Reactants combine and break apart to form products with entirely new properties. No atoms are lost — they're just rearranged.
A chemical reaction is a process in which substances (reactants) are transformed into new substances (products) with different chemical properties. During the reaction, chemical bonds in the reactants are broken and new bonds form to create the products.
The substances that are present before the reaction — the starting materials that react with one another.
The substances that are produced after the reaction. They have different properties to the reactants.
Chemical reactions are written as equations with reactants on the left and products on the right, separated by an arrow (→ meaning "produces"):
Example 1 — vinegar and eggshell:
You can observe this reaction: bubbles of CO₂ form, the eggshell dissolves, and the solution changes — these are signs that a chemical reaction has occurred.
Example 2 — CO₂ and lime water:
When you blow through a straw into clear lime water, it turns milky/white. This is the standard test for carbon dioxide.
In a physical change (like melting, dissolving or cutting), no new substances are formed and the change is usually reversible. In a chemical reaction, new substances with different properties are formed, and the change is usually difficult or impossible to reverse.
Fermentation is an example of a chemical reaction known across cultures for thousands of years. Yeast converts sugars into ethanol (alcohol) and carbon dioxide gas. This is used in brewing traditional beverages such as umqombothi (sorghum beer) — a process that also makes grains more digestible and nutritious.
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