Acid (Chemistry)
An acid is a substance that donates protons (hydrogen ions, H⁺) to another substance or accepts electron pairs, resulting in a sour taste, ability to turn blue litmus red, and a pH below 7 in aqueous solution. Acids play a fundamental role in chemical reactions, biological processes, and industrial applications ranging from digestion to manufacturing. Common examples include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and acetic acid (CH₃COOH) found in vinegar.
Key Formula
HA → H⁺ + A⁻
LaTeX: \text{HA} \rightarrow \text{H}^+ + \text{A}^-
| Symbol | Meaning | Unit |
|---|---|---|
| HA | Generic acid molecule | none |
| H⁺ | Hydrogen ion (proton) | none |
| A⁻ | Conjugate base anion | none |
Worked Example
Problem
A 0.1 mol/L solution of hydrochloric acid (HCl) completely dissociates. Calculate the concentration of H⁺ ions and the pH.
Solution
Step 1: Write the dissociation equation: HCl → H⁺ + Cl⁻ Step 2: Since HCl is a strong acid, it dissociates completely: [H⁺] = 0.1 mol/L Step 3: Calculate pH: pH = -log[H⁺] = -log(0.1) = -log(10⁻¹) = 1
Answer
[H⁺] = 0.1 mol/L; pH = 1
Common Acids — Strength, Formula, and Uses
| Acid Name | Formula | Strength | Common Use |
|---|---|---|---|
| Hydrochloric acid | HCl | Strong | Stomach digestion, industrial cleaning |
| Sulfuric acid | H₂SO₄ | Strong | Car batteries, fertiliser production |
| Nitric acid | HNO₃ | Strong | Explosives, fertilisers |
| Acetic acid | CH₃COOH | Weak | Vinegar, food preservation |
| Citric acid | C₆H₈O₇ | Weak | Citrus fruits, food flavouring |
| Carbonic acid | H₂CO₃ | Weak | Carbonated beverages, blood pH |
Interactive Tools
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Related Terms
Base (Chemistry)
A base is a substance that accepts protons (H⁺ ions) from an acid or donates electron pairs, producing hydroxide ions (OH⁻) in aqueous solution, a bitter taste, slippery feel, and a pH above 7. Bases neutralise acids in reactions that form salt and water, making them essential in biological systems such as blood buffering, as well as in industrial processes like soap making. Common examples include sodium hydroxide (NaOH), ammonia (NH₃), and sodium bicarbonate (NaHCO₃).
pH Scale
The pH scale is a logarithmic measure of the hydrogen ion concentration [H⁺] in a solution, ranging from 0 to 14 at 25 °C, where values below 7 indicate acidic conditions, 7 is neutral, and above 7 is basic or alkaline. Introduced by Danish chemist Søren Peder Lauritz Sørensen in 1909, the scale compresses a trillion-fold range of H⁺ concentrations into a convenient 0–14 range. pH measurement is critical in agriculture, biology, medicine, food science, and environmental monitoring.
Acid Dissociation Constant (Ka)
The acid dissociation constant (Ka) is the equilibrium constant for the ionisation of an acid in water, quantifying the extent to which an acid donates its proton to water at a given temperature. A large Ka (> 1) indicates a strong acid that dissociates almost completely, while a small Ka (< 10⁻³) indicates a weak acid with limited dissociation. Ka values are fundamental in predicting reaction directions, calculating pH of weak acid solutions, and designing buffer systems.
From Latin "acidus" meaning sour, derived from "acere" (to be sour). The term was formalised in chemistry by Antoine Lavoisier in the late 18th century, who incorrectly believed all acids contained oxygen (from Greek "oxys" = sharp/acid).