Nearly everything around is an acid or a base, with the exception of water. Water is neither acidic nor basic, but rather neutral. Acidic substances tend to taste sour, for example, citrus fruits like lemons, oranges and limes are all acidic and have a slightly sour taste. Bases on the other hand, taste bitter and tend to feel slippery. Bases are often used as cleaning products, such as soaps and detergents. (Just because we’re saying acids taste sour and bases taste bitter, doesn’t mean you should go test that theory. Don’t go tasting any cleaning products to see if this holds up.)

The pH scale was developed to measure how acidic or basic a substance is. This scale measures pH from 0-14. Acids have pH values between 0 and 7 and bases have pH values between 7 and 14. Pure water is neutral and has a pH of exactly 7.

Indicators are used to measure the pH of a substance. Often times, plant extracts will change color if exposed to acidic or basic conditions and can therefore be used to indicate if a substance is an acid or a base. Today’s experiment focuses on the use of litmus paper, a most basic pH paper dating back many years. Litmus is extracted from a specific lichen to form a litmus powder, which is then used in a chemical solution to treat paper, cut into strips, and ta-da! Litmus paper is made. We have three different types of litmus paper: blue, red and neutral.

Blue litmus paper will turn red in acidic solutions, and remains blue in alkaline solutions. Red litmus paper turns blue in alkaline solutions, and remains red in acidic solutions. Neutral litmus paper turns red in acidic solutions and turns blue in alkaline solutions. At neutral pH, none of the litmus papers change color.

measuring pH with litmus paper, litmus paper, litmus test paper

So let’s start off with a few questions:

  1. What color will blue litmus paper turn if it is dipped into a solution with a pH of 3?
  2. What color will blue litmus paper turn if dipped into a solution with a pH of 9?
  3. What color will red litmus paper turn if dipped into a solution with a pH of 2?
  4. What color will red litmus paper turn if dipped into a solution with a pH of 10?
  5. What color will neutral litmus paper turn if dipped into a solution with a pH of 4?
  6. What color will neutral litmus paper turn if dipped into a solution with a pH of 11?

Now, let’s test several substances using red and blue litmus paper.

Materials

  • Red litmus paper
  • Blue litmus paper
  • Stir rods
  • Small beakers
  • Lemon juice
  • Orange juice
  • Aspirin
  • Shampoo
  • Tap water
  • Bar of soap
  • Vinegar
  • Dish soap
  • Liquid hand cleaner
  • Window cleaner

Objectives & Procedure

Our objective is to classify each substance being tested as an acid or a base. Start by stating a hypothesis for each substance on whether it will be acidic, basic, or neutral. Record all of your findings in your lab notebook, using the chart below as a guide.

Substance being Tested Prediction

(Acid, Base, Neutral)

Red Litmus Test Result Blue Litmus Test Result Is it an acid or a base?

measuring pH with litmus paper, litmus paper, litmus test paper, pH test

  1. If the sample is solid, such as the aspirin, crush and mix it with water until it dissolves to make a liquid solution. Stir each solution with a different stir rod to ensure it is completely mixed together.
  2. Remove a piece of litmus test paper from the vial
  3. Holding one end of the litmus paper, dip the strip into a solution and remove. If a color change is to occur, it will happen immediately.
  4. Note the color change, if any, and record in your notebook.

Data Analysis

Answer the following questions after you have completed the lab.

  • What factors did you base your predictions on?
  • Were there any samples that did not change color on both the blue and red litmus papers? If so, what does this say about that sample’s pH?
  • Would you always be able to use just one of the litmus papers to determine whether an unknown substance was an acid, base, or neutral? Why or why not?

Going Further

If you mix an acid and a base, the result is a solution that is more neutral. This means that if you mix a substance with a pH of 3 and a substance with a pH of 9, then the resulting solution will have a pH closer to 7. Heartburn is caused when acid from the stomach rises up into the esophagus, which is the tube leading from the mouth to the stomach. To relieve the discomfort of heartburn people will consume an antacid like Tums. Design and carry out an experiment to test the pH of Tums and its effect on the pH of a known acidic solution when they are mixed.

You can use all of the materials listed above to carry out this additional experiment. Design a procedure, and carry it out to complete the investigation. Record your observations and discuss whether or not they were what you expected.