Compounds as Substances

Compounds are pure substances formed when two or more elements chemically combine. This process is known as a chemical reaction.
Each compound has a unique, fixed ratio of elements. For example, water (H2O) always consists of two hydrogen atoms for each oxygen atom.
Compounds may be formed through several types of chemical reactions, such as synthesis (combining elements), decomposition (breaking down a single compound), single displacement (one element replacing another in a compound), and double displacement (elements in two compounds switching places).
Once formed, a compound has properties that are different from those of its constituent elements. For instance, sodium is a soft, reactive metal and chlorine is a poisonous gas, but together they form sodium chloride – a stable and edible salt.
Compounds can only be separated into their element parts through chemical reactions, which involve breaking and forming bonds. This is distinct from mixtures, which can be separated by physical means such as filtering or distillation.
Each compound is represented by a chemical formula, which indicates the types and numbers of atoms it contains. For example, CO2 represents carbon dioxide, with one carbon atom and two oxygen atoms.
In chemical equations, compounds are often shown reacting with other compounds or elements. The use of symbols, subscript numbers (which indicate the number of atoms of each element), and plus signs (to separate reactants and products) allow for a concise representation of chemical reactions.
Balancing these chemical equations is an important skill, and involves adjusting the numbers of each type of atom so that they are equal on both sides of the equation.
Ionic compounds are formed from metals and non-metals, while covalent compounds are usually formed from non-metals only.
The physical properties of compounds, such as their boiling and melting points or their solubility in water, are determined by the types of bonds they contain and their molecular structure.