All liquids can be categorized as either acids or bases depending on their pH, which is a measure of a substance's acidity on the pH scale. The pH scale ranges from 0 to 14. Anything below 7 is acidic, anything above 7 is basic and 7 is neutral. The lower a substance's measure on the pH scale, the more acidic it is, and the higher the more basic it is. All substances are measured in comparison to distilled water, which has a neutral pH of 7.
An acid is a substance that has a measure of below 7 on the pH scale. The Arrhenius definition of an acid is a compound that contains hydrogen and can dissolve in water to release hydrogen ions into solution; therefore, acids are proton donors that increase the concentration of hydronium ions in solution.
The strength of an acid is measured by how easily the acid gives off a positive hydrogen atom, or proton, in water. The easier an acid dissociates and gives off a proton in water, the stronger the acid.
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Acids are corrosive to metals, sour to taste and turn litmus paper red. Common acids include hydrochloric acid, which is used in the digestion of food and has a pH of 1; vinegar, which has a pH of 2.9; and milk, which has a pH of 6.6.
A base is chemical that measures above 7 on the pH scale. The Arrhenius definition of a base is a compound that dissolves in water to release hydroxide ions (OH-) in solution. Bases are proton acceptors that increase the concentration of hydroxide ions and decrease the concentration of hydronium ions in solution. Bases turn litmus paper blue, and they are slippery like soap. Common bases include liquid drain cleaner, which has a pH of 14; milk of magnesium, which has a pH of 10.5; and baking soda, which has a pH of 8.4.