34. Acids & Bases: pH and Neutralisation

1. The pH Scale

The pH scale measures how acidic or alkaline a solution is, ranging from 0 to 14.

  • 0 - 6: Acidic (lower numbers are more strongly acidic).

  • 7: Neutral (e.g., pure water).

  • 8 - 14: Alkaline (higher numbers are more strongly alkaline).

2. Measuring pH

There are two main ways to measure the pH of a solution:

  • Chemical Indicators: Dyes that change color depending on the pH. Universal Indicator is commonly used because its color changes across the whole range (Red for strong acid, Green for neutral, Purple for strong alkali).

  • pH Probes: Dipped into a solution to provide a numerical reading on a meter. This is more accurate and precise than indicators because it removes human judgment of color shades.

3. Defining Acids, Bases, and Alkalis

  • Acids: Substances with a pH less than 7. In water, they release Hydrogen ions (H⁺).

  • Bases: Any substance with a pH greater than 7.

  • Alkalis: A specific subgroup of bases that are soluble in water. In water, they release Hydroxide ions (OH⁻).

4. Neutralisation Reactions

When an acid and a base react together, they neutralise each other to produce a salt and water.

Acid + Base → Salt + Water

  • Example: Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water.

  • Ionic Equation: The H⁺ ions from the acid and the OH⁻ ions from the base combine to form water:

    • H⁺ + OH⁻ → H₂O

  • The resulting products are neutral, with a pH of 7.

5. Common Acids and Bases

It is helpful to remember these common examples:

  • Common Acids: Hydrochloric acid (HCl), Sulfuric acid (H₂SO₄), and Nitric acid (HNO₃).

  • Common Bases: Usually hydroxides or carbonates, such as Sodium hydroxide (NaOH) or Calcium carbonate (CaCO₃).