Salinity & Water Density

Cold water is denser than warm water. Water with a high salt concentration is denser than water having a lower salt concentration. Hot water rises above colder water. Bodies of water having different temperatures can form layers according to their respective temperatures.

Water having a low salt concentration rises above water having a high salt concentration. Bodies of water having different salinities will form layers according to their respective salinities. Differences in temperature and salinity produce differences in water density. The denser water sinks beneath the less dense water. As this happens, the less dense water rises. Water temperature has a larger effect on water movement than 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.