Arrhenius Theory
Simple Explanation
The Arrhenius theory defines an acid as a substance that increases the concentration of H⁺ ions when dissolved in water, and a base as a substance that increases the concentration of OH⁻ ions when dissolved in water.
Why Do We Need It?
This was the first modern, testable definition of acids and bases, and it directly explains the classic properties everyone associates with them — acids turning litmus red and tasting sour, bases turning litmus blue and feeling slippery.
Why Does This Work?
Acids like HCl ionise in water to release H⁺ ions directly (HCl → H⁺ + Cl⁻); bases like NaOH ionise to release OH⁻ ions directly (NaOH → Na⁺ + OH⁻) — the defining behaviour of each class follows straightforwardly from what ions their dissolution actually produces.
Real-Life Example
Testing a solution with litmus paper
Litmus paper turns red in vinegar (acetic acid) and blue in ammonia solution (a base).
Vinegar releases H⁺ ions in water, satisfying the Arrhenius definition of an acid; ammonia solution generates OH⁻ ions in water, satisfying the Arrhenius definition of a base.
Practice
According to Arrhenius theory, what makes a substance a base?
EasyCommon mistake
Assuming Arrhenius theory can explain every acid-base reaction — it only applies to substances dissolved specifically in WATER, and cannot explain how a substance with no OH⁻ at all (like ammonia, NH₃) can still act as a base; that gap is exactly what the Brønsted–Lowry theory fixes.
Quick Review
- Arrhenius acid: increases [H⁺] in water.
- Arrhenius base: increases [OH⁻] in water.
- Only applies to aqueous (water-based) solutions.