Acid: a solution with an excess of H+ ions and pH below 7.
- Acids taste sour
- Acids are sticky
- Acids often react with metals
- Acids are good solvents, especially for dissolving salts
- Acids turn litmus paper or universal indicator paper red.
- Many biological molecules are acids, including fatty acids, amino acids, some vitamins
- Acids have many important industrial applications, including as catalysts and solvents.
Some Acids Found in Every Day Life
- HCl (hydrochloric acid): a strong acid that is involved in digestion
- H2SO4 (sulfuric acid): a strong acid used widely in industry and in car batteries
- HC2H3O2 (acetic acid): found in vinegar, adds flavor to many foods
- H3PO4 (phosphoric acid): strong acid, added to soda for flavor
Notice that these formulas start with H. This is because the cation (positive ion) in each of these ionic compounds is H+.
Base: a solution with an excess of OH– (hydroxide) ions and pH above 7.
- Bases taste bitter.
- Bases are slippery.
- Bases turn litmus paper or universal indicator paper blue.
Some Bases Found in Every Day Life
- NH3 (ammonia): used as a fertilizer, household cleaner, and food disinfectant. Also, many other nitrogen compounds are made from ammonia in industry.
- NaOH (sodium hydroxide, or lye, or caustic soda): used as a strong cleaner and also in many industrial applications, such as in aluminum production.
- Ca(OH)2 (calcium hydroxide, or slaked lime): used in water treatment (flocculation), used in food processing, used in industry
- Mg(OH)2 (magnesium hydroxide): used as an antacid (milk of magnesia); also used as a smoke suppressent / fire retardant.
Notice that some of these formulas include a metal and hydroxide (OH–) ions. That combination gives a basic solution, because it results in excess OH– ions in solution. Some bases, such as NH3, do not have (OH) in their formula, but instead they react with water molecules to form OH– ions when they are dissolved in a solution.
pH: scale used to measure acid-base behavior. Neutral solutions have a pH of 7, acids are less than 7, and bases are greater than 7.
Each number on the pH scale is 10 times more acidic or basic than the next number on the scale. For example:
•pH=3 is 10 times more acidic than pH=4
•pH=2 is 100 times more acidic than pH=4
•pH = 11 is 1000 times more basic than pH = 8.
Water – Acid – Base
Normally, acids and bases only exist in liquid water solutions. For example, H2SO4 doesn’t behave as an acid when it’s all by itself as a solid. When it’s mixed with water, however, its hydrogen ions separate, and then we have excess H+ ions in solution, which means the solution is an acid.
Water molecules can split to form the ions for both acid and base:
H2O -> H+ + OH–
The reverse of this reaction shows that if you combine acid and base, they neutralize each other and form water molecules:
H+ + OH– –> H2O