Salt and the freezing point of water

Introduction: (Initial Observation)

In winter time we spread salt in the roads and side walks to melt snow and ice or to prevent ice. How does salt melt ice or snow? Do other material do the same? Is salt creating heat to melt the snow? Is it changing the freezing point of water so water does not freeze at 0ºC (32º F)?

In this project we will perform an experiment to see if the salt has any affect on the freezing point of water.

Information Gathering:

Find out about freezing and freezing point. Read books, magazines or ask professionals who might know in order to learn about the effect of salt on the freezing point of water. Keep track of where you got your information from.
freezing point: the  temperature at which the solid and liquid phases of a substance are in equilibrium.

http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/meltpt.html

Changes in the physical properties of solutions caused by solutes is known as the colligative properties of solutions. Change in freezing point is one of the colligative properties. Wikipedia, the free online encyclopedia has a good description for how solutes affect the properties of solutions. See the following link for more details:

http://en.wikipedia.org/wiki/Colligative_properties

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 freezing 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.

This is how you may define the variables:

• The independent variable (also known as manipulated variable) is the amount of salt in water. The amount of salt in water may be reported as the percent of salt in water or as grams of salt per liter of water.
• The dependent variable (also known as responding variable) is the freezing point of water. Freezing point is the temperature in which water will freeze.
• Constants are the amount of water in each cup.
• Controlled variable is not needed because your experiment trials are performed at the same time or under the same environmental conditions.

Hypothesis:

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

This is a sample hypothesis:

Since salt is used to de-ice roads in the winter, then it probably reduces the freezing point because it keeps ice from forming.

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.”

Experiment 1: Test the effect of table salt (sodium chloride) on the freezing point of water.

Introduction: In this experiment we will make different solutions of table salt with different concentrations and test to see at what temperature they freeze.

Procedure:

1. Fill up 5 cups with 100 ml of water each. For the best results, you may use distilled water for this experiment. distilled water can be purchased from pharmacies. If you don’t have access to distilled water, use regular drinking water instead.
2. Number the cups from 1 to 5 and label them.
3. Do not add any salt to cup number 1. That will be your control.
4. Add 5 grams of salt to the cup number 2 and stir it.
5. Add 10 grams of salt to the cup number 3 and stir it.
6. Add 15 grams of salt to the cup number 4 and stir it.
7. Add 20 grams of salt to the cup number 5 and stir it.
8. Place one thermometer in each cup.
9. Place all the cups in the freezer and set your freezer to the coolest possible temperature.
10. Inspect the condition of each cup (Frozen or not) and record the temperature every 10 minutes. The temperature you record for each cup is the temperature in which the water is partially frozen.
11. Record the results in a table like this:

Notes:

• L – 12ºC is an example of recorded data and means the sample is still Liquid and it’s temperature is 12 degrees centigrade.
• F – 0ºC means the sample is partially or fully Frozen and the temperature is zero degree centigrade.
• If you don’t have 5 thermometers, use one to check the temperature of each cup. To avoid transferring salt from one cup to the other, you will need to clean the thermometer with water and paper towel after measuring the temperature of each cup.
• For these experiments you will need a thermometer that can show as low as -20ºC or 0ºF.
• C = Celsius Temperature Scale (Centigrade)
• F = Fahrenheit
• If you don’t have a scale to weight 5 grams s

Materials and Equipment:

This is a sample list of material:

1. Thermometer* (available at science suppliers),
2. salt,
3. water,
4. plastic or aluminum cup,
5. Freezer.

* 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 lowest temperature that the salt water reached?

2. Did the salt water solidify completely?

3. Is the freezing point for salt water below, the same as, or above the freezing point for pure water?

4. At what temperature does the pure water solidify?

5. Lake Winnipeg is a freshwater lake. If there was a salt lake located right beside Lake Winnipeg and it was the same size which lake would freeze first? Why?

6. Why is salt sprinkled on our streets in the winter?

You may also us the result of your experiments to make a bar chart. Each bar represents certain amount of salt and the height of that will represent the freezing point. Following is a bar chart made with non-real data. you will need to make your own, using the result of your own experiment.

Calculations:

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.

Conclusion:

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.

You may repeat such experiment on other types of salts such as Epsom salt (Magnesium sulfate) and Salt cake (Sodium sulfate).

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.

References:

List of References:

Salt and Freezing Point of water

Freezing point depression

Use of salt in Making ice cream

Science in ice cream

Sample Display: