Acid vs Base
Acids are substances that donate hydrogen ions (H⁺) in solution. Bases are substances that accept hydrogen ions, or equivalently, that produce hydroxide ions (OH⁻). The pH scale measures how acidic or basic a solution is — low values are acidic, high values are basic, and 7 is neutral. Mix an acid and a base in the right proportions and they neutralise each other.
Last reviewed on 2026-04-27.
Quick Comparison
| Aspect | Acid | Base |
|---|---|---|
| Definition (Brønsted-Lowry) | Donates H⁺ | Accepts H⁺ (or donates OH⁻) |
| pH range | Below 7 | Above 7 |
| Taste | Sour | Bitter |
| Feel | Reactive — can sting cuts | Slippery, soapy |
| Common examples | Lemon juice, vinegar, stomach acid, sulphuric acid | Baking soda, ammonia, soap, sodium hydroxide |
| Reaction with each other | Neutralisation: produces salt + water | Same |
| Indicator (litmus) | Turns red | Turns blue |
| Strength | Strong (HCl, H2SO4) to weak (acetic acid) | Strong (NaOH, KOH) to weak (ammonia) |
Key Differences
1. Defining behaviour
Acids donate hydrogen ions. In water, hydrochloric acid (HCl) splits into H⁺ and Cl⁻. The H⁺ — really H3O⁺ when it bonds to water — is what gives the solution its acidic properties.
Bases accept hydrogen ions. Sodium hydroxide (NaOH) splits into Na⁺ and OH⁻; the OH⁻ readily combines with H⁺ from acids to form water.
2. The pH scale
pH is a logarithmic scale of hydrogen-ion concentration. pH 0–6 is acidic; pH 7 is neutral; pH 8–14 is basic. Each pH unit represents a tenfold change in H⁺ concentration.
A solution at pH 4 has 100 times the H⁺ concentration of a solution at pH 6. Saying "slightly acidic" or "slightly basic" can hide much bigger differences than the small pH gap suggests.
3. Common examples
Acids: lemon juice (citric acid, pH ~2), vinegar (acetic acid, pH ~3), Coke (carbonic and phosphoric acid, pH ~2.5), stomach acid (HCl, pH ~1–2), sulphuric acid (battery acid, pH near 0).
Bases: baking soda (sodium bicarbonate, pH ~8), ammonia (pH ~11), bleach (sodium hypochlorite, pH ~12), oven cleaner (sodium hydroxide, pH ~14).
4. Sensory clues
Acids taste sour. The sour bite of citrus, vinegar, and unripe fruit comes from acid content. Strong acids burn — never deliberately taste them.
Bases taste bitter and feel slippery or soapy. The slippery feel of soap and bleach comes from saponification of skin oils. Strong bases are arguably more dangerous than strong acids because they continue penetrating tissue.
5. Neutralisation
Acid + base → salt + water. HCl + NaOH → NaCl + H2O. The H⁺ from the acid combines with the OH⁻ from the base to form water; the remaining ions form a salt.
In stomach upset, antacids (mild bases like calcium carbonate or magnesium hydroxide) neutralise excess stomach acid (HCl). The product is a salt and water; the discomfort eases.
6. Strength versus concentration
Strong acids (HCl, H2SO4, HNO3) ionise nearly completely in water. Weak acids (acetic, citric) ionise only partially.
Strong bases (NaOH, KOH) similarly ionise completely. Strength refers to how completely the substance ionises; concentration refers to how much is dissolved. A dilute strong acid can be milder than a concentrated weak acid.
When to Choose Each
Choose Acid if:
- Industrial processes — pickling, etching, batteries.
- Biological roles — stomach acid for digestion, citric acid in metabolism.
- Chemical synthesis where proton donors are needed.
- Cleaning hard-water deposits and rust.
Choose Base if:
- Industrial soap and detergent manufacturing.
- Neutralising acid spills and stomach upset.
- Biological roles — bicarbonate buffering blood pH.
- Cleaning organic residue (oven grease, drains).
Worked example
Vinegar (a weak acid) and baking soda (a weak base) react together. Add a spoonful of baking soda to vinegar and the mixture fizzes — the carbonic acid intermediate breaks down to CO2 (the bubbles) and water. The remaining sodium acetate is a salt. The same reaction, scaled up, is the principle behind some homemade volcanos and many cleaning hacks.
Common Mistakes
- "Strong = concentrated." Different concepts. A strong acid is one that fully ionises; concentration is how much is dissolved.
- "All acids burn skin." Strong concentrated acids do; weak dilute acids (vinegar, lemon juice) don't — though they can sting cuts.
- "Bases are safer than acids." Often more dangerous to skin and eyes — strong bases are highly damaging because they keep penetrating tissue.
- "Pure water has no H⁺ ions." Pure water self-ionises slightly to H⁺ and OH⁻ in equal small amounts, giving pH 7.