Acids vs. Bases
In chemistry, we can classify many substances into two important categories: acids and bases. They are essentially chemical opposites.
Properties of Acids
An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. This is what gives them their characteristic properties:
Taste: They taste sour (though you should NEVER taste a chemical in the lab!). Think of the taste of lemons (citric acid) or vinegar (acetic acid).
Feel: Strong acids can cause a burning sensation.
Reactions: They react with most metals to produce hydrogen gas. They also react with bases to neutralize them.
Indicators: They turn blue litmus paper red.
Properties of Bases
A base (also called an alkali) is a substance that produces hydroxide ions (OH⁻) when dissolved in water.
Taste: They taste bitter.
Feel: They feel slippery or soapy. Soap and many cleaning products are bases.
Reactions: They react with acids to neutralize them.
Indicators: They turn red litmus paper blue.
The pH Scale
How do we measure how acidic or basic a substance is? We use the pH scale. The pH scale is a measure of the concentration of hydrogen ions (H⁺) in a solution.
The scale runs from 0 to 14.
Acids have a pH less than 7. The lower the number, the stronger the acid. A pH of 1 is a very strong acid.
Bases have a pH greater than 7. The higher the number, the stronger the base. A pH of 14 is a very strong base.
A substance with a pH of exactly 7 is neutral. It is neither acidic nor basic. Pure water is a perfect example of a neutral substance.
It's important to know that the pH scale is logarithmic. This means that a change of 1 on the scale is actually a 10-fold change in acidity. For example, a solution with a pH of 3 is 10 times more acidic than a solution with a pH of 4.
Indicators
An indicator is a special chemical that changes color in the presence of an acid or a base. Indicators allow us to visually determine the approximate pH of a solution.
Litmus Paper: This is a common indicator. Blue litmus paper turns red in an acid. Red litmus paper turns blue in a base.
Universal Indicator: This is a mixture of several indicators that shows a whole spectrum of colors, allowing you to estimate the specific pH value, not just whether it's an acid or a base. For example, it might turn bright red in a strong acid, green in a neutral solution, and dark purple in a strong base.