Grade Level: 5th - 8th; Type: Chemistry/Meteorology
The goal of these experiments is to compare the freezing temperature of salt water and freshwater, learn about supercooling and condensation nuclei.
- Can you change the freezing point of water?
- What is latent heat and how does it affect the freezing of water?
- What is a supercooled solution?
- What role do condensation nuclei play in the freezing process?
- How does supercooling relate to meteorology?
You may already know that salt water has a lower freezing point than fresh water.You can create a supercooled solution of fresh water that does not freeze at 32 degrees Fahrenheit by bathing a scrupulously clean plastic cup of fresh water in a solution the salt water that is below 32 degrees.This will not work if the materials are contaminated with dust or if there are impurities in the water which is why this experiment calls for distilled water and why you are asked o rinse the plastic cup with the distilled water before placing it into the cold salt water.Impurities- or even rough spots on the plastic cup – act as condensation nuclei, triggering the formation of ice crystals.
Normally, we associate the freezing of water with the decrease in the temperature of the water. However, in this experiment, latent heat is released to the air when molecules in water moves from to a less ordered state (such as found in liquids) to a rigidly ordered state (like ice). The freezing of the supercooled water releases latent heat, which is why the temperature increases when the supercooled water freezes. Supercooled liquids are sometimes found in the troposphere where temperatures may be between–10 and 0 degrees C.
- Distilled water
- Small plastic cup
- Thermometer that reads 20 to 40 degrees Fahrenheit or -10 to 10 degrees Centigrade
- Used dust rag
- Using your distilled water, fill several clean ice trays and put them in the freezer.You will need enough ice to fill your bowl almost to the top.
- Pour distilled water into a bowl until the bowl is half full.
- Add 1 tablespoon salt to the water and stir until the salt is dissolved.Keep adding salt in tablespoon increments until no more salt will dissolve.Make sure your measuring spoon is scrupulously clean.
- Fill the bowl with your distilled water ice cubes until the water and ice reach the top of the bowl.Wait five minutes for the temperature to equilibrate and then take the temperature of the water.The temperature should be below freezing.
- Rinse your plastic cup with distilled water to remove any possible traces of dust from the cup.Fill the cup with fresh distilled water.
- Put the plastic cup of fresh distilled water into the bowl of salt water. Make sure that the salt water does not contaminate the fresh distilled water in the cup.
- Wait five minutes for the temperature of the fresh distilled water and the temperature of the salt water to equilibrate. Measure the temperature of the fresh water. It should be below freezing. This water is supercooled.
- Gently shake a used dust rag over the fresh water. The object is to add a few pieces of dust to the fresh water. Once the dust is in the water, you should see ice crystals begin to form around the dust. Measure the temperature of the fresh water as it freezes.Does the temperature increase or stay the same?
Terms/Concepts: Freezing point; Supercooled solution; Latent heat; Molecular motion as a function of state
- Supercooling http://en.wikipedia.org/wiki/Supercooling
- Freezing Supercooled Water Puzzles Scientists http://www.rsc.org/chemistryworld/News/2010/February/04021002.asp
- Experimental Explanation of Supercooling http://www.sciencedaily.com/releases/2010/04/100421133114.htm