Temperature, Salinity, and Deep Ocean Currents

Objectives:

  • Salt water and cold waterare denser than fresh water and warmer waters. Salt water and cold water aretherefore heavier and sink.
  • When ocean water evaporates near the equator, the water becomes more dense because most of the salt remains in the water. The ocean water also becomes saltier and denser near the poles as salt remains behind after freezing as well. The densest water is found at the poles because it is both salty and cold.
  • Deep ocean currents are driven by thermohaline circulation or convection currents that redistribute the heat absorbed at the equator around the planet. Our current weather and climatic patterns are dependent upon this circulation.

Background:

Ocean currents arise in many different ways. For example, wind pushes the water along the surface to form wind-driven currents. Deep ocean currents are caused by differences in water temperature and salinity. This activity can be done in front of the class as a demo or students can follow along with you as you do it with them.

Materials:

  • 8 Baby food jars
  • 1 Laminated index card (or 4 unlamented index cards)
  • Table salt
  • 2 Colors of food coloring
  • Stir stick
  • Dish pan (for spills)
  • Towels
  • Hot plate
  • Container to heat water
  • Ice
  • Cooler

Part I - Salt Water vs. Fresh Water Activity:

  1. Prepare ahead of time4 baby food jars with water. Dissolve the salt in two of the jars and add blue food coloring. Don’t mark the jars or tell students that there is any difference between the two bottles of water other than color. Add a drop of red food coloring to the other 2 jars. You should have 4 jars left for Part II.
  2. Place a 3 x 5 index card on top of the salt water and carefully invert it. Place the salt water jar on top of the fresh water container and have someone carefully remove the card. Ask students what they think will happen then observe the results.
  3. Use the second set of jars to repeat the experiment. This time, invert the fresh water jar over the salt water jar. Remove the card and ask them again what they think will happen? Observe the results.
  4. Take the inverted freshwater jar on the salt water jar and turn them horizontally.

Questions for Class Discussion:

Is salt water heavier or lighter (higher or lower in density) than fresh water? Make sure that you explain your answer in terms of the results that you obtained from your experiment. If evaporation causes surface water to be salty, where would you expect ocean water to be very dense? Is there any other process other than evaporation that could leave salt behind in the water? Where would this occur? Does the density of ocean water have any relationship to the temperature of ocean water?

Part II - Cold Water vs. Warm Water Activity:

  1. Fill 4 jars with regular water. Add blue food coloring to two of them and put them on ice to cool. This time students will kind of be on to you so if you want to mark the jars for this experiment to make it more concrete for students you can.Mark the jar "Cold Water", if you wish. Add a drop of red food coloring to the other 2 jars, label them warm water, and place them in a pan of hot water on the hot plate to heat up (be careful not to get them too hot)
  2. Place a 3 x 5 index card on top of the cold water and carefully invert it. Place the cold water jar on top of the hot water container and have someone carefully remove the card. Observe the results.
  3. Use the second set of jars to repeat the experiment. This time, invert the warm water jar over the cold water jar. Remove the card, and observe the results.
  4. Take the inverted warm water jar on the cold water jar and turn them horizontally.

Questions for Class Discussion:

Is cold water heavier or lighter (higher or lower in density) than warm water? Make sure that you explain your answer in terms of the results that you obtained from your experiment. Where would you expect ocean water to be very cold or dense? Where would you expect it to be warm? Where is ocean water the densest?

Results:

  1. When the cold water is placed on top of the warm water the water in the two jars will mix

and the cold water will sink to the bottom.

  1. When the warm water is placed on top of the cold water the warm water will sit on top of

the cold water and no mixing will occur.

  1. When the higher concentration salt water is placed on top of the lower concentration salt

water the water in the two jars will mix and the saltier water will sink to the bottom.

  1. When the lower concentration salt water is placed on top of the higher concentration salt

water the less salty water will sit on top of the more salty water and no mixing will occur.

Explanation of Results:

When the jar of cold water is turned upside down on the jar of warm water, the water will mix

because the cold water is more dense than the warm water. The molecules in the jar of cold water

are much more highly concentrated than the molecules in the jar of warm water, meaning there

are more water molecules packed into the space of this jar than in the jar of warm water. The

greater number of molecules in the cold water makes this water heavier than the warm water so it

will sink into the bottom jar, mixing with the warmer water. After this mixing occurs the higher

density water is sitting in the bottom jar and the lower density water has risen to the top.

However, some of the heat energy in the warm water will be transferred to molecules of cold

water so the water in both jars will become mixed to a certain extent and the colors of blue and

red will mix. When the warm water is placed on top of the cold water, the denser water is already

on the bottom so the less dense warm water will just sit on top of the more dense water. The jar

of red water will stay red and the jar of blue water will stay blue.

The same results will occur using the salt water and the fresh water. The more salt in the water the greater its density will be because there are more molecules packed into a given area. The higher concentration salt water will be heavier than the fresh water so when salt water is placed on top of fresh water the two will mix and the saltier water will sink to the bottom. Whenfresh water is placed on top of salt water the higher density water is already on the bottom and is held there by gravity so no mixing occurs.