Identify an Unknown Chemical Mixture

Overview

Students will learn about the scientific method and will explore chemical reactions. Initially, this activity allows students to use the scientific method to examine and identify a set of (nontoxic) unknown substances. Once the characteristics of these substances are known, the students can use the information to drawinference to identify unknown mixtures of these materials.

Time Required: 3 hours or three one-hour sessions

Grade Level: 5-7

Objectives

To practice using the scientific method. To learn how to record observations and apply the information to perform more complex tasks.

Materials

Each group will need:

• plastic cups
• magnifying glass
• 4 unknown powders in 4 plastic baggies:
  • sugar
  • salt
  • baking soda
  • corn starch

For the entire class:

• water
• vinegar
• heat source
• iodine solution

  Activities

  Remind students that they should never taste an unknown substance. Review the steps of the scientific method. Although the unknown powders are similar in appearance, each substance has characteristic properties that make it distinguishable from the other powders. Explain how the students can use their senses to examine the powders and record properties. Have them use sight (magnifying glass), touch, and smell to examine each powder. Observations should be written down. Students may be asked to predict the identity of the powders. Introduce heat, water, vinegar, and iodine. Explain the concepts chemical reactions and chemical change. A chemical reaction takes place when new products are made from the reactants. Signs of a reaction could include bubbling, temperature change, color change, smoke, or change in odor. You may wish to demonstrate how to mix chemicals, apply heat, or add indicator. If desired, use containers with labeled volume measurements to introduce students to the importance of recording quantities used in a scientific investigation. Students can put a specified amount of powder from the baggie into a cup (e.g., 2 scoops), then add vinegar or water or indicator. Cups and hands are to be washed between 'experiments'. Make a chart with the following:

• What was the appearance of each powder?
• What happened when water was added to each powder?
• What happened when vinegar was added to each powder?
• Did all powders produce the same response?
• What happened when iodine solution was added to each powder?
• Why do you think this happened?
• If you predicted the identity of the powders, were your predictions correct? If not, how were they different?
• What are true identities of the mystery powders A-D?
• How did you determine the correct answer?

  Now, give the students a mystery powder made up using at least two of the four pure subtances. They are to test this mixture using the procedures they used on the pure substances. In addition, they may wish to design new experiments.

  Assessment

  Students may be evaluated on their ability to correctly identify the final unknown mixture. Points may be awarded for teamwork, staying on task, submission of data or a lab report, and ability to follow directions and follow safety rules.