Elemental Composition of Pure Substances

A pure substance is a substance that is made up of only one type of particle; it has a fixed composition with distinct chemical and physical properties.
Pure substances include elements and compounds. An element consists of only one type of atom, while a compound consists of two or more types of atoms bonded together.
An element is a substance that cannot be broken down into simpler substances by chemical means. Each element is distinguished by its atomic number, the number of protons in its nucleus.
Every element has a unique symbol, usually derived from its modern name or its Latin name. For example, oxygen is ‘O’, carbon is ‘C’, and gold is ‘Au’ (from the Latin Aurum).
Compounds are pure substances composed of two or more elements combined in a fixed, definite proportion. They can only be separated into their constituent elements through chemical reactions.
The properties of a compound are often different from the properties of the elements it contains. For example, sodium (a reactive metal) and chlorine (a poisonous gas) react together to form sodium chloride (table salt), which has completely different properties.

Chemical Formulas

A chemical formula shows the proportion of atoms of each element present in a compound. For example, in water (H2O), there are two hydrogen atoms for every one oxygen atom.
The formula also indicates the element’s oxidation states. For instance, in sulfuric acid (H2SO4), the sulfur is in the +6 oxidation state.
A molecular formula shows the actual number of atoms of each element in a molecule of a compound, while an empirical formula shows the simplest ratio of atoms in a compound.
Structural formulas give a graphic representation of the molecular structure, showing how the atoms are connected in a molecule.

The Law of Constant Composition or Law of Definite Proportions states that a given compound will always have the same proportion of element types by mass, regardless of the source of the compound or how it was prepared.
This law is a fundamental principle of chemistry and helps in understanding the nature and behaviour of pure substances.

Chemical reactions involve restructuring of the atoms in reactants to produce new substances (products). During a chemical reaction, the bonds between atoms in reactants break, and new bonds form in products.
Reactants and products in a chemical reaction are represented by a chemical equation, which shows the number and type of atoms in reactants and products.

The Law of Conservation of Mass states that mass cannot be created or destroyed in an isolated system. Therefore, the total mass of reactants in a chemical reaction always equals the total mass of the products.
This principle is fundamental in balancing chemical equations. Each atom that appears in the reactants must appear in the products, and in the same amount.