Summary

In this lab, students will identify an unknown substance as the dangerous chemical X or a common household product. Students will perform tests on the physical and chemical properties of different substances to determine the identity of the unknown.

Grade Level

Middle school

NGSS Alignment

This lab will help prepare your students to meet the performance expectations in the following standards:

• MS-PS1-2. Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
• Scientific and Engineering Practices
  • Planning and Carrying Out Investigations
  • Constructing Explanations

Objectives

By the end of this lab, students should be able to

• Identify and differentiate between physical and chemical properties of matter.

Chemistry Topics

This lab supports students’ understanding of

• Physical change
• Chemical change

Time

Teacher Preparation: 1 hour

Lesson: 50 minutes

Materials

For each group:

Unknown sample (see identities in teacher notes)

• Cabbage juice for pH test
• Biuret reagent (1% strong base followed by CuSO₄ (aq))
• Vinegar
• Iodine

Safety

• Always wear safety goggles when working in a chemistry lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.

Teacher Notes

• This activity is engaging, quick, inexpensive, and requires students to use deductive reasoning to solve a problem. I use it after a similar lab that uses only physical properties to identify an unknown. It’s a nice parallel to draw because in this lab students identify substances using physical and chemical properties.
• Students should understand that reactivity and flammability are independent of the size of sample.
• Unknown identities:
  • A: Baking Soda
  • B: Corn starch (Chemical X)
  • C: Salt
  • D: Gelatin
  • E: Baking powder

Suggested lab implementation:

• Engage: To engage students, ask them to list some examples of physical change. Sample answers: tearing paper, crumbling cookies, melting ice. Also ask students to list some examples of chemical change. Sample answers: sugar caramelizing, vinegar and baking soda reacting to produce a gas, iron rusting.
• Explore: Students complete the lab activity as describe din the procedures of the student handout.
• Explain: Enforce the understanding that the cabbage juice reacts with acids and bases differently but does not react with neutral substances.
• The biuret solution will react with certain substances and turn purple. If it stays blue there was no reaction since color change is a sign of a chemical change.
• The vinegar produces bubbles when it reacts. The bubbles are a sign of a chemical reaction.
• The iodine turns black when it reacts with certain substances.
• Extend: Dalton's Atomic Theory Research: In 1808, John Dalton proposed what is now known as the law of conservation of mass. Students can research Dalton and his career. Task them with making a timeline to highlight his achievements.