Law of definite proportions facts for kids

The law of definite proportions, also known as Proust's law, is a basic rule in chemistry. It says that a specific chemical compound always has the same elements combined in the same exact amounts by mass. This is true no matter where the compound comes from or how it was made.

For example, pure water is always made of hydrogen and oxygen. About ⁸/₉ of water's mass is oxygen, and the remaining ¹/₉ is hydrogen. This ratio never changes for pure water. This law helps us understand how chemicals combine.

History of the Law

The idea of constant proportions was first clearly stated by a French chemist named Joseph Proust in 1797. He was working in Segovia, Spain, at the time. Before Proust, other chemists like Joseph Priestley from England and Antoine Lavoisier from France had also noticed similar things.

Proust explained that elements combine in very specific ways. He said that metals like iron always combine with oxygen in fixed amounts. This was a new idea because, back then, chemists were still figuring out the difference between a true chemical compound and a simple mixture.

The Big Debate

When Proust first shared his law, it wasn't immediately accepted by everyone. Another French chemist, Claude Louis Berthollet, disagreed. Berthollet thought that elements could combine in many different proportions. This debate showed that chemists were still learning how elements join together to form new substances.

Dalton's Atomic Theory

The law of definite proportions became much clearer thanks to John Dalton and his atomic theory. Starting in 1803, Dalton suggested that all matter is made of tiny, unseen particles called atoms. He said that each element has its own type of atom. When compounds form, these atoms combine in simple, fixed ratios. This idea perfectly explained why the law of definite proportions is true.

Prout's Hypothesis

An early idea related to this law was Prout's hypothesis, proposed by English chemist William Prout. He thought that the hydrogen atom was the basic building block for all other atoms. This led to the "whole number rule," which suggested that atomic masses should be whole number multiples of hydrogen's mass.

However, later measurements by chemists like Jöns Jacob Berzelius showed that this wasn't perfectly true. For example, the atomic mass of chlorine was found to be 35.45, not a whole number. This difference was later explained by the discovery of isotopes, which are atoms of the same element with slightly different masses.

When the Law Isn't Perfect

While the law of definite proportions is very useful, it's not always perfectly true for every single compound.

Non-Stoichiometric Compounds

Some compounds are called non-stoichiometric compounds. This means their elemental makeup can change slightly from one sample to another. They don't always follow the law of definite proportions perfectly.

For example, a type of iron oxide called wüstite (FeO) can have a slightly different amount of iron. Instead of being exactly FeO, its formula might be closer to Fe_0.95O. This means it can have between 0.83 and 0.95 iron atoms for every oxygen atom. Proust's tools weren't precise enough to notice these small differences.

Isotopes and Mass

Also, the specific types of isotopes in an element can vary depending on where it came from. Since isotopes have slightly different masses, this can cause a tiny change in the overall mass ratio of a compound. This effect is usually very small, except for hydrogen and its isotopes. Scientists use these tiny variations in isotopes for things like radiometric dating, which helps them figure out the age of rocks or ancient objects.

Many natural polymers, like DNA or proteins, also vary in their exact makeup. They are not usually considered "pure chemical compounds" in the same way simple compounds are.

See also

In Spanish: Ley de las proporciones constantes para niños