Convection and salinity: How does salinity affect the movement of water?Featured science projectScience project video

popular science fair projects
Complexity level:
5
Project cost ($):
Time required:
Material availability:
Safety concerns:
This science fair project involves a heat source and water. Care should be taken when handling these.

Abstract

Convection is the phenomena characterized by the movement of molecules in gases and liquids. Heat causes molecules to be active, while cooler temperatures result in less activity. Convection can be observed in volcanoes, in the ocean, in cooking, in heating your home and even in decorative lava lamps. In this science fair project you will observe how the addition of salt to water effects convection.

Objective

Understanding how salt affects convection currents in water.

Background

In water, convection occurs when warmer water at the bottom, rises to the surface. If the water cools near the surface it will eventually sink back again to the bottom. This rising and falling of water becomes a cycle.

Density is a measure of the amount of mass an object of a given volume, possesses. At the molecular level, water molecules are further apart from each other at warmer temperatures (as compared to lower temperatures). At warm temperatures, the water becomes less dense. At lower temperatures, water molecules are closer together, becoming denser. This is why warm water rises; it has a lower density than cooler waters.

Salinity is the amount of salt dissolved in water. Salinity affects water density. Refer to resources under the Bibliography section for more information on salinity and density.

Scientific Terms

Convection, Density, Salinity

Materials

  • Two large beakers or a heat-proof jar
  • Food coloring
  • Liquid soap
  • Two hot plates
  • Water
  • Salt

Procedure

convection and salinity science fair project

  1. Place the two beakers or jars on the hot plates, side by side.
  2. Fill both containers half-way with water.
  3. In the second jar, add salt to the water and stir until all the salt is dissolved. Keep adding salt until you see grains of salt at the bottom of the glass that do not dissolve despite your stirring.
  4. Add food coloring to thenwater and mix thoroughly.
  5. Pour liquid soap into both containers, enough to raise the total fluid level to three quarters of the containers' volume.
  6. Turn on the hot plates to a low heat setting and observe what happens.
  7. In which container is the movement of the soap more rapid?

Discussion

How is density affected by salinity?

Questions & Answers

What causes liquids to rise to the surface?

As the temperature of the water increases, it becomes less dense. A lower density means the water becomes less heavy, so it moves toward the surface.

What causes liquids to sink to the bottom?

As the temperature of the water decreases, it becomes denser. A higher density means the water becomes heavier, so it sinks toward the bottom.

What determines the rate of this movement?

While temperature can affect the rate of movement, so does salinity. Ocean water has salt so it has a greater salinity than fresh water, which has little or no salt. The higher the water's salinity, the heavier is the water.

Make it Your Own

Try the science experiment with hot and cold water, instead of fresh and salt water.

References

? "What is Convection" at the Wise Geek website http://www.wisegeek.com/what-is-convection.htm

? "What is density?" at the Whyzz web site http://whyzz.com/what-is-density

? "What is Salinity" at the Wise Geek website http://www.wisegeek.org/what-is-salinity.htm

Videos