- University of Virginia
- Physics Department
Convection Currents and Density
A Physical Science Activity
2003 Virginia SOLs
- Plan and conduct investigations in which valid conclusions are made after
- Present experimental results in appropriate written form;
- Investigate and understand the heat transfer concept of convection;
- Explore relationships between convection currents and fluid density.
- What happens when you use ice cubes to cool a drink? After a few minutes,
the liquid feels cooler than before you added the ice. What exactly happens
to the particles that make up the ice cube when it melts? How does convection
aid in the cooling of your drink?
- clear plastic shoe box or a small aquarium
- immersion heater
- sealed plastic bag of ice cubes
- slide projector
- overhead projector
- Pour water into the shoebox until it is 2/3 full.
- Place the immersion heater at one end of the box.
- Place the bag of ice cubes at the other end against the outside wall.
- Using the slide projector cast the shadow of the box on a wall. Students
will see the vertical convection currents as the water is heated.
- (Optional) Repeat the demonstration on an overhead projector. Students should
notice that the primary convection currents are set up horizontally.
Liquids and gases differ from solids because they can flow. Anything that can
flow is referred to as a "fluid."
Both fluids and solids can gain or lose heat by one of three methods: Radiation,
Conduction, or Convection. Radiation is the transfer of energy through electromagnetic
waves; an example of this is the heat we feel from the sun, or the heat we can
feel coming off a red hot stove. Conduction is the transfer of heat or energy
between two objects that are in contact with each other. For instance, if you
were to touch a red hot stove with your finger, the burn you would receive would
be from conduction. Convection is the transfer of energy by the movement of
The best way thermal energy (heat) is transferred in fluids is by convection.
The particles move from one location to another, carrying energy with them.
When heat is added to a fluid, its particles begin to move faster. Particles
in a fluid have considerable freedom to move, and they tend to move further
apart as they gain more energy. When a pot of water is heated, the stove burner
heats the bottom and as it heats it expands and becomes less dense. Cooler,
denser water at the top sinks and pushes the hot water upward. As the hot water
rises, it cools, becomes denser and sinks, forcing the warm water to rise again.
This movement causes convection currents. These currents transfer thermal energy
from warmer to cooler parts of the fluid. Gradually all the water in the pot
To print out the Student Copy only, click
Materials (for each group)
- 2 ice cubes (strongly dyed with food coloring - dark blue works well)
- fresh warm water
- a ruler
- 250 ml beaker
- salt (about 70 grams)
- stirring rod
- Obtain an ice cube that has been strongly dyed with food coloring.
- Using tongs, gently place the ice cube in a beaker ¾ full of warm, fresh
water. Keep the beaker and water as still as possible.
- Observe the ice - water mixture for several minutes. Write a detailed description
of your observations.
- After you have made your observation in #3, measure the temperature at the
surface, at the bottom of the mixture, and 3 measured levels in between. Record
the temperatures in the data table. If you take these measurements too soon,
you may tend to stir the water mixture. If you wait too long, temperature
equilibrium may have already occurred. You may need to do the experiment twice
(observing one and measuring temperature in the other).
- Empty the beaker and fill it to the same level as previously with warm water.
- Add salt to the water, stirring vigorously as you pour until no more salt
- Repeat steps 3 and 4 using salt water in place of fresh water.
To print out the Data Sheet only, click
Observations of Fresh Water:
Observations of Salt Water:
- Predict what would happen to the coloring and temperature of the liquid
if you left the beaker containing fresh water and a colored ice cube untouched
for 30 minutes.
- Test your prediction.
- Relate this activity to ocean currents. Why do fish go down to the bottom
during cold weather?
- Think about ocean currents. Where do you think the water is the coldest?
Students with Special Needs
All students should be able to participate in this activity.
Click here for further
information on laboratories with students with special needs.
Answer the following questions:
- As the ice melts, what happens to the colored meltwater in the sample of
- In the salt water?
- Do you observe convection currents in this activity?
- Of the 2 liquids used, which do you think is denser?
Answers to Questions:
1. The meltwater will eventually move down into the container of water.
2. The meltwater will stay on top of the saltwater for a longer time than it
did for the pure water.
3. Convection currents should be able to be seen.
4. Saltwater is denser.