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Changing Water's Boiling Point Mark as Favorite (8 Favorites)

LAB in Physical Properties, Concentration, Colligative Properties, Boiling Point, Accuracy, Graphing, Molality, Error analysis, Boiling point elevation. Last updated May 30, 2017.

Summary

In this lab, students will explore colligative properties in a quantitative approach.

Grade Level

High school

Objectives

By the end of this lesson, students will

  • Understand that adding a solute to water changes the boiling point of water.
  • Know that depending on how much solute is added, the boiling point is effected differently.

Chemistry Topics

This lesson supports students’ understanding of

  • Colligative properties
  • Solutions
  • Boiling point
  • Molarity

Time

Teacher Preparation: 15 minutes

Lesson: 90 minutes

Materials

For each group:

  • 500-mL beakers (5)
  • NaCl (noniodized)
  • Distilled water
  • Hot plate or Bunsen burner
  • Thermometer or temperature probe
  • 250-mL beaker
  • Beaker label or grease pen
  • Weighing boat
  • Stirring rod

Safety

  • Always wear safety goggles when working in the lab.
  • Always be aware of an open flame. Do not reach over it, tie back hair, and secure lose clothing.
  • 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 lab works with a small percent error.
  • It is a cheap and relatively quick experiment.
  • You can also shorten this by assigning different solutions to different "groups" if time is an issue.
  • Students can rinse the salt water down the sink.
  • Student Activity A requires the students to complete calculations and figure out the procedures they will follow to prepare the various NaCl molality solutions required in the experiment.
  • Student Activity B is a more traditional “cookbook” lab that provides the calculations for your students.
  • Student Activity C has incorporated NGSS-based science and engineering practices.
  • Find the Boiling simulation from Purdue used in part C here

For the Student

Lesson

Purpose
To determine how adding a solute to water will affect the boiling point of water.

Safety

  • Always wear safety goggles when working in the lab.
  • Always be aware of an open flame. Do not reach over it, tie back hair, and secure lose clothing.
  • 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.

Procedure

  1. Make at least 300 mL of the following NaCl solutions: 0 m, 0.5 m, 1.0 m, 1.5 m, and 2.0 m solution. Make sure you are using non-iodized NaCl and distilled water.
  2. Record the boiling point of a 100-mL sample of each solution three times.
  3. Determine the change in boiling point using the table below.
  4. Clean up your workspace.

Results


Sample

0 m

0.5 m

1.0 m

1.5 m

2.0 m

1

2

3

Average

Concentration

Average BP (°C)

Change in BP (°C)

0.00 m

0.25 m

0.50 m

0.75 m

1.00 m

Analysis

  1. Graph your data and use a proportional fit for the data.
  2. Determine water’s KB value from your graph.
    a. Take the slope of the line (A) and divide it by the number of particles that are in NaCl. This is your experimental KB.
    b. You can do this because the equation to get the new boiling point for a liquid is: ∆TB =imKB
    c. Since the change in boiling point is your y variable and the molality is your x, that leaves iKB as the slope: A = iKB
  1. Calculate the percent error knowing that the correct value for water’s KB is 0.512 °C/m.

Resource updated on 7-22-16 by AACT.