What are acids and bases?
In the widely used Brønsted–Lowry definition, an acid is a substance that donates hydrogen ions (H⁺, which in water become hydronium ions, H₃O⁺), and a base is a substance that accepts them. Many bases release hydroxide ions (OH⁻), which combine with H⁺ to form water. Even pure water contains a tiny, equal amount of both ions, because a few water molecules constantly swap H⁺ with each other.
The pH scale
pH measures how acidic or basic a solution is. At 25 °C, pure water has pH 7 (neutral); lower values are acidic and higher values are basic, or alkaline. The scale is logarithmic: each step of 1 is a tenfold change in hydronium concentration. Lemon juice at pH 2 has about 100,000 times more H₃O⁺ than pure water. The simulator shows the balance of ions with dots on a compressed scale, because the real ratio can be as large as 10¹⁴.
Indicators
An indicator is a dye that changes colour over a particular pH range. Chemists choose an indicator whose colour change matches what they want to detect.
| Indicator | Colour change | Approximate pH range |
|---|---|---|
| Methyl orange | Red → yellow | 3.1–4.4 |
| Litmus | Red → blue | 4.5–8.3 |
| Bromothymol blue | Yellow → blue | 6.0–7.6 |
| Phenolphthalein | Colourless → pink | 8.2–10 |
| Universal indicator | Red → orange → yellow → green → blue → violet | 0–14 |
Neutralization
When an acid and a base react, H⁺ and OH⁻ combine to make water, and the remaining ions form asalt. For hydrochloric acid and sodium hydroxide:
HCl + NaOH → NaCl + H₂O (H⁺ + OH⁻ → H₂O)Titration
A titration measures the concentration of an acid (or base) by adding a solution of known concentration until the reaction is exactly complete — the equivalence point. For a strong acid and strong base, the pH changes slowly at first, then jumps sharply from about 4 to 10 within a fraction of a millilitre around the equivalence point, and then levels off again. An indicator such as phenolphthalein, which changes colour in that jump, shows when to stop.
Formulas
pH = −log₁₀[H₃O⁺] pOH = −log₁₀[OH⁻][H₃O⁺] × [OH⁻] = K_w = 1.0 × 10⁻¹⁴ so pH + pOH = 14 (at 25 °C)At equivalence: moles of acid = moles of base → C_a·V_a = C_b·V_bKey terms
- Strong acid / strong base
- Ionizes (or dissociates) completely in water, like HCl or NaOH.
- Weak acid / weak base
- Only partly ionizes, like the acetic acid in vinegar.
- Hydronium ion
- H₃O⁺, the form hydrogen ions take in water.
- Equivalence point
- The point in a titration where exactly enough base has been added to react with all the acid.
- Buffer
- A solution that resists changes in pH, such as the bicarbonate system in blood.
Real-world applications
- The human body keeps blood between about pH 7.35 and 7.45 using buffers (an educational example only — see our medical disclaimer).
- Agriculture: farmers test soil pH and add lime to reduce acidity, because many crops grow best between pH 6 and 7.
- Environment: acid rain and ocean acidification — seawater pH has fallen from about 8.2 to about 8.1 since the Industrial Revolution — affect lakes, forests and shell-building organisms.
- Everyday life: antacids neutralize excess stomach acid, and swimming pools are kept near pH 7.2–7.8.
- Industry and labs: titrations are used to check the concentration of everything from vinegar to medicines.
Common misconceptions
- “Diluting an acid enough makes it basic.” Dilution only brings pH closer to 7; it never crosses to basic.
- “pH 4 is twice as acidic as pH 8.” The scale is logarithmic: pH 4 has 10,000 times more H₃O⁺ than pH 8.
- “All acids are dangerous.” Many are weak and harmless — citric acid in fruit, for example. Strength and concentration both matter.
- “Neutralization makes pure water.” It also produces a salt.
What the model simplifies
Everyday pH values are typical approximations; real products vary. Dilution treats each solution as a simple excess of strong acid or base, so it ignores buffering and weak-acid behaviour (vinegar, for instance, would rise in pH more slowly). The titration uses a strong acid and a strong base at 25 °C, for which the calculation is exact.
Safety: never taste or mix household chemicals. Bleach mixed with ammonia or acids releases toxic gases.
Key takeaways
- Acids donate H⁺; bases accept H⁺ or release OH⁻.
- pH = −log[H₃O⁺]; each pH unit is a factor of 10.
- Indicators change colour over specific pH ranges.
- In a strong acid–strong base titration, pH jumps sharply at the equivalence point.