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Salinity & Water Density

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Author: Randall Frost, Ph.D.

Cold water is denser than warm water. Water with a high salt concentration (salinity) is denser than water that has a lower salt concentration. Warm water rises above denser colder water, and bodies of water that have different temperatures can form layers according to their respective temperatures. Likewise, denser water with high salinity sinks below water with low salinity.

Problem:

How does the addition of salt to hot water affect its mixing with colder water?

Materials:

  • Wide mouth quart jar
  • Kitchen spoon
  • Salt
  • Food coloring
  • Drinking glasses
  • Thermometer

Procedure:

  1. Read about the effects of temperature and salinity on ocean water circulation.
  2. Formulate a hypothesis to explain how differences in salinity affect the circulation of hot and cold water.
  3. Have ready a 1 quart wide mouth (clear glass) jar.
  4. Add 8 oz. of room temperature tap water to the jar. Measure the temperature.
  5. Heat another 8 oz. sample of tap water until it is warm. Measure the temperature.
  6. Add several drops of food coloring to the warm water and stir it around.
  7. Carefully pour the warm water into a kitchen spoon placed over the jar, and allow the hot water to slowly spill over from the spoon into the jar.
  8. Note whether the colored warm water mixes or forms a layer with the uncolored room temperature water.
  9. Empty the jar, and add 8 oz. of room-temperature tap water to it.
  10. Heat another 8 oz. sample of tap water to the same temperature used in the first part of the experiment. Add food coloring.
  11. Add 1/2 teaspoon of salt to the warm water and stir the solution until the salt dissolves.
  12. Carefully pour the warm salt water into the jar containing the room temperature tap water using the kitchen spoon to slow the transfer. Note any layering or mixing behavior.
  13. Repeat these steps, gradually adding more salt to the hot water used in the experiment until complete mixing is observed.
  14. Repeat the entire experiment, using warm water at a different temperatures.
  15. Prepare a table comparing the amount of salt dissolved in the hot water with the amount of mixing observed for different temperatures.
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