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Salt and the boiling temperature of water

Salt and the boiling temperature of water

Introduction: (Initial Observation)

At home hot water is used for cooking and heating systems such as hot water radiators. In laboratories hot water is used in water baths. Hot water has many industrial applications as well.

One problem with water is that it never gets hotter than 100º Celsius. Any additional heat will only cause more evaporation. Being able to control or modify the boiling point of water may be helpful for any applications requiring heat transfer.

In this project you will study the effect of table salt on the boiling temperature of water. Report your results in a table and draw a graph to visually display your results.


If you have any questions, click on the “Ask Question” button at the top of this page to send me your questions. I may respond by email, but often I update this page with the information that you need.

You will also need to know:

Project Advisor

Information Gathering:

Find out about boiling and boiling point point. Read books, magazines or ask professionals who might know in order to learn about the effect of salt on the boiling point of water. Keep track of where you got your information from.

matter: anything that occupies space and has mass.

mass: the quantity of matter contained by an object. Mass is measured in terms of the force required to change the speed or direction of its movement.

liquid: the state in which matter takes the shape of its container, assumes a horizontal upper surface, and has a fairly definite volume.

boiling point: the temperature at which the vapor pressure of a liquid equals the pressure of the gas above it.

temperature: measure of the hotness or coldness of a body.

pressure: force exerted on a unit area. The SI unit of pressure is the Pascal (Pa).

gas: the state in which matter has neither definite volume nor shape.

boiling-point elevation: the elevation of the boiling point of a liquid by addition of a solute.

Effect of air pressure

A liquid boils when its vapor pressure becomes equal to atmospheric pressure. Low atmospheric pressure causes the boiling point to go down; high pressure drives it up. Atmospheric pressure varies a bit from day to day, depending on the weather, and it varies from place to place, depending on the altitude.

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

The purpose of this project is to determine the effect of salt on the boiling point of water.

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

  • The independent variable (also known as manipulated variable) is the amount of salt.
  • Dependent variable (also known as responding variable) is the boiling point of water.
  • Controlled variables are the air temperature and pressure. Perform all your experiments in the same day while the air pressure and temperature will not be subject to noticeable changes.


Based on your gathered information, make an educated guess about the effect of salt on boiling point of water.

Following are two sample hypothesis:

Sample hypothesis 1:

I hypothesize that salt will reduce the boiling point of water. My hypothesis is based on my information that salt reduce the freezing point of water and it is used as an anti freeze in winter.

Sample hypothesis 2:

I hypothesize that salt will increase the boiling point of water. My hypothesis is based on my information that salt does not boil as easy as water, so when mixed with water it may make it hard for water to boil as well.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”

Introduction: In this experiment you will test the effect of table salt (sodium chloride) on the boiling point of water. You may repeat this experiment with other solutes such as sugar, Epsom salt (Magnesium sulfate) and Salt cake (Sodium sulfate). Experiment involve preparing salt-water solutions with different amounts of salt; heat them to the boiling temperature and then measure and record the temperature while the solution is boiling.


  • Fill up a glass beaker or a small pot with 100 ml distilled water.
  • Place a thermometer in the water several centimeters from the bottom of the pot. Make sure you are using a thermometer with at least one degree markings to insure accurate measurements.
  • Begin to heat the water. Take temperature readings every 10 seconds.
  • Continue reading the temperature until it remains constant for at least four measurements. This is the boiling point.
  • Repeat the steps 1 to 4; however, each time add a different amount of salt to the water. Suggested amounts of salt are 5, 10, 15, 20 and 25 grams as shown in the following table.



Your results table may look like this:

grams of salt added to
the 100 ml of water
Boiling Temperature of the salt-water solution








  • Use tap water or drinking water if you don’t have access to the distilled water
  • If your pot or beaker are big and you need to do your experiment with more water, increase the amount of salt at the same ratio.
  • C = Celsius Temperature Scale (Centigrade)
  • F = Fahrenheit
  • If you don’t have a scale to weight 5 grams salt, use one small tea spoon. That will hold approximately 5 grams of salt.
  • The first experiment with pure water is also the control for your other experiments.
  • 102ºC and 106ºC in the above table are possible answers reported by other students. Please note that they may be wrong or inaccurate!

Make a bar graph:

For each of the six solutions that you test make a vertical bar (so your graph will have 6 vertical bars). The height of each bar will represent the boiling temperature of one specific solution. The name of the bar will be the amount of salt added.

The bar graph in the right is for a similar experiment with only 3 different solutions. 0 is for no salt, 1 is for 1 table spoon and 2 is for 2 table spoon salt in one quart of water.




Materials and Equipment:

This is a sample list of material:

  1. Thermometer* (available at science suppliers),
  2. salt,
  3. water,
  4. Glass beakers or metal pots,
  5. Electric stove (hotplate)

* Glass and dial thermometers shown above are available at MiniScience.com and klk.com. Either of the two models may be used for freezing temperatures. Dial thermometers last longer; however, glass thermometers are more accurate.

Results of Experiment (Observation):

The above table will be completed and used as the result of your experiment. You may also write in a paragraph or two the result. What you write may be an answer to the following questions:

1. What was the highest temperature that the salt water reached?

2. At what temperature does the pure water boil?

If the thermometer extends beyond the outside of the pot it reads a higher temperature. Heat from the stove burner makes the thermometer read higher. Keep the thermometer over the pot when making temperature measurements.


No calculation is required

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Using the trends in your experimental data and your experimental observations, try to describe the effect of salt on freezing point of water. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.


Visit your local library and find books about salt, water, general chemistry, physical chemistry or chemical physics. Look for chapters that discuss changes in physical properties of a substance when mixed with other substances.

List the books and the online resources that you use in this part of your report.