Acid-base reactions involve the transfer of protons from one substance to another.

An acid is defined as a proton (H+ ion) donor; a base is therefore a proton (H+ ion) acceptor.

Acids in solution will have pH less than 7; bases in solution will have pH greater than 7.

The value of 7 for the neutrality of the pH scale is derived from the auto-ionization of water:

H2O(l) --> H+(aq) + OH-(aq)

Where the molar concentration of H+ is 1.0 x 10-7 M at 25 oC.

When converted to the log base-10 scale, the pH of pure water at this temperature is 7.0.


See the pH scale for more information.

There are substances that can act as both acids and bases (like water) - these are called amphoteric substances.


Common Acids

Here are some common acids and their formulas:
  • hydrochloric acid, HCl
  • nitric acid, HNO3
  • sulfuric acid, H2SO4
  • phosphoric acid, H3PO4
  • carbonic acid, H2CO3
  • acetic acid, HC2H3O2
  • citric acid, H3C6H5O7

Note that all of their chemical formulas begin with H's.

The general rule of thumb when naming an acid is that if the polyatomic ion ends in "ate" (as in, you ate an acid), the name of its acid ends in "ic acid" (as in, acids taste sour so you'd say ick! acid!).

The polyatomic ions that end with "ite" will have acids ending in "ous acid". The nitrite ion (NO2-) would be nitrous acid (HNO2).


Common Bases:

The most common bases contain the hydroxide ion (OH-). There are other basic polyatomic ions, however.
  • Lye, or caustic soda, is actually sodium hydroxide, NaOH.
  • Bleach contains sodium hypochlorite, a basic compound.
  • Ammonia (NH3) is also basic in nature. Its acidic counterpart is the ammonium ion (NH4+).
  • Lime (powdered limestone) is a base, and is made of calcium carbonate.
  • Baking soda is also basic (sodium bicarbonate).