Montshire Minute: Acids and Bases

Originally aired during the week of August 23, 1999

Monday

O.K. Mr. Science person, today we're going to test liquids for acididity. Your first job is to stick this juicy slice of lemon in your mouth. Then, wash your mouth out with soap. Not interested? Well, in the past scientists have used taste as one way to distinguish acids from bases. The English chemist Robert Boyle established the following "taste criteria" for acids and bases: Acids taste sour and bases or "alkalies" (like those found in soap) feel slippery on the tongue. The name "acid" comes from the Latin word acidus meaning "sour." Citric acid is what makes lemons tart and sour. Alkalies in soap are more commonly known as bases because they are the "base" for making certain salts. There's some pretty interesting chemistry involved with acids and bases, too. Acids and bases can neutralize each other: when combined, acids lose their acidity and bases become less "alkaline" or basic.

Tuesday

Some neutralization reactions are just delicious. Don't believe me? As Tina Seelig writes in The Epicurean Laboratory, cooks are really doing acid-base chemistry when they bake. Let's say you're whipping up a batch of pancakes for breakfast. When you mix an acidic ingredient like buttermilk with a basic ingredient like baking soda, a neutralization occurs, producing carbon dioxide, gas, and water. The carbon dioxide bubbles expand when the batter is poured on a hot griddle, making the batter rise. Leavening agents like baking soda get most of the credit for making pancakes rise, but you actually need an acidic ingredient like buttermilk, yogurt, or even honey to make the carbon dioxide bubbles. In recipes that lack acidic ingredients, baking powder is used rather than baking soda. Baking powder is baking soda with a acid-like cream of tartar-already built in.

Wednesday

Scientists use the pH scale to tell if a liquid is more of an acid or more of a base, just as we use the Fahrenheit or Celsius scale to measure temperature. The range of the pH scale is one to fourteen from very acidic to very basic. Carbonated beverages like soda are more acidic with a pH of 3 while detergents are more basic with a pH around 10 and 11. Pure water is a neutral solution-it has a pH of 7. You can also use indicator paper or "litmus paper" to estimate the pH of a solution. The paper is made with litmus, an organic dye that turns different colors. A very acidic solution is red, a very basic solution is blue, and a neutral solution is green. By dipping strips of litmus paper in different solutions (say, lemonade or milk), the color of the strip tells if you're dealing with an strong acid, base, or something in between.

Thursday

Most people don't keep pH meters sitting around the house the way we do thermometers to tell us the temperature. But it's pretty easy to make your own color pH indicator. Indicators are organic dyes that change color when an acid or base is added. To make your own indicator just follow this recipe: Chop up a red cabbage and put it in a saucepan. Add enough water to cover the cabbage and boil it for 20 minutes. After cooling the boiled mixture for about 30 minutes, remove the cabbage and keep the dark liquid. The dark liquid will be your indicator. You can test this "cabbage-juice indicator" by pouring a 1/4 teaspoon of an acid (like vinegar or lemon juice) into a container and adding a spoonful of the indicator. Repeat the process with a 1/4 teaspoon of a base (say baking soda) in a separate container. What happens?

Friday

After eating a big dinner, you might take an antacid tablet with sodium bicarbonate in it to settle your upset stomach. The base in the antacid tablet neutralizes some of the acid in your stomach and should make you feel better. With the proper lab equipment, scientists can find the best brand of antacid by doing what's called a titration. First, a known amount of antacid is dissolved in water with a few drops of acid indicator. More acid is is added a little bit at a time, until there is slight color change due in the acid indicator. This indicates that all the sodium bicarbonate in the antacid has been neutralized. From now on, the solution will become more acidic with each additional drop of acid, and the color change will become more intense. A scientist can repeat this experiment with different antacids and determine which one neutralizes the most acid.