Chemical composition is the arrangement, type, and ratio of atoms in molecules of chemical substances. Chemical composition varies when chemicals are added or subtracted from a substance, when the ratio of substances changes, or when other chemical changes occur in chemicals. Chemical formulas show this information.
The chemical composition of a substance determines the intramolecular forces and properties of the substance. This means that the way atoms are put together in something determines the color, density, strength, texture and other properties of the thing. Chemists can use tests to learn the chemical composition of a substance, including a pH test, flammability test, heavy metal test etc.
A particular chemical has a particular ratio of its atoms, allowed by their valence (chemistry). An example is Hydrogen Fluoride, or HF, which has 1 Hydrogen atom in ratio to every 1 Fluorine atom. This substance has strong intramolecular forces of attraction because it is a hydrogen bond.