Chemistry is often described as the study of matter and the changes it undergoes. To describe these changes accurately and universally, scientists use a specialized language of symbols and formulas. This system allows us to represent the composition of substances and the stoichiometry of reactions with precision. The IGCSE 0654 Core syllabus requires a clear understanding of chemical formulas, the construction of word and symbol equations, and the fundamental law that governs all chemical transformations.

A chemical formula is a notation that shows the number and type of atoms present in a molecule or a formula unit.

• Element Symbols: Each element is represented by a unique one- or two-letter symbol (e.g., $H$ for Hydrogen, $Na$ for Sodium).
• Subscripts: Small numbers written to the right of a symbol indicate the number of atoms of that element. For example, $H_2O$ contains two hydrogen atoms and one oxygen atom.
• Molecular Formula: Specifically defines the exact number of atoms of each element in one molecule of a compound.

A chemical equation is a shorthand representation of a chemical reaction, showing the transformation of reactants (starting materials) into products (final substances).

Word Equations

Word equations provide a qualitative description of a reaction.

• Format: Reactant A + Reactant B $\rightarrow$ Product C + Product D
• Example: Hydrogen + Oxygen $\rightarrow$ Water

Symbol Equations

Symbol equations provide a quantitative and more detailed description using chemical formulas.

• Example: $2H_2 + O_2 \rightarrow 2H_2O$

The most fundamental rule of chemical reactions is that mass is conserved. Atoms are neither created nor destroyed during a chemical change; they are simply rearranged into new combinations.

Balancing Equations

Because of the conservation of mass, a symbol equation must be balanced. This means that the total number of atoms of each element must be the same on both the reactant side (left) and the product side (right).

• Coefficients: Large numbers placed in front of a formula (e.g., the 2 in $2H_2$) are used to balance the equation. They multiply the entire formula that follows.

To provide a complete description of a reaction, chemists often include state symbols in brackets after each formula:

• (s): Solid
• (l): Liquid
• (g): Gas
• (aq): Aqueous (dissolved in water)

Example: $Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)$

Mastery of formulas and equations is the primary skill in chemistry. By utilizing this standardized notation, we can accurately track the movement of atoms through complex reactions, ensuring that the fundamental laws of physics and chemistry are upheld in every calculation and experiment.