Conservation of mass

The law of conservation of mass is a fundamental principle in physics and chemistry. It states that in any closed system, mass must remain constant over time. A closed system is one that is isolated from all incoming or outgoing transfers of matter. This law implies that mass cannot be created or destroyed. Instead, mass can only be rearranged in space. The entities that make up the mass can also change their form. This principle is a cornerstone for many scientific fields. It is essential to chemistry, mechanics, and fluid dynamics.

In chemical reactions, this law dictates how substances interact. We refer to the starting materials as reactants. The substances created by the reaction are called products. According to the principle, the total mass of the reactants must equal the total mass of the products. For example, consider the reaction of methane and oxygen. One molecule of methane and two molecules of oxygen react together. This process produces one molecule of carbon dioxide and two molecules of water. While the molecules look different, the total number of atoms remains the same. There are four hydrogen atoms, four oxygen atoms, and one carbon atom throughout the entire process.

Scientists use mathematical tools to describe how mass moves. In fluid mechanics, this is often expressed through the continuity equation. This equation describes the relationship between density, time, and flow velocity. For a given closed surface, the change in mass over a time interval equals the mass that moves through that surface. If matter enters the surface, the value is positive. If matter leaves the surface, the value is negative. For a whole isolated system, the sum of the masses of all components does not change. This mathematical approach helps engineers solve complex problems using a method called mass balance.

The history of this discovery spans many centuries and cultures. As early as 520 BCE, Jain philosophy stated that matter cannot be created or destroyed. Ancient Greek philosophers like Empedocles also argued that nothing can come from nothing. Later, around the 3rd century BCE, Epicurus suggested the totality of things would always remain as it is. In the 18th century, scientific progress accelerated. The Russian scientist Mikhail Lomonosov formulated the law in 1756. He used this principle to show that the phlogiston theory was incorrect. Later, Antoine Lavoisier confirmed the law through refined experiments in the late 18th century.

Lavoisier's work was vital for the transition from alchemy to modern chemistry. Before his time, it seemed like mass was lost during combustion. For instance, a piece of wood weighs less after it burns. This happens because gases escape into the atmosphere. This effect was hard to measure until the invention of the vacuum pump in the 17th century. Lavoisier used sealed glass containers to perform his tests. He found that the weight of the sealed container did not change during the reaction. His quantitative experiments proved that combustion involves oxygen. This discovery led to new ideas like John Dalton's atomic theory.

Modern physics changed our understanding of mass through special relativity. In 1905, Albert Einstein suggested that mass and energy are equivalent. This means mass can be converted into electromagnetic radiation. In most chemical reactions, the energy changes are too small to measure mass changes. However, in nuclear reactions, the changes are significant. For example, when an electron and a positron meet, they can annihilate. This process creates two photons and is the mechanism used in PET scans. This demonstrates that mass and energy are linked through the equation E=mc².

The concept of mass conservation is now part of a larger idea. Scientists now use the principle of mass-energy equivalence. This unifies the conservation of mass and the conservation of energy. In systems with large gravitational fields, general relativity must be considered. In these cases, mass and energy are defined relative to a frame of reference. While the classical law works for most everyday objects, the modern view is more complex. Understanding these connections allows us to study everything from tiny particles to the entire universe.