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.

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Experiment: Salt and Solubilities

“It is an everlasting covenant of salt before the Lord for both you and your offspring.”  Numbers 18:19

The presence of salt in the ocean has always been very interesting to ocean goers, especially since there seems to be no reason why the sea should be salty and not alkaline, or full of some other set of chemicals.  But the reality of it is that the salt in the ocean effects the physical characteristics as well as the chemical characteristics of the ocean and its life.

This experiment serves to illustrate some of the properties of salty water.  As you do the experiment, be sure to record it in your notebook.  Use the scientific method.  If you like, a report can be made using the critical thinking questions included in the procedure.

Salt and Solubilities

One of the basic reasons for a salty sea is the fact that sodium chloride is able to be dissolved in water.  But that saltiness depends upon the temperature of the water.   When water is cold less salt will dissolve in it.  When water is hot, more salt will dissolve.  The Gulf of Mexico is of higher salinity water than is the Arctic Ocean.  The Gulf of Mexico has a higher temperature than does arctic water.

You can test the solubility of salt in water in your kitchen.

Materials:

  • Salt
  • Distilled water
  • Thermometer (for older students)
  • Canning jars
  • Heat source
  • Measuring spoons

Procedure:

  1. Make ice cubes from the distilled water.
  2. Fill three canning jars with water: one with water at room temperature, another with water which has ice in it, and the last with boiling water.  Older students should use more jars with more different temperatures of water in them.
  3. Add salt by teaspoon to each of the jars, stirring between each teaspoonful until dissolved.   Count the teaspoons.  Stop adding salt when it no longer dissolves.
  4. Which jar of water dissolved the most salt?   Explain how the increase in energy of the water molecules affected the dissolution process?
  5. Make a graph showing the relation of water temperature to the amount of salt dissolved in the water.
  6. Does the salt precipitate out when the hot water cools?
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