an adult
lemon juice
red and blue litmus paper (obtain from a science teacher or a science supply company) blackberries bowl dinner fork strainer cups or glasses red cabbage non-aluminum pot and cover
Place a drop of lemon juice on your tongue. Do you think lemon juice is an acid or a base? Why?
To test your hypothesis about lemon juice, dip pieces of red and blue litmus paper into the liquid. What do you find?
Litmus paper is not the only way to test for acids and bases. There are many acid-base indicators. You can easily make your own.
Put five or six blackberries in a bowl. Crush them thor- oughly with a fork until you have something resembling blackberry jam. Then pour the “jam” into a strainer and col- lect the dark red juice in a cup or glass. Save any juice you do not use here. You will use it in the next experiment to make indicator sticks.
110
water
stove
tongs
container with lid
refrigerator
white vinegar
glass jars or test tubes
eyedropper
household ammonia
substances for testing: apple juice, grapefruit
juice, rubbing alcohol, powdered cleanser, salt and sugar solutions, aspirin, wood ashes, baking soda, baking powder, lime (calcium oxide), citric acid or Kool-Aid or Tang drink mix crystals, milk of magnesia, washing soda, ginger ale, tonic water, seltzer water, pickle juice, olive juice
You can also use red cabbage juice as an acid-base indicator. Remove a few leaves from a red cabbage. Break the leaves into small pieces and place them in a non-aluminum pot together with enough water to just cover the leaves. Put a cover on the pot and
ask an adult to help you heat the pot until the water is boiling. Reduce the heat, but continue boiling for about half an
hour. Then turn off the heat and let the water cool to room tem- perature. Using tongs, remove the cabbage leaves. Pour the cabbage juice solution into a container. Cover it and put it in a refrigerator.
Vinegar is a solution of acetic acid (C2H4O2), which, as you probably know, has a sour taste. To confirm that vinegar is an acid, dip a piece of blue litmus paper into a few milliliters of white vinegar. How does this help you identify vinegar as an acid?
Now add a few drops of the cabbage extract to the white vinegar in a small glass jar or test tube. What is the color of the cabbage juice indicator in an acid?
When ammonia gas (NH3) dissolves in water, some of it reacts with the water to form ammonium (NH4+) and hydrox- ide ions (OH−), as shown by the chemical equation below:
NH3 + H2O ➝ NH4+ +OH−
To confirm that an ammonia solution is basic, dip a piece of red litmus paper into a few milliliters of a household ammonia solution. How can you tell that the ammonia solution is a base?
Next, add a few drops of the cabbage juice to a few milliliters of ammonia. What is the color of the cabbage juice indicator in a base?
Add a few drops of the indicator to some tap water. What is the color of the indicator in a neutral solution?
Repeat the experiment using the berry juice extract. What is the color of the berry juice indicator in an acid? What is the color of the berry juice indicator in a base? In a neutral solution?
Using litmus paper, berry juice, and cabbage juice, test the following substances to determine whether each is an acid, a base, or neutral: lemon juice, apple juice, grapefruit juice, rub- bing alcohol, powdered cleanser (in water), salt and sugar solutions, crushed aspirin dissolved in water, wood ashes mixed with water, solutions of baking soda and baking powder, lime (calcium oxide) in water, citric acid or Kool-Aid or Tang drink mix crystals dissolved in water, milk of magnesia, washing soda dissolved in water, ginger ale, tonic water, seltzer water, pickle juice, and the juice from a jar of olives.
Which of these substances are acids? Which are bases? Which are neutral?
Which natural indicator juice, berry or cabbage, do you prefer? Why do you prefer one over the other?
G Turmeric, a common spice, can also be used as an
acid-base indicator. Prepare an extract of turmeric by mixing 1/4 teaspoon of turmeric with a 1/4 cup of rub-
bing alcohol. Add a few drops of the turmeric indicator to acids, bases, and neutral substances. How does it compare with other indicators you have tried?
G Investigate other common indicators that can be found in
many science rooms or obtained from a science supply company. These include phenolphthalein, methyl orange, methyl red, bromthymol blue, congo red, indigo carmine, and alizarin yellow. Other than color, how do they differ?
G Read the label on a bottle of vitamin C tablets. Crush one
of the tablets into a powder and dissolve it in water. (Depending on the type of tablet, the mixture may be cloudy because some of the ingredients may not be sol- uble.) Divide the solution into three parts. Predict the color of litmus paper, drops of cabbage juice, and drops of berry juice when added to a solution of vitamin C.
G Prepare a cup of hot tea. Then add a few drops of lemon
juice to the tea. What evidence do you have to suggest that tea is an acid-base indicator?
Experiment 5.3