Does Fresh or Salt Water Freeze Faster?
This lab is a perfect go-to for those looking for a low-prep experiment. Our FREE lab sheets let students practice the scientific method as they walk through answering the question, “Does Fresh or Salt Water Freeze Faster?” We use this experiment while investigating our Earth’s Water Unit. While answering the question, “Does Fresh or Salt Water Freeze Faster,” students will learn the difference in their freezing points.
*Don’t miss the Floating Egg Experiment with FREE lab sheets to investigate the difference in density between fresh and salt water. Our Salt Water Density Experiment is fun too!
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Important Terms and Facts to Introduce
Before conducting the experiment, there are a few things you should cover with your students to create a meaningful learning experience. First, spend time talking about the terms below. We discuss the meaning of salinity and the freezing point of both fresh and salt water amongst other terms. Ocean water freezes just like freshwater, just at a lower temperature.
– Salt is also called Sodium Chloride.
– Salinity describes the amount of salt dissolved in water. Saline water is water with salt dissolved in it.
– Saturation describes the maximum amount of a substance within another substance.
– Concentration refers to the amount of a substance in a defined space.
– Solution is a type of homogeneous mixture composed of two or more substances.
– Solute is a substance dissolved in another substance, known as a solvent.
– Freezing Point of Fresh Water is 32 degrees Fahrenheit or 0 degrees Celsius.
– Freezing Point of Salt Water is 28.4 degrees Fahrenheit or -1.8 degrees Celsius.
Does fresh or salt water freeze faster? Lab Sheets
Write Hypothesis
After reviewing with students, guide them to write a hypothesis for the posing question.
For example, their hypothesis could read like this:
- If I freeze a cup of salt water and a cup of fresh water, then the _______________ water will freeze faster.
Students will plug in the words, “fresh” or “salt,” in the blank of the hypothesis.
Identify Variables
Manipulated Variable– variable that is different in the experiment. It is the thing that is manipulated. In this case, the manipulated variable is that salt is added to the water to create saline water.
Responding Variable– variable that is being tested. It responds to the manipulated variable. Hence, how long it takes to freeze.
Controlled Variable– variable(s) in the experiment that do not change or remain the same. In this lab, the amount of water, same freezer, and same cups (try to use clear plastic containers).
Gather Materials
The materials list is likely to all be found within your kitchen. I love experiments that do not require a tricky supply list. So, gather 2 plastic cups/containers of equal size, water, salt, measuring cup, salt, TBSP, and lab sheet. (Download Below!)
Does Fresh or Salt Water Freeze Faster? Lab Procedures
The lab sheet will guide your students through the steps of the experiment. In this experiment, students will fill up two plastic containers with 1 cup of water each. Be sure to label each container with an F for fresh and a S for salt. You don’t want to get confused about your samples.
Once you have identified the container as salt, put 2 tablespoons of salt into the water. Mix it thoroughly until dissolved. Next, put the two cups into the freezer. Be sure to note the time, as students will need to record their observations within the data table on the second page of their lab sheet. Students will simply mark the stage that each sample is in at each time check. Be prepared to record data over the next couple of hours.
Drawing Conclusions
It’s very important that before, during, and after an experiment that you are explaining the science behind the experiment. If you skip this step, then the lab becomes merely a fun hands-on activity with no real science connections. The experiments are meant to help the students get past the surface and begin to soak in the abstract and unseen science behind it.
For most of us who are using a residential freezer, the recommended temperature is set at 0 degrees Fahrenheit. At this temperature, you would think that the salt water should have no problem freezing. We left our salt water in the freezer for 2 days and it never froze. At best, it remained slushy. The freezing point for salt water is 28.4 degrees Fahrenheit, which is well above the temperature of the freezer. So, why didn’t it freeze in time?
What's the Science behind it?
I was baffled by this and my curiosity got the best of me. It was time to do some research into the why. My research led me to the understand that the freezing point for the salt water we created (2 tablespoons of salt to 1 cup of water) was lower than the reported 28.4 degrees Fahrenheit. See, when salt molecules are combined with water molecules, the salt keeps the water molecules from packing together as in a solid. It works to prevent ice from forming.
At TemperatureAsk, there is an educational article written called, Does Adding Salt to Water Keep it from Freezing? The author wrote that when…
“…salt dissolves into water molecules it spreads them out, preventing them from bonding together and forming ice crystals. The more salt in the solution, the lower the freezing point will be. That’s why rock salt is used to melt ice on roads — it can lower the freezing point all the way down to -20° F/-28.9° C. That’s cold enough to turn most ice into a slushy mixture, even in very cold weather.”
In conclusion, our concentration of salt to water was high enough to lower the temperature to a point that it would not freeze in a residential freezer. Hence, the salt water we created had a lower freezing point than 0 degrees Fahrenheit.
More Free Resources!
John 4:14 – But whosoever drinketh of the water that I shall give him shall never thirst; but the water that I shall give him shall be in him a well of water springing up into everlasting life.
