Acid vs Base
Acid
In the Brønsted–Lowry definition, an acid is a substance that donates a hydrogen ion (a proton, H+) to another substance in solution. Aqueous acids have pH values below 7. Common laboratory examples include hydrochloric acid and sulfuric acid.
Example: Hydrochloric acid, HCl: it donates its proton to water to form hydronium ions (H3O+), which is why stomach acid and many cleaners are acidic.
Base
In the Brønsted–Lowry definition, a base is a substance that accepts a hydrogen ion (a proton, H+) from another substance. Aqueous bases have pH values above 7. Common examples include sodium hydroxide and ammonia.
Example: Sodium hydroxide, NaOH: its hydroxide ion (OH-) readily accepts protons, which is why it is used in drain cleaners and soap making.
Key differences
| Aspect | Acid | Base |
|---|---|---|
| Brønsted–Lowry role | Donates a proton (H+) to another species. | Accepts a proton (H+) from another species. |
| pH of aqueous solution | Below 7 (the lower the number, the more acidic). | Above 7 (the higher the number, the more basic or alkaline). |
| Taste and feel | Sour taste (citric acid in lemons); acids do not feel slippery. | Bitter taste and slippery feel (soap); bases turn litmus paper blue. |
| Reaction with indicators | Turns blue litmus paper red; phenolphthalein stays colorless. | Turns red litmus paper blue; phenolphthalein turns pink. |
| What they produce in water | Increases the concentration of H3O+ (hydronium) ions in solution. | Increases the concentration of OH- (hydroxide) ions in solution. |
| Neutralization product | An acid neutralized by a base produces water plus a salt. | A base neutralized by an acid produces the same water-plus-salt products. |
The same substance can act as an acid in one reaction and a base in another — water does this, which is why it is called amphiprotic. Also, 'acid vs base' describes a role in a proton-transfer reaction, not a permanent label: whether something donates or accepts a proton depends on what it is reacting with. Finally, strength is separate from concentration: a dilute strong acid can have the same pH as a concentrated weak acid.