Category: earth science

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Experiment: Salt Water and Heat Convection

In our series of experiments on the ocean, we’ve learned about Salt and Solubilities, the Density of Sea Water,  and Salt Water and Buoyancy.

Now we take a look at convection within salt water.

When we discuss heat transfer, we think of the three types: conduction, convection, and radiation.

Conduction takes place between two systems in contact with each other.

Convection occurs when heat is transferred by mass motion of molecules within a fluid.

Radiation is a form of heat transfer that occurs through electromagnetic wave propagation.  Usually we think of the rays of the sun as a major example of radiation.

All three types of heat transfer can be present simultaneously.  As you do this experiment on convection, see if you can also identify the other two types of heat transfer.

Materials:

  • food coloring
  • distilled water
  • salt
  • freezer tray for ice cubes
  • two glass jars

Procedure:

  1. Mix up some of the distilled water with a few drops of food coloring.
  2. Freeze this into cubes.
  3. Fill two glass jars 3/4 full with distilled water.
  4. Add some salt to one of the jars and label.
  5. Place a colored cube into each jar of water.  Observe the movement of the colored water as it melts into the warmer water.
  6. Leave the jars undisturbed as you watch.  Can you identify convection currents?  Where is the colored water going?  Is the colored cold water heavier or lighter than the warmer clear water?  Are the jars the same?  (Remember the Salt Water Density experiment..)
  7. Draw your jars, labeling them as to salt or fresh water.
  8. Write a conclusion statement about your finding.  Note: Yes, the presence of food coloring has affected your experiment, but please assume a small effect, and draw your own conclusions about the relative densities of these two solutions.  You can minimize the coloring effect by using a smaller amount of it.

Remember, as with all experiments, if you require your student to do a writeup, be sure to use the scientific method. Even if you do no writeup, as in the case of younger students, be sure to discuss the steps of the scientific method as you go.

Leave a comment and let us know what ocean experiments you have been doing!

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Experiment: Density of Sea Water

Milford Sound

In our study of the ocean, we come to another interesting fact about salt water.

It is more dense than fresh water.  This means that there is more mass in a volume of it than there is in fresh water.  More mass means that gravity has more effect upon it.

Salt water is heavier than fresh.

You can test this in your kitchen lab.

Materials:

  • a transparent container
  • food coloring
  • distilled water
  • salt
  • plastic wrap (may use paper)

Procedure:

  1. Into the glass container, put the water and enough salt so that the water is saturated and there is salt on the bottom of the jar.
  2. Add a few drops of food coloring.  Just for a moment enjoy the sight of the color diffusing into the water.  Stir it up to finish the mixing of the color.
  3. Carefully, place the plastic wrap on the surface of the colored water.
  4. Slowly pour some distilled water onto the plastic wrap.
  5. Slide the plastic wrap out from the jar smoothly so that the two bodies of water do not mix.
  6. Do you notice a clear layer of water standing above the colored water?  If you are careful not to agitate the water, it will remain so for a number of hours.  Gradually, the color will become uniform throughout the jar as the molecules of the salt diffuse throughout.

How does this effect show up in the world?

In Texas’ Galveston Bay, rain (fresh water) falls onto the surface of the water and forms a layer of fresh water on top.

Where a stream meets the Pacific Ocean (or Atlantic) the stream water will tend to stay at the surface of the water until the two are mixed by tidal or wave action.  Milford Sound, in New Zealand, is fed by fresh water streams, and maintains a constant fresh water layer several feet deep above the salt water.

Have you encountered any other examples of this phenomenon in your homeschool adventures?  Leave a comment and tell us about it!

 

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Experiment: Salt Water and Buoyancy

This experiment serves to illustrate some of the properties of salty water.  As you do this experiment, be sure to record it in your notebook.  Use the scientific method in a writeup of the experiment.

A characteristic of salty water is that in it things float better than they do in fresh water.  In a fresh water lake you would float a little lower than you would in the ocean.  The more salty the water the higher you would float.  You would float higher in the very salty Great Salt Lake than you would in the less salty ocean.

You can demonstrate this in your kitchen.

Materials:

  • Salt
  • Distilled water
  • 2 clear glass containers
  • a wooden block

Procedure:

  1. Place equal amounts of distilled water into the glass containers.
  2. Into one of the glass containers, place four tablespoons of salt.  Mark the container so that you know which one has the salt.
  3. Mark your wooden block with a fine tipped indelible marker so that it is marked vertically into half centimeters.
  4. Place the block into the distilled water carefully so that the block is upright in the water.  You should be able to read the markings on the side of the block to determine the depth at which the block floats.
  5. Repeat using the salty water.
  6. Compare the results.  In which type of water did the block float the highest: salty or distilled?

I tried this with a marshmallow.  The marshmallow in the salty water floated slightly higher than the marshmallow in the distilled water.  But since I had not marked the marshmallow first, the results were hard to see and the marshmallow started to dissolve.

This experiment works well with fresh eggs.  When placed in fresh water, an egg sinks to the bottom of the glass.  When placed in a glass of water with 1 tablespoon of salt dissolved in it, the fresh egg will float.