Convection of Liquids Tube

Introduction

Convection occurs all around us in our everyday lives, from the air surrounding us to the contents of the food we cook on the stove. Use this demonstration to demonstrate to students the transfer of energy through convection of a liquid.

Concepts

• Density
• Convection
• Heat transfer

Materials

Safety Precautions

Wear chemical splash goggles whenever chemicals, heat or glassware is used. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The solution of water and food dye may be rinsed down the drain according to Flinn Suggested Disposal Method #26b.

Procedure

1. Attach a buret clamp to the support stand.
2. Fill the convection of liquids tube with cool tap water.
3. Clamp the convection of liquids tube to the support stand.
4. Position the Bunsen burner underneath the lower right-hand corner of the tube. The top of the burner should be about 5" below the tube (see Figure 1).

4. Light the burner then quickly place one drop of food dye in the top of the tube.
5. Observe the convection of liquid.
6. Use the Convection of Liquids Worksheet for class discussion.

Student Worksheet PDF

11936_Student1.pdf

Teacher Tips

• It is not necessary to fill the convection of liquids tube with ice water. Ice water may be used, however, to demonstrate a slightly different convection rate.

Sample Data

Observations

Answers to Questions

1. Describe what is occurring in the convection of liquids tube during the demonstration.

   The burner is placed under one corner of the tube and heats the water above it, causing the molecules to move faster and become less dense. The cool water on the other side has greater density and sinks to the bottom down the other side of the tube.

2. Name at least two other examples of convection in everyday life situations.

   House furnaces, ocean currents, wind currents and hot air balloons are a few examples.

3. Did the movement of the water change over time? If so, how?

   The rate of flow decreased as the temperature of the water in the tube approached equilibrium.

4. Other than convection, what other method of heat transfer was taking place in this demonstration?

   Heat was transferred by conduction from the flame to the glass tube and then to the water in the tube.

Discussion

Heat energy can be transferred by three methods—conduction, convection and radiation. Conduction is the transfer of heat through direct contact. Radiation occurs when energy is transferred by electromagnetic waves, such as the sun heating the Earth. Convection is the transfer of thermal energy by the movement of molecules from one part of a fluid (gas or liquid) to another.

The molecules in a gas or a liquid are in constant, random motion and can move easily from one location to another. (This is in contrast to the properties of molecules in a solid, which are held in fixed locations.)The motion of molecules in a liquid or gas results in convection when heat energy is applied, Consider, for example, convection in a boiling a pot of water on the stove. Thermal energy from the stove is transferred to the bottom of the pot and then by conduction to the water in the bottom of the pan. The water molecules move faster as their energy increases. The faster the molecules move the farther apart they become. The water near the top of the pan has not obtained as much thermal energy as the water in the bottom of the pan. The cooler water molecules are closer together and more dense than the warm water. This results in the less-dense warm water rising to the top of the pot and the more-dense cool water sinking to the bottom (see Figure 2). The cool water, now on the bottom of the pan, is heated and rises and the cycle continues.

This same phenomenon occurs in nature in air and water. It occurs when a cool more-dense fluid displaces a warmer less-dense fluid. For example, during the day the water of the lake is cooler than the land. During the day the sun heats the air above the nearby land causing the molecules to move faster and become less dense. Cool air off of the lake flows towards the land, displaces the warm air and creates a cool breeze. As this new cool air is heated from the warmth of the ground, it rises and the cycle repeats.

References

Introduction to Physical Science. Glencoe McGraw Hill: New York; 2002; pp 292–293. Science Spectrum, Physical Science: Holt, Rinehart and Winston: Orlando; 2008; pp 480–481.