Acids and bases can be defined based on the ions they produce when dissolved in water:

Acids: Substances that produce hydrogen ions (H⁺) in aqueous solutions. The more H⁺ ions a substance produces, the more acidic it is. Bases: Substances that produce hydroxide ions (OH⁻) in aqueous solutions. Bases that are soluble in water are called alkalis. When an alkali dissolves in water, it creates an alkaline solution. The more OH⁻ ions a substance produces, the more basic (or alkaline) it is.

Common Examples of Acids and Bases

• Acidic Substances: Examples include vinegar, citric acid, and milk (although milk is only very mildly acidic).
• Basic (Alkaline) Substances: Bases are often found in cleaning products. Common examples include bleach and ammonia, both of which are strongly basic compounds.

Understanding the nature of acids and bases is fundamental to grasping the pH scale, as it is the concentration of H⁺ and OH⁻ ions that determines the pH and whether a substance is acidic, neutral, or alkaline.

The pH Scale is a widely used method for measuring the acidity or alkalinity (basicity) of a substance. The scale indicates the concentration of hydrogen ions (H⁺) in a solution, with pH being a logarithmic measure of this concentration. The lower the pH value, the higher the concentration of H⁺ ions, making the solution more acidic.

pH can be measured using a universal indicator, which is a chemical mixture that changes colour depending on the pH of the solution. When added to an acid or base, the indicator produces a colour that corresponds to a specific pH value on the pH scale. For example:

Neutralisation is a chemical reaction between an acid and a base that results in the formation of a salt and water.

The general word equation for this type of reaction is: $Acid+Base→Salt+Water$

For example:

• When hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), sodium chloride (NaCl) and water (H₂O) are formed: $HCl+NaOH→NaCl+H₂O$

Salts are often insoluble in water, meaning they do not dissolve easily and may form a precipitate. A precipitate is a solid that forms in solution and settles at the bottom of the reaction vessel. This can be separated from the solution using filtration.