Acid Rain Experiments – Experiment 2 – Determining pH of Common Substances

In this experiment you will use a pH indicator to measure the pH of some fruits, common beverages, and borax. Borax is a cleaning agent that some people add to their laundry detergent. It is available at grocery stores. Many foods and household cleaners are either acids or bases. Acids usually taste sour, and bases bitter. Household cleaners are poisons so you should never taste them.

Materials

pH paper and color chart (range pH 2 to 12) or garden soil pH testing kit
3 fresh whole fruits (lemon, lime, orange, or melon)
3 beverages (cola, carbonated non-cola, milk)
1/8 teaspoon borax
measuring spoons (1/4 and 1/8 teaspoons)
4 small, clear cups or glasses
1 clean stirring spoon
notebook and pencil
paring knife

Instructions

Cut each fruit in half, drying off the knife after each cut.
Place an unused strip of pH paper half-on and half-off the inside of the cut fruit. Leave until wet (about 2 seconds). Immediately compare with the color chart. Write down the approximate pH value of the fruit. (If using a garden soil pH tester kit, squeeze 1/4 teaspoon of juice from the cut fruit into the test container, and add 1/4 teaspoon of the test solution. Cover the test container and shake once or twice to mix, or stir if necessary. Compare with the color chart provided in the kit, and record the result.)
Repeat the same process for the other 2 fruits.
Label the 3 cups: one cola, another non-cola, and the third milk.
Pour each liquid into an appropriately labeled cup.
Dip an unused strip of pH paper into the cola, compare with the color chart, and record the result. Repeat the same process for the remaining beverages. Be sure to use a clean, unused strip of pH paper for each one. (If using a garden soil pH tester kit, pour 1/4 teaspoon of cola into the test container, and add 1/4 teaspoon of the test solution. Tightly press your finger over the top of the test container and shake once or twice to mix, or stir if necessary. Compare with the color chart provided in the kit, and record the result.)
Add 1/8 teaspoon borax to 1/4 cup distilled water and stir for about 2 minutes. Dip an unused strip of pH paper in the borax mixture, compare with the color chart, and record the result. (If using a garden soil pH tester kit, pour 1/4 teaspoon of the borax/water mixture into the test container, and add 1/4 teaspoon of the test solution. Tightly press your finger over the top of the test container and gently shake, or stir if necessary. Compare with the color chart provided in the kit and record the result.)

Questions and Answers

Are lemons, limes and oranges acids or bases?

These fruits all contain acids and taste sour. Lemons and limes have pH values near 2. Oranges may be slightly less acidic than lemons and limes, but your pH indicator may not be accurate enough to show the difference.

Are colas and non-colas acids or bases?

They are both acidic, primarily because they both contain carbon dioxide to make them fizz, and carbon dioxide and water produce carbonic acid. The pH of these beverages varies with the amount of carbon dioxide and other ingredients in them, but it is usually below 4.

Was the milk acidic or basic?

Milk can be slightly basic or slightly acidic depending on its age and how it was processed at the dairy.

Was the borax/water mixture acidic or basic?

Borax contains a strong base and will turn most pH indicators blue. The approximate pH of the borax/water mixture is 9. Its alkaline properties make it an excellent cleaning agent, which is why some people use it to wash clothes.