Summary

In this lab, students observe and describe the shape, size, and arrangement of salt crystals at various magnifications under a microscope and then compare the properties of each microscopically.

Grade Level

Middle School and High School

NGSS Alignment

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

• HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.
• Scientific and Engineering Practices:
  • Asking Questions and Defining Problems
  • Developing and Using Models
  • Analyzing and Interpreting Data
  • Obtaining, Evaluating, and Communicating Information

Objectives

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

• Understand the properties of different salts
• Observe and describe the shape, size, and arrangement of salt crystals under a microscope
• Compare and contrast the different types of salt crystals

Chemistry Topics

This lab supports students’ understanding of:

• Ionic Bonding
• Physical Properties
• Solubility
• Observations

Time

Teacher Preparation: 20 minutes
Lesson: 40–80 minutes(depending on student interest and enthusiasm)

Materials

• Either Stereo or Compound Microscopes (pairs of students may share if necessary)
• Microscope slides (5+ per group)
• Water (with eyedroppers)
• 2 small pieces of polarizing film per microscope
• Large collection of different types of salt in a shared area
  • Examples: table salt, colorful salts, Epsom salts, Rock salt, ionic compounds such as iron(III) chloride, cobalt(II) chloride or other salts with transition elements as cations work well. Colorful salts such as copper (II) chloride can be used as well.
• Optional: other non-ionic samples (such as sugar, sand, dust)

Safety

• Microscope safety should be reviewed (how to adjust without breaking slides, cleaning, etc.)
• Always wear safety goggles when handling chemicals in the lab.
  
• Wash your hands thoroughly before leaving the lab.
• Follow the teacher’s instructions for the cleanup of materials and disposal of chemicals.

Teacher Notes

• Start by introducing the concept of salts and their properties. Discuss how salts are formed and the different types of salts (examples: table salt, Epsom salt, rock salt, other salts with transition elements as cations, or colorful salts)
• New teachers may find background information about salts and their uses helpful.
• It’s suggested that teachers organize the salt samples in a shared location and allow students to choose from the samples available for their investigation. It’s suggested that students observe at least 3 different samples.
• Students should choose a sample and prepare the samples of salt for viewing by following the procedures on the student handout.
• Students will first observe a sample under the microscope by placing a small amount of the salt on a slide, without a cover slip or water at low magnification (40x-100x).
• They will record observations related to the shape, size, and arrangement of the crystals and/or take a picture by placing the camera lens from a phone over the eyepiece.
• Next students will increase the magnification to a higher level (100x-400x) Students will record their observations and/or take a picture of the salt crystals, paying attention to any new details about the sample that they can see.
• Finally, students will investigate the solubility of the salt by adding a drop of water to the crystals on the slide. It may be beneficial to have a partner place a drop while another partner is observing the microscopic image.
• Students will repeat the process for as many different salt samples as necessary (suggested minimum is at least 3 samples).
• It might be helpful for students to share slides with other groups as time allows.
• After completion, students should compare and contrast the different types of salt crystals. What differences do they notice in the size, shape, and arrangement of the crystals? Examples of microscopic images are shown below for teacher reference. Additionally, an answer key document is available for teachers to reference.