Acids and bases
- An acid produces hydrogen ions, H+, in water. Acids taste sour and react with many metals.
- A base produces hydroxide ions, OH−, in water. Bases taste bitter and feel slippery.
HCl → H+ + Cl−
NaOH → Na+ + OH−
A broader definition covers more cases: an acid is a proton donor and a base is a proton acceptor. A hydrogen ion is just a proton. Ammonia, NH3, has no OH in its formula but is a base because it accepts H+ from water.
In water, H+ attaches to a water molecule to form the hydronium ion, H3O+. The two symbols are used interchangeably.
- A strong acid or base breaks apart completely into ions. HCl, HNO3 and H2SO4 are strong acids. NaOH and KOH are strong bases.
- A weak acid or base ionizes only slightly. Acetic acid (in vinegar) and ammonia are weak.
Worked example: classify HNO3 and Ca(OH)2
HNO3 releases H+ in water, so it is an acid. Ca(OH)2 releases OH−, so it is a base.
Common mistake
Confusing strong with concentrated. Strength is about how completely the substance ionizes. Concentration is about how much is dissolved. A dilute solution of a strong acid and a concentrated solution of a weak acid are both possible.