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Dehydration of Hydrated Salt Mark as Favorite (4 Favorites)

LAB in Observations, Percent Composition, Quantitative Chemistry, Measurements, Chemistry Basics. Last updated November 02, 2022.

Summary

In this lab, students are introduced to chemical measurement in a hands-on investigation using a heat source and a hydrated compound. Students will determine the percentage water lost, by mass, from a hydrated compound during the heating process. Additionally, students will analyze and interpret their results in a claim, evidence, reasoning format.

Grade Level

High School

NGSS Alignment

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

  • Scientific and Engineering Practices:
    • Using Mathematics and Computational Thinking
    • Analyzing and Interpreting Data
    • Engaging in Argument from Evidence

Objectives

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

  • Explain the purpose of using cobalt chloride test strips.
  • Interpret the meaning of a color change when using cobalt chloride test strips.
  • Correctly collect mass measurements in a lab setting.
  • Determine a percentage by mass by analyzing collected data.
  • Write a conclusion using the claim, evidence, reasoning format.

Chemistry Topics

This lesson supports students’ understanding of:

  • Quantitative Chemistry
  • Percent Composition
  • Measurement
  • Observations

Time

Teacher Preparation: 30-45 minutes

Lesson: ~1 hour

Materials

  • Hot plate (Microwave oven, or Bunsen burner)
  • Crucible and Lid
  • Crucible Tongs
  • Spatula
  • Digital Balance (centigram or milligram)
  • Copper (II) Sulfate pentahydrate
  • Cobalt chloride test paper
  • Graduate Cylinder
  • Distilled Water
  • Forceps

Safety

  • Safety goggles and chemical-resistant lab aprons must be worn in this lab.
  • Use scoopulas or other transfer instruments; do not allow chemicals to touch skin or be ingested by nose or mouth.
  • Reference the SDS for Copper (II) Sulfate pentahydrate
  • If a white smoke is detected from the heating crucible, immediately discontinue heating, and inform the lab instructor.
  • Be aware that the crucible and lid will be extremely hot and can cause burns if touched. Only use the crucible tongs to transport the crucible and to remove the lid.
  • Wash your hands thoroughly before leaving the lab.
  • Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.

Teacher Notes

  • To learn more about this lab, read the associated article, Safely Introducing Students to the Chemistry Lab by Modifying a Classic Investigation, featured in the November 2022 issue of Chemistry Solutions.
  • This lab is intended to be implemented near the beginning of the school year, as an opportunity for students to observe a very simple but engaging chemical reaction that will help master some unfamiliar, but safe, skills.
  • Teacher preparation of chemicals, hot plates, crucibles, and lab balances should take 30-45 minutes; the lab itself should last 25-30 minutes, plus time for cool-down, clean-up and completion of the Analysis section.
  • Magnesium sulfate and calcium chloride hydrates deliver reproducible results, but both reactant and product are white, meaning that the only macroscopic change observable would be a small change in the crystalline form. Cobalt chloride gives a color change, (but it is much more expensive than the copper (II) sulfate) and, in my experience, can easily be overheated, producing chlorine gas. For these reasons, it’s suggested to use cobalt chloride test paper as part of the investigation, because students can make a further “discovery when they see that the paper changes visible color when exposed to water vapor or liquid water.
  • It is suggested that preparation for the investigation begin in a class meeting immediately prior to the lab, and should include a review of safety considerations, lab procedures, and the objective. The investigation should be presented as one of discovery, allowing students to offer questions as they read the lab procedure.
  • Teachers should demonstrate to students how to correctly use tongs to transfer the crucible and remind students that neither the crucible nor its lid should be directly touched after heating.
  • Note that in the “Materials” section of the student handout, the prefix “penta” for the hydrate has been omitted. This is intentional so that the composition of the hydrate isn’t inadvertently given to students who understand the meaning of the prefix.
  • This lab was designed to be used with a hot plate heat source, rather than a Bunsen burner with the goal of reducing the carbon footprint of the lab. Additionally, since it’s intended to be implemented early in the school year, a hot plate might be more appropriate for students new to the chemistry lab, rather than using a Bunsen burner. Alternatively, a microwave oven could be used for heating (~15 minutes), and a Bunsen burner (~5 minutes) to achieve the same results.
  • An Answer Key document is available for teacher reference.

For the Student

Background

As part of your introduction to measurement, you have the opportunity to observe quantitative aspects in a laboratory experience. Quantitative investigations involve questions such as “how much?” In this case, your focus will be “how much of this hydrated salt is lost when it is heated?”

Copper sulfate hydrate is a spectacularly blue salt that has been used to clear sewer lines of roots and other biological material. It is a hydrated salt, which means there is water trapped inside the salt crystals.

In your investigation, you have a single objective: determine what percentage of mass is lost from copper sulfate when it is heated at a temperature that should drive off the water, leaving a dehydrated copper sulfate.

In the process, you will gain experience measuring the mass of objects with a sensitive balance, and you will be able to compare your results with those of other students to see if any amount of water can combine with the salt to form the hydrate, or only a predictable amount can combine to form the hydrate.

Materials

  • Hot plate (Microwave oven, or Bunsen burner)
  • Crucible and Lid
  • Crucible Tongs
  • Spatula
  • Digital Balance (centigram or milligram)
  • Copper (II) Sulfate (hydrate)
  • Cobalt chloride test paper
  • Graduate Cylinder
  • Distilled Water
  • Forceps

Safety

  • Safety goggles and chemical-resistant lab aprons must be worn in this lab.
  • Use scoopulas or other transfer instruments; do not allow chemicals to touch your skin or be ingested by nose or mouth.
  • If a white smoke is detected from the heating crucible, immediately discontinue heating, and inform the lab instructor.
  • Be aware that the crucible and lid will be extremely hot and can cause burns if touched. Only use the crucible tongs to transport the crucible and to remove the lid.
  • Wash your hands thoroughly before leaving the lab.
  • Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.

Procedure

  1. Secure a clean crucible, with a lid. Place 1-3 mL of distilled water in it. Secure a dry piece of cobalt chloride test paper. Heat the water but do not boil it. When it is quite hot, probably after 2-3 minutes, use forceps to hold the test paper over the crucible in the vapor stream and record any color change. Make preliminary conclusions about your observation.
  1. Cool the crucible. Wash it out thoroughly, dry it as well as you can. Determine its mass (including the lid). Record it.
  1. Use careful transfer and massing techniques to place about 1.5 grams of the hydrated salt in the crucible. Then find the mass of the crucible with the lid and the hydrate. When adding or removing crystals, first remove the crucible from the balance. Don’t add chemicals to a vessel while it is on a balance.
  1. Heat the crucible, covered, with the hydrate in it on a hot plate for 10 minutes. Handle the crucible with crucible tongs only. At the halfway point of heating, carefully remove the lid and use forceps to hold the dry cobalt chloride test paper in the vapor stream. Note color changes, if any. Note any changes you see in the salt, carefully. Put the lid back using crucible tongs and finish the heating. Note any decrepitation (popping as steam breaks open the crystal) or other evidence of decomposition. As soon as the heating is finished, use the crucible tongs to transfer the crucible to the table to cool.
  1. Use the electronic balance to determine the mass of the crucible, lid and dehydrated salt combination. Record your result.

Data and Observations

Create a data table to record necessary information from the experiment as well as observations:

Analysis

  1. From your preliminary laboratory work with the cobalt chloride test strips, determine the most likely identity of the volatile substance that was driven from the salt by heating. Write your statement using a claim, evidence and reasoning format.
  1. Determine the following (show work where necessary):
    1. Mass of crucible, lid, and hydrated salt
    2. Mass of crucible, lid, and dehydrated salt
    3. Mass of hydrate alone
    4. Mass of dehydrated salt alone
    5. Mass of volatile substance driven off
    6. Percent mass of volatile substance in the salt
  1. Compare the percent of volatile substance in your crystal with the results from other lab groups. Can you make a general conclusion about the percent volatile substance in this hydrated salt?

Extension

  1. You determined a change of at least one property of the salt during the heating. What does that tell you about the structure of the salt and how the water affects that structure?