A Dense Subject

Developer:

Grade Levels:

Discipline:

Physical Science

Objectives:

  1. Students will learn the effect of salinity on the density of water.
  2. Students will learn the effect of temperature on the density of water.
  3. Students will apply learnings to the movement of water in the ocean.

Vocabulary:

DENSITY = mass per unit volume

MASS = the amount of matter in an object.

VOLUME = the space that an object occupies.

Background:

One physical characteristic of sea water is density. Density is the mass of the seawater compared with an equal volume of fresh water. Whether a substance sinks or floats in a liquid is determined by its density. A substance less dense than its surroundings tends to move upward; in other words, it floats. If its density is greater than its surroundings, it will sink. Temperature, salinity and pressure control seawater density.

PART I - SALINITY AND WATER DENSITY

Materials
Part I:

beaker


eyedropper

water

food coloring salt graduated cylinder

Procedure
Part I :

  1. Fill a beaker with 150 ml of fresh water (tap water).

  2. Fill an eye dropper half-full of colored, salt water.

  3. Gently squeeze out the colored, salt water into the beaker of fresh water.

  4. Observe. Answer Question #1 below.

  5. Reverse the procedure. Fill a beaker with 150 ml of salt water.

  6. Fill any eye dropper half-full of colored, fresh water.

  7. Gently squeeze out the colored, fresh water into the beaker of salt water.

  8. Observe. Answer Question #2 below.

PART II - TEMPERATURE AND WATER DENSITY

Materials
Part II:

clear, rectangular dish

two hot plates

water

ice food coloring

Procedure
Part II :

  1. Place a clear, rectangular dish with fresh (tap) water in it on two hot plates.

  2. Add a chunk of ice at one end of the dish.

  3. Turn on the hot plate at the other end of the dish.

  4. Add one drop of food coloring at the warm end to make water movement more visible.

  5. Observe. Complete the information below.

Explanation:

Questions
Part II: :

  1. Make a sketch of the demonstration showing the movement of the water using arrows. Label your sketch including which end of the dish was hot and whch was cold.

  2. Is ice more or less dense than water? Explain.

  3. Does the warmed water rise or sink?

  4. Is the warmed water more dense or less dense than the colder water? Explain.

  5. Does the cooled water rise or sink?

  6. Is the cooled water more dense or less dense than the warmed water? Explain.

  7. Which water is more dense, water from the North Pole or the Equator?

  8. As water from the North Pole moves south, would you expect it to sink or rise? Explain.

  9. What was the problem you were investigating in this demonstration?

PART III - A WATER SYSTEM LIKE THE OCEAN

Materials
Part III:

beaker

Eye Dropper

eye dropper

water

salt food coloring graduated cylinder

Procedure
Part III: :

  1. Pour 75 ml of very salty water into a beaker.

  2. Carefully and gently pour 75 ml of fresh water down the side of the beaker containing salt water so the salt water will be disturbed as little as possible. After any small water movements have stopped, you should have a two-layer system with salt water beneath fresh water. (You will not be able to see this distinctly.)

  3. Fill an eye dropper half-full of colored, slightly salty water

  4. Gently squeeze it into the fresh water.

  5. Observe the results. Make a sketch per Question #1.

  6. Fill an eye dropper half-full of colored, slight salty water.

  7. Extend the eye dropper down into the very salty water and gently squeeze out the colored, slightly salty water.

  8. Observe the results. Make a sketch per Question #2.

Explanation:

Questions
Part III: :

  1. Make a sketch showing how the drop of colored, slightly salty water moved when it was added to the fresh water.

  2. Make a sketch showing how the drop of colored, slightly salty water moved when it was added to the very salty water.

  3. Which water will sink, the more dense or less dense water?

  4. Does ocean water become more or less dense when the salinity is decreased by adding fresh water?

  5. Will water from the Mississippi River stay on top or sink as it flows into the Gulf of Mexico? Why?

  6. Would the water from the Laguna Madre during summer (with higher salinity than the Gulf of Mexico) stay on top or sink as it enters the Gulf of Mexico? Explain.

Conclusion:

Explain the relationship between the density (due to temperature and salinity) of water and the moverment of water in the ocean.


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