Chemical properties are expressed by chemical equations. Both physical and chemical properties can be predicted based on the elemental composition and the structure of a substance. 3.3 Elements and compounds. Elements are substances that cannot be broken down into other substances by chemical means. Oxygen is an example of an element.
Compounds are substances containing different atoms always combined together in a specific ratio.
H2O. Substances can be broken down into their constituting elements. 3.4 Mixtures and pure substances. In a pure substance , all of the atoms or molecules are of the same type. Pure substances cannot be separated , only broken down or decomposed. Mixtures are combinations of substances, of any ratio, that do not react with each other.