Freeze Your Fruit With Science!
Pop Science Quiz: What happens to water when it reaches 0°C (32°F)?
Answer: It freezes!
But does water always freeze when it reaches 0°C (32°F)?
Believe it or not, water can sometimes be cooled to temperatures below its freezing point and still remain liquid. In this state the water is supercool.
How can this happen? In this activity you will create your own supercool water, and initiate its transition from liquid to solid. All while making a tasty snack!
This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.
When water is warm or even room temperature, the water molecules are moving around, bumping into each other. The warmer the water, the higher the energy of the molecules and the more they collide with one another. In contrast, as water cools, the molecules move slower until they eventually stop moving altogether. In a frozen state, water molecules are locked together in a crystalline array. These crystals are thought to form around ‘impurities’ in the water, such as dust and minerals. Without the presence of these impurities, the water can be supercooled past its freezing point.
In this activity you will create a ‘bath’ using salt and ice. Adding the salt results in a bath with a temperature below water’s freezing point, a phenomenon known as freezing point depression. You will use this bath to supercool the pure water it surrounds. Then you will perform some supercool experiments on this water!
Observations and Results
In this activity you made water REALLY cold – in fact, you made it supercool! To understand what this means, imagine the molecules that make up the water in your cup. When water is warm or room temperature, those molecules move around, bouncing off each other and the side of the cup.
However, when you remove the heat energy from the cup, those molecules slow down and eventually stop moving. When they stop moving, the molecules form crystals, like those you see creating frost on your windows, or drifting from the sky as a snowflake. Crystals don’t just form on their own. Before you have a crystal, you need a ‘seed’, something for the water molecules to hold onto as they arrange themselves. The water you used in your cups was distilled water, and therefore generally free of contaminants such as dust and microbes. As a result, there wasn’t anything in the water for the molecules to crystallize around. Therefore, instead of crystalizing when it reached its freezing point, the water remained liquid.
This changed when you added your fruit, and your ice cube, to the water! Both the fruit and ice acted as the ‘seed’ in your supercool water, and gave the water molecules something to crystalize around. In the case of the fruit, the ice crystals you noticed on the surface of the fruit acted as the seed. And in the second cup, the ice itself was the seed!
More to Explore
Megan Arnett, PhD, Science Buddies
Science Buddies |
Supercool water, crystallization, freezing, molecules, Freezing Point Depression
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