Mystery Powders: An Introduction to Physical and Chemical Properties
In this classroom guided inquiry unit, students will complete various tests of five mystery powders to develop an understanding of physical and chemical properties. The five mystery powders include: baking soda, sugar, cornstarch, salt, and plaster of paris. Students will record their observations on their recording sheet throughout the lesson. Throughout the investigations, students will develop hypotheses as to what they think each mystery powder is and also what type of reaction each powder will have when mixed with different liquids, including water, vinegar, iodine and when heat is added. Students will draw a conclusions about each powder based on their investigations.
- Key Concepts:
- Students will learn that a physical change happens when some properties change (such as shape), but the material itself is the same before and after the change. The change can be undone.
- Students will learn that a chemical change occurs when a substance present at the beginning of the change are not present at the end; new substances are formed. The change cannot be undone.
Context for Use
Resource Type: Activities:Classroom Activity
Grade Level: Intermediate (3-5)
Description and Teaching Materials
3 pounds salt
5 pounds sugar
4 pounds baking soda
4 pounds cornstarch
5 pounds plaster of paris
Small clear cups
Wooden stir sticks
Mystery Powders Mixtures sheets
Science notebooks and pencils
Heat source: hot plate, candles, or Sterno
- Day 1: Physical Properties: Introduce Mystery Powders by telling students that over the course of the next two weeks they will become detectives. Explain they will be developing experiments in order to crack the case of the five mystery powders. Show students the unknown powders and let them speculate what they think they might be. Warn students of the dangers of tasting unknown substances. Today students will describe the powders' physical properties on the recording sheet by describing color, texture, and shape. Students will then develop a hypothesis of what they think each mystery powder is. Students will record their hypotheses in their Science notebooks. Have students share their hypotheses and ask students what tests they could do to find out about each powder.
- Day 2: Mixing with Water: Students will mix each of the mystery powders with water to discover what happens. They will record their observations on the recording sheet. Encourage students to try different amounts of water to see the difference in results. Share results of water tests. Results should include: baking soda turns a milky color and gets sticky, sugar dissolves, cornstarch turns to a soft solid, salt dissolves, and plaster absorbs water and hardens.
- Day 3: Testing with Heat: Now that students have become familiar with the powders, they can try a few more tests. To complete the heat tests I would set-up one teacher station, and students will record the results they observe on their recording sheet. I will use a hot plate, candles, or cans of Sterno. Put the powder in a little cup made from aluminum foil and hold it over the heat source with a wooden clothespin. Heat the powders for a few minutes, or until no more changes occur. If you like, you can demonstrate how caramel is made by melting sugar in a pan. When it turns brown pour it into cups to harden and the students can enjoy a candy treat! There will be no change for the baking soda and plaster of paris. The cornstarch will turn brown and smell like burnt toast. Sugar will melt, bubble, smoke, caramelize, turn black, and finally harden. The heat test is a good one to detect sugar.
- Day 4: Testing with Iodine: For the iodine tests, have students cover their desks with newspaper for easier clean-up. Have students put a little of each powder, some diluted iodine in a dropper bottle, and toothpicks for mixing. Students (in pairs) will then drop iodine on each of the mystery powders. They will record their reactions on the recording sheet. The results for the iodine testing include plaster turning a mustard yellow color, baking soda turns brown, cornstarch starts out red, then ends black, sugar turns purple, and salt turns multi-colored. There may be some disagreement about which powders change since only a small amount of starch is necessary to give a black color, some contamination may occur from mixing up the mixing sticks.
- Day 5: Testing with Vinegar: Just like with the iodine tests, distribute little cups with each of the powders. Have students add a few drops of vinegar to each cup and record their observations on the recording sheet. The results include: cornstarch thickens like glue, then hardens, baking soda fizzes, foams, bubbles, and makes noise, sugar partially dissolves, plaster bubbles, melts, hardens, and salt shows no change.
- Day 6 and 7: Before class, make up several mystery powder mixtures. Label them with the numbers of each of the powders included. Allow students to test mixtures using any of the tests from the previous week. One idea you could do the second day is have each partner group make up a mixture on their own, and exchange it with another group's to try to solve the mystery. One way students can test their results is by attempting to create a matching sample and conducting the same test on each until they find a match. Have students record their results on the Mystery Powders Mixtures sheet. To wrap-up the unit, have each group of detectives write their final conclusions in their science journals about each mystery powder (what they think it is and what they know about it).
This unit has been modified from Delta Education, Inc., Mystery Powders Teacher's Guide, Published by Delta Education, Inc., Nashua, NH, 1986. Recording Sheet for Mystery Powders Investigations (Microsoft Word 26kB Aug25 08) Recording Sheet for Mixing Powders (Microsoft Word 29kB Aug25 08)
Teaching Notes and Tips
- II.A.1 Structures of Matter
- The student will know that heating and cooling may cause changes to the properties of a substance.
- I.B.2 Scientific Inquiry
- The student will participate in a controlled scientific investigation. The student will collect, organize, analyze and present data from a controlled experiment.