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Density - Liquid Rainbow (Rena Benor)

Author

Rena Benor

Principle Illustrated
1.  Different densities of water, or solutions, will stratify to form layers.
2.  Density of ocean water is affected by temperature and salinity.

3.    In the oceans, the deep, bottom layer is colder and saltier than the surface.
4. Cold water with dissolved salts (higher salinity) is denser than warm water without dissolved salts.

Vocabulary

density: Mass per given unit of volume.

mass: The property of a body that causes it to have weight in a gravitational field.

salinity: Saltiness or dissolved salt content of a body of water.

stratification: Physical layering of the water column resulting from density differences caused by salinity or

temperature variation.

Standards

Density and Buoyancy
8. All objects experience a buoyant force when immersed in a fluid. As a basis for
understanding this concept:
a. Students know density is mass per unit volume.
b. Students know how to calculate the density of substances (regular and irregular
solids and liquids) from measurements of mass and volume.
d. Students know how to predict whether an object will float or sink.

Questioning Script

Prior knowledge & experience:

Ocean water has salt in it. It is easier to float in the ocean, even though their own weight does not change.  There must be something about salt water that makes it easier to float.

Root Question

What do you think is going to happen when we combine these liquids in the straw?

Prediction, question, etc.

Target response:

What is the correct answer

Common Misconceptions:

Students may think:

  • Objects sink in water because they are “heavier” than water.
  • An object floats because it has air in it.
  • Students often confuse density and thickness,
  • An object floats because it is “lighter” than water.
  • All liquids mix.

Background Information

Ocean Literacy: Most of Earth's water (97%) is in the ocean. Seawater has unique properties: it is saline, its freezing point is slightly lower than fresh water,its density is slightly higher, its electrical conductivity is much higher, and it is slightly basic.

Big Idea: When solutions of two different densities meet, the lower density (less dense) solution will move on top of the higher density (more dense) solution, resulting in layering or stratification of the solutions. Density is an important feature of seawater since many physical and biological processes are affected by it, such as heat movement around the globe, which influences climate. Also, feeding and reproduction by marine organisms.

Background

Density is a property of matter  that deals with the relationship between weight and volume. How can two objects that are the same size have different weights? The answer has to do with their density. An object's density is determined by comparing its mass to its volume. If you compare a rock and a cork that are the same size (they have equal volume),which is heavier? The rock is, because it has more mass. Thus the rock is denser than the cork because it has more mass in the same volume.

The densities of many fluids can be easily changed. Do objects float the same way in fresh water as they do in saltwater? If you have the same amount of each, saltwater weighs more than fresh water. Salt water is described as being more dense than fresh water. In the case of ocean water, heating, cooling, and salinity all influence density.

Circulation in the ocean depends in part on differences in density of the water. Water with more salt is denser (heavier) and sinks while fresh water is less dense and "floats" on the surface. These buoyancy differences can result in the separation of water into layers (stratification)within an estuary or ocean.

Density driven currents are an important feature in coastal waters, affecting the physical, chemical, and biological dynamics in the ocean. Many marine organisms use density currents formigration, reproduction, and feeding.

Materials:

4 lg containers, food coloring (4 colors), transparent drinking straws, pickling salt (preferred),    5 vials or test tubes per student group (4 for solutions and 1 for waste)

Preparation:

Before the students arrive, the teacher will prepare 4 solutions, each with a different density, as follows:

#1: 1 gallon water + 0 cups of salt + bottle of yellow food coloring

#2: 1 gallon water + 1/2 cup of salt + bottle of green food coloring

#3: 1 gallon water + 1 cup of salt + bottle of red food coloring

#4: 1 gallon water + 2 cups of salt + bottle of blue food coloring

Mix the solutions thoroughly, until all salt is dissolved. The solutions must be distinctly colored. Use the entire contents of one of the small bottles usually sold in sets of four at the grocery store. Pickling salt is preferred for this activity because it does not have any additives and will not make cloudy solutions. Clear or translucent drinking straws must be used so that the colors of the different solutions can be observed when in the straw.


Photographs and Movies
References

          GEMS science lesson plan

 

 

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