Identifying Unknown Substances Physical Science Lab
Students will test four unknown substances (baking powder, baking soda, cornstarch and powdered sugar) and determine the identity of the substances based on how they react with iodine and vinegar.
When iodine is added, there are two possible outcomes:
- the substance will react with the iodine and turn blue/black
- the substance will not react with the iodine, and the iodine will simply stain it brown
When vinegar is added, there are two possible outcomes:
- the substance will react with the vinegar and produce gas bubbles
- the substance will not react with the vinegar, and will simply become wet and dissolve
Students will determine the identity of the substances based on the following information:
Baking powder: vinegar causes bubbles, iodine turns blue/black
Baking soda: vinegar causes bubbles, iodine does not react
Cornstarch: vinegar does not cause bubbles, iodine turns blue/black
Powdered sugar: vinegar does not cause bubbles, iodine does not react
Ice cube trays
Plastic or paper cups
Dropper bottles with lids, or small containers and eye droppers
Label 8 sections of each ice cube tray as follows: Iodine 1, Iodine 2, Iodine 3, Iodine 4, and Vinegar 1, Vinegar 2, Vinegar 3, Vinegar 4
Ahead of time, place the baking powder, baking soda, cornstarch and powdered sugar into plastic or paper cups and label them Unknown 1, Unknown 2, Unknown 3, and Unknown 4. Make a key for which unknown is which substance, but keep the identities hidden from the students. Keep the unknowns at the front of the classroom and have a student from each group come up and place about half a teaspoon of each unknown into the specified section of the ice cube tray. Students should have two samples of each unknown in the ice cube tray, one to test with vinegar and one to test with iodine.
Caution students to take care not to spill or mix up the samples, and do not taste any samples under any circumstances.
Supply each group with a dropper bottle of iodine and a dropper bottle of vinegar.
Students will add 2-3 drops of vinegar to one sample of each unknown and record the results as "bubbles" or "no bubbles."
Students will add 2-3 drops of iodine to the other sample of each unknown and record the results as "turned black" or "did not turn black."
Caution students to be careful not to spill the iodine, it may stain clothing.
When students have completed the experiment and recorded their results, have them rinse out and dry the ice cube trays.
Results and Conclusion
Based on the observed data, students will identify the four unknown mystery substances.
Vinegar bubbles and Iodine turned black = baking powder
Vinegar bubbles and Iodine did not turn black = baking soda
Vinegar no bubbles and Iodine turned black = cornstarch
Vinegar no bubbles and Iodine did not turn black = powdered sugar
Have students answer the following question:
Why was it necessary to use more than one chemical reaction to identify the unknowns?
Answer: One reaction does not give us enough information to differentiate between all four unknowns; we won't be able to tell the substances apart because more than one will have similar results.