The basic cognition of chemistry lies in the fact that pure substances consist of completely identical MOLECULES which are composed of ATOMS of the elements. This cognition results from the basic laws of chemistry. We need to imagine that the element consists of completely identical atoms. Atoms of different elements are different. In chemical reactions atoms combine into molecules in such a manner that the number of different atoms stays in the same proportion all the time. In the case of water each atom of oxygen binds two atoms of hydrogen.
Atoms and molecules are the basic ingredients of substances. Atoms and molecules of the same substance have in various circumstances the ability of forming either gas, liquid or solid.
Characteristics of substances depend upon the linkage between atoms and molecules.
