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Education: Student Outcomes |
Click here to search for other assets Can Seawater Freeze? Grade level: K-4 Theme: water cycle Activity: http://aquarius.nasa.gov/sea_water_freeze.html Big Idea Salt causes water to freeze at a lower temperature. The oceans do not freeze (except in extreme polar areas) because of the salinity (i.e., concentration of salts).
Key Concepts Dissolving any substance in pure water raises or lowers the freezing and boiling point.
When water freezes - goes from the liquid state to the solid state - its molecules go from a disorganized state to an organized state.
When water freezes to a solid, molecular motion slows down enough that the molecules become permanently fixed in an orderly arrangement called a crystal.
The individual particles that make up salt (known as ions) arrange themselves around the water molecules. In doing so, they shield the water molecules from interactions among themselves, making it less likely that they will find each other and form ice.
The water molecules have to be slowed down even more in the presence of salt in order to form a solid. So you have to go to a lower temperature in order to freeze water that contains salt.Salt is excluded in the formation of ice; therefore ice made from salt water is essentially salt-free.
Salt water is denser than freshwater because of its salt content, and, as a result, an object is more buoyant in the ocean than in a freshwater lake. The density of salt water depends on its salinity and varies around the world. Background Under normal conditions, ordinary fresh water freezes and melts at 0°C, or 32°F. If water and ice are at 0°C, which is the melting point of ice and the freezing point of water then molecules of ice are escaping into the water (melting), and molecules of water are being captured on the surface of the ice (freezing). (more) Student Outcomes After completing this activity, students should be able to: Compare the basic properties of fresh and salt water (e.g., density, ability to dissolve salt, freezing point). (W: K-4) Key: C = climate / O = ocean circulation / T = 21st century technology / W = water cycle |
