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Chemical Indicators

based on 19 ratings
Author: Cy Ashley Webb

Grade Level: 4th - 8th; Type: Chemistry

Objective:

The goal of this experiment is to make different chemical indicators and use them to identify solutions containing vitamin C, starch or that have a high pH. Students will consider how analytical chemists identify the properties of matter.

Research Questions:

  • What substances can be used to identify starches?
  • Vitamin C? Basic solutions?
  • What other types of chemical indicators do chemists routinely use?
  • What properties of matter are useful in identifying the matter?
  • What is Benedict’s solution? Bromophenol blue? How are they used as chemical indicators?

Chemical indicators are substances that are used to identify a property of matter or to identify the material itself. Common chemical indicators are Benedict’s solution, which indicates the presence of some sugars, and bromophenol blue, which is used to measure pH. In this experiment students will make their own chemical indicators to identify basic solutions and test for the presence of starches and vitamin C.

Materials:

Most ingredients are probably available at home already. Tincture of iodine is available in good drug stores. Potato starch is available in the kosher food section of some grocery stores.

  • Lemon juice
  • Tincture of iodine
  • Cornstarch or potato starch
  • Purple cabbage
  • Beakers (or empty mason jars)
  • Jars with lids (clean spaghetti sauce jars or large apple sauce jars are good)
  • Teaspoons
  • Notebook and pencil
  • Measuring cup

Experimental Procedure:

  1. Measure two teaspoons of corn starch or potato starch and put them in a one quart mason jar. Measure one quarter of water. Add 2 tablespoons of water that you just measured and stir until it is a thick paste. Dilute this paste with 2 more tablespoons of water and stir. Dilute the paste one more time with 2 tablespoons of the water and stir. Add the rest of the water and stir well and cover. Using masking tape, label this bottle “stock starch solution.” You will be able to detect the presence of starch with one of your indicators.
  2. Measure one quart of water and put into a one-quart mason jar. Using masking tape, label this bottle “stock water solution.”
  3. Measure one quart of water and put into a one-quart mason jar. Add 4 tablespoons of bottled lemon juice. Using masking tape, label this bottle “stock lemon juice solution.” Lemon juice contains vitamin C. You will be able to detect vitamin C using one of your indicators.
  4. Measure one quart of water and put into a one-quart mason jar. Add 4 tablespoons of sodium bicarbonate (baking soda). Using masking tape, label this bottle “stock bicarb solution.” Sodium bicarbonate is mildly basic. You will be able to detect basic solutions using one of your indicators.
  5. Chop four or five large red cabbage leaves into small pieces and put into a small saucepan. Cover with 1½ cups water and simmer for twenty minutes. Transfer the colored water into a jar. Label the jar “Base indicator (blue).
  6. Pour one cup of the stock starch solution into a small bottle such as a small jelly jar. Add 10 drops of tincture of iodine. If the resulting solution is not dark blue, continue adding the tincture of iodine drop by drop until it is a dark blue. Label this container “Vitamin C indicator.”
  7. The tincture of iodine can be used straight out of the bottle as a starch indicator.
  8. Test out the indicators that you made in Part II. Using one your stock solutions, put two tablespoons of liquid from each stock solution into a baby food jar or small beaker. Repeat so that you have four baby food jars with two tablespoons of liquid, each containing a different stock solution. These are your test samples.
  9. Put one teaspoon of the vitamin C indicator into each of the four samples. What did you observe? Does the indicator remain blue or does it become completely clear? Create a table showing what you observed in each of four different test samples.
  10. Repeat step 8 so that you have a new set of test samples from a different stock solution. Put one teaspoon of the base indicator into each of the four samples. What did you observe? Does the indicator remain reddish or does it turn blue? Add your observations to the chart you created. 
  11. Repeat step 8. Put 2-3 drops of the tincture of iodine solution into each test sample. What did you observe? Does the sample remain clear, or perhaps slightly yellow or does it turn dark blue? Add your observations to the chart you created.

Terms/Concepts: Chemical indicator; pH; Acid and base 

References:

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