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Changing Water's Boiling Point (17 Favorites)

LAB in Physical Properties, Concentration, Colligative Properties, Boiling Point, Accuracy, Graphing, Molality, Boiling Point Elevation, Error Analysis. Last updated June 22, 2022.


Summary

In this lab, students will explore colligative properties in a quantitative approach. They will measure the effect of increasing the molality of a salt solution on the solution’s boiling point, and they will graph their data and use the slope of the line of best fit to calculate the boiling point elevation constant of water. There are three versions of the lab, some of which provide more structure and guidance and others of which are more student-driven.

Grade Level

High School

NGSS Alignment

This lab will help prepare your students to meet the following scientific and engineering practices:

  • Scientific and Engineering Practices:
    • Asking Questions and Defining Problems
    • Using Mathematics and Computational Thinking
    • Analyzing and Interpreting Data
    • Planning and Carrying Out Investigations

Objectives

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

  • Experimentally determine the boiling point elevation constant of water.
  • Evaluate the accuracy of their experimental value against the accepted value.
  • Explain how adding different amounts of solute to water changes the boiling point of water.
  • Design an experiment to measure the effect of the concentration of a solute on a solution’s boiling point. (Student Activity C only)

Chemistry Topics

This lab supports students’ understanding of:

  • Colligative properties
  • Solutions
  • Boiling point
  • Molarity
  • Experimental Design (Student Activity C)

Time

Teacher Preparation: 15 minutes

Lesson: 60–120 minutes (depending on which version of the lab you do)

Materials

For each group:

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

Safety

  • Always wear safety goggles when working in the lab.
  • Exercise caution when using a heat source. Hot plates should be turned off and unplugged as soon as they are no longer needed.
  • 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

  • Students will be graphing their experimental data and using the slope of the line of best fit to calculate the boiling point elevation constant for water. Be sure students are familiar with proper graphing techniques. The AACT Graphing Simulation could be a useful resource to review before doing this lab.
  • Be sure to use pure NaCl (i.e. don’t use iodized salt that you might find at the grocery store), since the molar mass (needed to calculate molality) and the i value won’t be quite the same as if it were pure NaCl, and it will skew the calculations.
  • This lab works with a relatively small percent error.
  • This is a cheap and relatively quick experiment, and it is easy to prep and clean. Students can just dispose of the salt water down the sink when they are done and clean any equipment they used with soap and water.
  • Be sure that students are aware that the “change in boiling point” calculation compares the boiling point of each salt solution to the boiling point of pure water (the 0 m “solution”).
  • There is some information about boiling point elevation included in the background information, but you may want to introduce students to the concept of molality and the equation for boiling point elevation and its variables before performing this lab.
  • Students are instructed to measure the boiling point of 5 solutions of different concentrations. To have more data points, you could have all the groups report their data for each concentration to share with the class, which would limit the effect of outliers on their calculations. Alternatively, you could have each group do multiple trials for each concentration (repeating steps 1-3 in Student Activity A, or steps 1-6 in Student Activity B). In that case, be sure to increase the amount of solution each group is instructed to prepare, allow more time for repeated trials, and add lines to the data table for each trial and an average.
  • You can also shorten this by assigning one concentration to each group and have students share data if time is an issue, though this allows more room for error with fewer data points. Another way to save time would be to prepare the solutions for students in advance of the lab period, though this puts more of the burden on the teacher and does not give students the same visual understanding of how much salt is in each solution if they are not preparing them themselves.
  • Student Activity A requires the students to complete calculations in the prelab questions to help them determine how to prepare the various NaCl molality solutions required in the experiment. You may want to check the prelab questions or review them as a class before students start preparing their solutions. To increase the challenge further, you could remove the explicit instructions for how to use the line of best fit to calculate KB.
  • Student Activity B is a more traditional “cookbook” lab that provides the correct amounts of salt for each molality for your students.
  • Student Activity C has incorporated NGSS-based science and engineering practices and requires students to design and execute the experiment themselves with limited guidance. Students will need to write up a lab report – if they have not done full lab write ups before, you will want to review the requirements with them (included at the end of Student Activity C), and perhaps show them a sample lab report. For more information about introducing students to formal lab reports, you may wish to consult the following resources:
  • Since Student Activity C is a lot more student-centered and has students posing a question and designing procedures to answer that question, it will take substantially longer than the other versions of this lab. Designing an experiment can be very challenging for students, as many of them have never had to do it before, which makes it a very valuable experience for them to practice this important scientific skill.
  • Be sure that you approve students’ procedures before they start conducting their investigation if you use Student Activity C. Make sure they are taking the appropriate safety precautions. Note that you should primarily be checking for safety – as long as they are not completely off-base, it’s ok if there are minor flaws in their experimental design! They will learn more from making the mistake and having to fix it than from you pointing it out!
  • For all versions of the student activity, students are required to graph their data and use a line of best fit to calculate the KB value for water. They will get the best results if they use a digital temperature probe (rather than a thermometer) to collect more accurate data and Excel or a similar graphing program to create their graphs, as it will generate a line of best fit for them. They can also use the “Set Intercept” function to set the y-intercept as 0, since they should anticipate that 0 g of salt will change the boiling point by 0°C, as that is the baseline. This will make it so the equation for the line of best fit has a y-intercept of 0 and is in the form y = mx.

For the Student A

Background

Solutions are homogeneous mixtures of solute and solvent. The solvent is the most abundant substance in a solution. In a liquid solution, the solvent does the dissolving. The solute is the other substance in a solution. In a liquid solution, the solute is dissolved. It is possible for a solution to have more than one solute, air is an example, but a solution can have only one solvent.

The boiling point of a substance is defined as the temperature at which a liquid becomes a gas. When a substance boils, the molecules gain enough energy to “break free” of the other molecules and escape as a gas. Dissolving a solute in a solvent increases the boiling point. This boiling point elevation is a colligative property, which means that it is dependent only the amount, not type, of solute that is dissolved in the solvent. Keep in mind that when ionic solids are dissolved in water they break up into ions. So NaCl will break up into two ions, MgCl2 will break up into three ions, and AlCl3 breaks up into four ions. The number of ions that the substance breaks up into is referred to as the Van’t Hoff factor. The equation for calculating boiling point elevation is ∆TB =imKB, where TB is the change in the boiling point (°C), i is the Van’t Hoff factor, m is the concentration (molality, m), and KB is the boiling point elevation constant for the solvent (°C/m).

Prelab Questions

  1. Define molality.
  2. Calculate the masses of NaCl and water (room temperature water has a density of about 0.998 g/mL) you would need to prepare 100 mL of each NaCl solution at the following molalities:
    1. 0 m NaCl
    2. 0.5 m NaCl
    3. 1.0 m NaCl
    4. 1.5 m NaCl
    5. 2.0 m NaCl
  1. How will you know when a solution has reached its boiling point?

Objective

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

Materials

  • 250-mL beakers (5)
  • NaCl (non-iodized)
  • Distilled water
  • Hot plate (or Bunsen burner and ring stand) to heat solutions to boiling
  • Tongs or hot hands to move hot beakers
  • Thermometer or temperature probe
  • Beaker label or grease pen
  • Weighing boat
  • Balance
  • Stirring rod

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Exercise caution when using a heat source. Hot plates should be turned off and unplugged as soon as they are no longer needed.
  • Wash your hands thoroughly before leaving the lab.
  • Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.

Procedure

  1. Prepare 100 mL of each of the following NaCl solutions using your calculations from the prelab questions: 0 m, 0.5 m, 1.0 m, 1.5 m, and 2.0 m. Make sure you are using non-iodized NaCl and distilled water.
  2. Heat each solution until it boils and record the boiling point.
  3. Determine the change in boiling point for each concentration of the solution and record it the table below.
  4. Clean up your work station. Salt water solutions can be poured down the drain. Thoroughly wash and dry all equipment.

Results

Concentration (m)

0.0 m

0.5 m

1.0 m

1.5 m

2.0 m

Boiling Point (°C)






Change in B.P. (°C)






Analysis

  1. Use your data to graph the relationship between concentration (x) and change in boiling point (y). You may use a program such as Excel to create your graph or you may neatly draw it on graph paper. Attach a copy of your graph when you turn in your lab.
  2. Determine the equation of the best fit line from your graph.
  3. Determine water’s KB value from your graph:
    1. Recall the equation for boiling point elevation: ∆TB =imKB
    2. Since the change in boiling point is your y variable and the molality is your x, that leaves iKB as the slope. Call the slope A: ∆TB/m = iKB = A
    3. Take the slope of the line and divide it by the Van’t Hoff factor, i, to determine your experimental KB: KB = A/i . Show your calculations in the space below.
  1. Calculate the percent error compared to the accepted value for water’s KB, 0.512 °C/m.
  2. How do you think the temperature change would be affected if you prepared solutions of CaBr2 (of the same molalities) instead of NaCl? Why?

Conclusion

Write 1–2 paragraphs responding to the following questions to summarize what you have learned in this lab:

  • Make a general statement about what happens to the boiling point of a solution as concentration increases, citing data from your experiment as evidence.
  • Research and find an application of this statement. Cite your sources.
  • Give one possible lab error that would have resulted in the percent error calculated in question 4.

For the Student B

Background

Solutions are homogeneous mixtures of solute and solvent. The solvent is the most abundant substance in a solution. In a liquid solution, the solvent does the dissolving. The solute is the other substance in a solution. In a liquid solution, the solute is dissolved. It is possible for a solution to have more than one solute, air is an example, but a solution can have only one solvent.

The boiling point of a substance is defined as the temperature at which a liquid becomes a gas. When a substance boils, the molecules gain enough energy to “break free” of the other molecules and escape as a gas. Dissolving a solute in a solvent increases the boiling point. This boiling point elevation is a colligative property, which means that it is dependent only the amount, not type, of solute that is dissolved in the solvent. Keep in mind that when ionic solids are dissolved in water they break up into ions. So NaCl will break up into two ions, MgCl2 will break up into three ions, and AlCl3 breaks up into four ions. The number of ions that the substance breaks up into is referred to as the Van’t Hoff factor. The equation for calculating boiling point elevation is ∆TB =imKB, where TB is the change in the boiling point (°C), i is the Van’t Hoff factor, m is the concentration (molality, m), and KB is the boiling point elevation constant for the solvent (°C/m).

Prelab Questions

  1. Define molality.
  2. How will you know when a solution has reached its boiling point?

Objective

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

Materials

  • 250-mL beakers (5)
  • NaCl (non-iodized)
  • Distilled water
  • Hot plate (or Bunsen burner and ring stand) to heat solutions to boiling
  • Tongs or hot hands to move hot beakers
  • Thermometer or temperature probe
  • Beaker label or grease pen
  • Weighing boat
  • Balance
  • Stirring rod

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Exercise caution when using a heat source. Hot plates should be turned off and unplugged as soon as they are no longer needed.
  • Wash your hands thoroughly before leaving the lab.
  • Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.

Procedure

  1. Make the following sodium chloride solutions by thoroughly mixing 99.8 g distilled water with the amount of non-iodized NaCl listed below in a clean, dry 250 mL beaker. Label each beaker with the appropriate molality:
    1. 0.0 m solution: 99.8 g distilled water + 0 g NaCl
    2. 0.5 m solution: 99.8 g distilled water + 2.91 g NaCl
    3. 1.0 m solution: 99.8 g distilled water + 5.83 g NaCl
    4. 1.5 m solution: 99.8 g distilled water + 8.74 g NaCl
    5. 2.0 m solution: 99.8 g distilled water + 11.7 g NaCl
  1. Use a hot plate to heat the 0.0 m sample until it comes to a rolling boil. (Recall that the temperature will stay constant once the water reaches its boiling point.)
  2. Use a temperature probe or thermometer to measure the boiling point of the solution. Be sure that the probe or thermometer does not touch the bottom or sides of the beaker. Record the boiling point in the table below.
  3. Determine the change in boiling point for each concentration of the solution and record it the table below.
  4. Using tongs or hot hands, remove the beaker from the hot plate and set it aside to cool. (Remember that hot glass and cold glass look the same, so be careful!)
  5. Repeat steps 2–5 for the remaining four solutions (0.5 m, 1.0 m, 1.5 m, 2.0 m).
  6. Clean up your work station. Salt water solutions can be poured down the drain. Thoroughly wash and dry all equipment.

Results

Concentration (m)

0.0 m

0.5 m

1.0 m

1.5 m

2.0 m

Boiling Point (°C)






Change in B.P. (°C)






Analysis

  1. Use your data to graph the relationship between concentration (x) and change in boiling point (y). You may use a program such as Excel to create your graph or you may neatly draw it on graph paper. Attach a copy of your graph when you turn in your lab.
  2. Determine the equation of the best fit line from your graph.
  3. Determine water’s KB value from your graph:
    1. Recall the equation for boiling point elevation: ∆TB =imKB
    2. Since the change in boiling point is your y variable and the molality is your x, that leaves iKB as the slope. Call the slope A: ∆TB/m = iKB = A
    3. Take the slope of the line and divide it by the Van’t Hoff factor, i, to determine your experimental KB: KB = A/i . Show your calculations in the space below.
  1. Calculate the percent error compared to the accepted value for water’s KB, 0.512 °C/m.
  2. How do you think the temperature change would be affected if you prepared solutions of CaBr2 (of the same molalities) instead of NaCl? Why?

Conclusion

Write 1–2 paragraphs responding to the following questions to summarize what you have learned in this lab:

  • Make a general statement about what happens to the boiling point of a solution as concentration increases, citing data from your experiment as evidence.
  • Research and find an application of this statement. Cite your sources.
  • Give one possible lab error that would have resulted in the percent error calculated in question 4.

For the Student C

Background

In this investigation, you will collect evidence that will show you the effect that solution concentration has on its boiling point. The boiling point of a substance is defined as the temperature at which a liquid becomes a gas. A substance boils when the molecules have enough energy to overcome the vapor pressure of the surrounding air and change from a liquid to a gas. When a substance boils, the molecules gain enough energy to “break free” of the other molecules and escape as a gas. Keep in mind that substances with higher intermolecular forces boil at a higher temperature than those with lower intermolecular forces. To read more about this process and see a simulation, go to: http://www.chem.purdue.edu/gchelp/liquids/boil.html.

Solutions are homogeneous mixtures of a solute and solvent. Dissolving a solute in a solvent increases the boiling point. In this investigation the solvent will be water and the solute will be sodium chloride (NaCl). When you add salt to water, sodium chloride dissociates into sodium and chloride ions. These charged ions interfere with the intermolecular forces between water molecules. Since the water molecule is a dipole, the oxygen is more negative and is attracted by the positive sodium ions. The hydrogen side is more positive and is attracted to the chloride ions.

Boiling point elevation is a colligative property, which means that it is dependent only the amount, not type, of solute that is dissolved in the solvent. Keep in mind that when ionic solids are dissolved in water they break up into ions. So NaCl will break up into two ions, MgCl2 will break up into three ions, and AlCl3 breaks up into four ions. The number of ions that the substance breaks up into is referred to as the Van’t Hoff factor. The equation for calculating boiling point elevation is , where DTB is the change in the boiling point (°C), i is the Van’t Hoff factor, m is the concentration (molality, m), and KB is the boiling point elevation constant for the solvent (°C/m).



Asking Questions

After you view the simulation above and read through the information, develop one or more scientific questions about boiling point elevation that you can answer in this investigation.

Preparing to Investigate

Before beginning the investigation, thoroughly review the rest of this document, including the outline on the last page for guidance on the lab report that you will submit after completing your investigation.

Once you have identified a question you will investigate, develop procedures that you will follow to gather data about the effect that solution concentration has on its boiling point. Make a note about safety concerns that are relevant for your procedures. Be sure to include a list of materials and equipment you will need. You must have your teacher approve your procedures before you begin. Additionally, think about the observations you will make and the data that you will need to collect during each part of the investigation and prepare appropriate data and observation tables.

Gathering Evidence

  • Wear goggles and a lab apron throughout the investigation.
  • When using a temperature probe, be sure that it does not touch the bottom or side of the container.
  • Hot and cold glassware look alike. Take caution when using glassware that has been heated.
  • Pour all liquid waste in the waste beaker located in the back of the lab.
  • Since you will be looking at the effect of the concentration of a solution on its boiling point, you should be sure to collect data about the boiling point of pure water.
  • Prepare at least three different NaCl solutions, each with a different molality.
  • Decide how many times you need to repeat each trial.

Analyzing Evidence

  1. What was the molality of each of the solutions you prepared? Please show all support calculations.
  2. What was the average change in the boiling point (°C) for each of the solutions when compared to the boiling point of pure water?

Interpreting Evidence

  1. Use your data to graph the relationship between concentration (x) and change in boiling point (y).
  2. Determine the equation of the best fit line from your graph.
  3. Use the equation of your best fit line and the equation  to calculate the boiling point elevation constant (KB) of water. Include units.
  4. Calculate the percent error compared to the accepted value for water’s KB, 0.512 °C/m.

Making Claims

  1. Make a general statement about what happens to the boiling point of a solution as concentration increases, citing data from your experiment as evidence.
  2. Research an everyday application of this statement. Cite your sources.
  3. In your own words and using evidence from the investigation, answer the scientific question(s) that you posed at the beginning of the investigation.

Reflecting on the Investigation

  1. Think about your experimental design for the investigation. Did you need to make any modifications so that your data produced accurate results? Explain.
  2. Give one possible lab error that would have resulted in the percent error calculated in question 6.
  3. Suppose that a classmate suggests that the concentration of a solution must also affect its freezing point. Describe a procedure that you would use to test this hypothesis.



Lab Report

You will write up a lab report describing what you did in your investigation and what you learned from it. Your report will need to include everything someone would need if they wanted to replicate your experiment, as well as an analysis of your data. You need to include the following sections:

Background

Include 1–2 paragraphs of background information relevant to the investigation you have planned. This information may come from external research, class notes, your textbook, etc. and should include brief explanations of important terms and concepts.

Problem/Objective

Write a brief scientific question (problem) or statement (objective) of what you plan to investigate in this experiment.

Materials

Make a bulleted list of the chemicals, glassware, and other equipment needed to carry out your investigation.

Safety

Make note of any safety precautions you will need to observe.

Procedure

List the step-by-step instructions that would allow another person to repeat your experiment.

Data

Create a data table to record all your data and observations in an organized manner.

Calculations

Show any calculations that you do with your data, including any graphs. This section should address the questions in the “Analyzing Evidence” and “Interpreting Evidence” sections above. Be sure that your work is well organized and easy to follow, and that you include units where needed.

Analysis

Address the questions in the “Making Claims” section above, and include any other relevant analysis of your data that you’d like to include. This should be one of the longer written sections of your report (several paragraphs), describing how you are interpreting the data you collected and the calculations you performed. You should cite specific data/calculations to support each claim you make in this section.

Conclusion

Address the questions in the “Reflecting on the Investigation” section above. This should be a couple of paragraphs reflecting on the larger themes of how you designed the experiment and how you could improve or expand on it in the future, what the overall results were, and how you answered the scientific question (from the “Problem/Objective” section) you set out to investigate. You can also discuss implications this experiment may have for future research.