed; it only changes form.
A roller coaster is one of the clearest everyday examples. At the top of the first hill, the cars have a large amount of stored energy due to their height (potential energy). As they roll downhill, that stored energy converts into the energy of motion (kinetic energy) and the cars accelerate. At the bottom of the hill, nearly all the stored energy has become motion. As the cars climb the next hill, kinetic energy converts back into potential energy and the cars slow down. This is why roller coasters almost always start with the tallest hill: the ride needs that initial store of energy to power the rest of the course. Some energy is lost to friction and air resistance along the way, but it doesn’t disappear. It transforms into heat, warming the tracks and the air slightly.
The same logic applies everywhere. A lightbulb converts electrical energy into light and heat. Your body converts the chemical energy in food into movement, body heat, and cellular processes. In every case, the total energy in a closed system stays the same.
Conservation of Momentum
Momentum is the quantity of motion an object has, determined by its mass and velocity. The conservation of momentum states that within a system, the total momentum remains constant. It is neither created nor destroyed, only transferred between objects through forces.
Think of two billiard balls colliding. Before the collision, one ball is moving and the other is still. After the hit, the first ball slows down (or stops) and the second ball starts moving. The motion transferred from one to the other, but the total momentum of both balls combined stayed the same. This principle is what makes Newton’s cradle work: the steel ball on one end swings in, stops, and the ball on the opposite end swings out with nearly the same speed.
Conservation of Electric Charge
Electric charge follows its own conservation law: the total net charge in a closed system always remains constant. Charge can move from one object to another, but it cannot be created or destroyed. When you shuffle across a carpet and shock a doorknob, electrons transferred from the carpet to your body. No new charge was made. It simply moved.
This holds true at every scale, from static electricity in your living room to reactions inside particle accelerators. Before and after any interaction, if you add up all the positive and negative charges, the total is always the same.
How Mass and Energy Connect
In everyday chemistry and physics, mass and energy appear to be conserved separately. But Albert Einstein showed that mass and energy are actually two forms of the same thing, related by the equation E = mc². The “c” in that equation is the speed of light (about 300 million meters per second), and because it’s squared, a tiny amount of mass converts into an enormous amount of energy.
This is what powers nuclear reactions. In a nuclear reactor or a star like the sun, small amounts of mass are converted into vast quantities of energy. The mass doesn’t violate conservation; it simply transforms into energy. So at the deepest level, the conservation law really governs mass-energy as a combined quantity. For ordinary chemical reactions (cooking, rusting, burning), the energy changes are so small that the corresponding mass change is undetectable, which is why Lavoisier’s original law of conservation of mass works perfectly in a chemistry lab.
Why Conservation Laws Matter
Conservation laws are not just abstract rules. They set hard limits on what is physically possible. The most famous example: perpetual motion machines. Inventors have tried for centuries to build devices that run forever without any energy input. Every single design fails, because it would violate the conservation of energy. A truly perpetual machine would need to produce energy from nothing, and the laws of physics don’t allow that. Even a hypothetical frictionless, perfectly silent machine would still radiate energy away because its moving parts contain charged particles, and accelerating charges emit radiation. It would eventually slow and stop.
Engineers rely on these laws constantly. Balancing a chemical equation is really just applying conservation of mass: every atom on the left side must appear on the right. Designing a bridge means accounting for how forces (and therefore momentum) transfer through the structure. Electrical circuit design depends on charge conservation to predict how current flows.
The Role of Isolated Systems
Conservation laws apply within a defined system, and the type of system matters. An isolated system exchanges neither matter nor energy with its surroundings. In such a system, mass, energy, momentum, and charge all remain perfectly constant. A closed system allows energy to cross its boundaries but not matter, so mass is conserved inside it while energy can flow in or out. An open system exchanges both.
In practice, perfectly isolated systems don’t exist. But the closer a system is to being isolated, the more precisely conservation laws predict what happens inside it. When physicists say energy is conserved, they mean that any energy that seems to “disappear” from one place can always be accounted for somewhere else, whether as heat, sound, light, or another form. Nothing is truly lost.