A discussion of acidity and basicity (or alkalinity) is a good topic for a beginning home chemistry lesson. Acids have a characteristic sour taste and smell. In chemical terms, they are described as emitting hydrogen (H+) ions. Bases, at the other end of the pH spectrum, have an acrid, bitter taste and smell and are defined chemically as emitting hydroxide (OH-) ions. Rader's Chem4Kids.com has a good explanation of ions and ion emission.
The Acid-Base Continuum
The acidity or basicity of a solution is expressed as a pH score (pH stands for "potential of hydrogen"). Acids test low, as a pH of 0-7, with 0 being the most acidic. Bases test high, as a pH of 7-14, with 14 being the most basic. Water is neutral, at 7.
We began this discussion referring to the taste and smell of acids and bases. But as a rule, DON'T TASTE OR SMELL a solution for testing purposes. Some household chemicals, such as ammonia, are toxic and even smelling them deeply could make you swoon. Others, like lemon juice, are safe to ingest, but very unpleasant. Litmus paper is one of the oldest methods used to test the acidity or basicity of a solution. Blue litmus paper turns red when put in an acidic solution. Red litmus paper turns blue when put in a basic solution. Neutral litmus paper is purple and stays that way when you put it in water, which is neutral.
Common Household Acidic Solutions
Vinegar and lemon juice are the classic household solutions used in school chemistry lessons for testing acids. Other citrus fruits such as grapefruit, limes and oranges are also acidic. Wine is acidic, as are most carbonated beverages.
Common Household Basic Solutions
Pour baking soda into water and you have a basic solution. Ammonia and washing soda are also bases, as is hydrogen peroxide. Milk of magnesia, a medicine for easing stomach acid, is a base. So is household chlorine bleach. (But don't try to test it with litmus paper. It will just turn it white.) Blood is slightly basic, so most meat products are, too.
Mixing or Neutralizing Acids and Bases
If you mix a strong acid such as vinegar with a strong base such as baking soda, they will react rather explosively. (They form the main ingredients for homemade "volcanoes.") The resulting compound will be a salt, which is neutral. This reactive quality of acids and bases is one reason why cleaning supplies often carry a warning not to mix solutions--because if you mix acidic and basic solutions, the result can produce toxic fumes.