Looking ahead to chemical reactions, an important concept to understand is The Law of Definite Composition. To understand this, it is a good idea to begin by reviewing Dalton's Atomic Theory (the parts he got right!):
Dalton noticed that all compounds have something in common. No matter how large or small the sample, the ratio of the masses of the elements in the compound is always the same. Dalton's theory was developed based on this observation.
Dalton proposed the theory that all matter is made up of individual particles called atoms which cannot be divided.
All elements are composed of atoms.
All atoms of the same element have the same mass, and atoms of different elements have different masses.
Compounds contain atoms of more than one element.
In a particular compound, atoms of different elements always combine in the same ratios.
Dalton compared the mass ratios of elements when they reacted. He found that (for instance) no matter how much sodium he started with, if the mass of the chlorine didn't change, the exact same amount of sodium would be used up in the reaction.
So suppose he had sixty grams of sodium but only needed one mole (22.990 grams)? After the reaction was complete, he would have left over sodium. In fact, he would have exactly 37.01 grams of sodium left over.
The reason behind this is that compounds form in fixed, set ratios. Salt is always in a ratio of one atom of sodium to one atom of chlorine (1:1 ratio of atoms), or 22.99 grams of sodium (Na) to 35.453 grams of chlorine (Cl). Water is always in a ration of two hydrogen (H) atoms to one oxygen (O) atom (2:1 ratio of atoms). Every compound is made up of a fixed, specific ratio of the atoms that make it up. And because atoms of any element have the same atomic mass, compounds form in not only fixed ratios of the number of atoms, but also in fixed ratios of masses.
This Law of Definite Composition (which, by the way has exceptions) can be cited asevidence that supports the idea that some pure substances are combined of elements in a definite ratio.
The Law of Definite Composition means that, when a chemical reaction is going on, when one of the elements is used up, the reaction ends, and any portion of the other element that is left over is—well, it is left over.