Heat Convection in Fluids

Introduction

Heat transfer by convection in fluids occurs continuously, but often goes unnoticed. Help students understand how convection currents form in both liquids and gases with these two hands-on demonstrations.

Concepts

• Convection
• Heat transfer
• Thermal equilibrium

Materials

Safety Precautions

To avoid burns, use caution when working with a flame or hot water and when handling hot glassware. Wear safety glasses and heat-resistant gloves. 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. Completely cooled candles and clay may be placed in the trash according to Flinn Suggested Disposal Method #26a.

Prelab Preparation

Convection of a Liquid

Rubber Stopper Assembly
It is important to take steps to ensure that the glass tubing is safely inserted into rubber stoppers. Lubricate both the end of the tubing and the hole of the stopper with glycerin or petroleum jelly. Always protect your hands with a towel or leather glove. Never try to force glass tubing into a too-small hole. The Glass-a-Matic Hand Saver (Flinn Catalog No. AP4599) is a device that makes inserting glass tubing into a rubber stopper safe and easy.

1. Insert the 2½" piece of glass tubing through the top into one hole of the two-hole rubber stopper.
2. Starting from the bottom, insert the 5" piece of glass tubing into the other hole of the stopper. Push the glass tubing through the stopper until about a half centimeter of tubing protrudes from the top (see Figure 1).
   {12665_Preparation_Figure_1}

Convection of a Gas

1. Form a marble-sized piece from the stick of clay.
2. Press the clay into the center of the plastic weighing dish. Make sure the 12" plastic tube will fit over the piece of clay (see Figure 5 in the Procedure section).

Foil-Wrapped Divider

1. Cut one T-shape from the laminated template. Save the remainder of the template for additional dividers as needed.
2. From the 12" x 18" piece of aluminum foil cut one 3" x 12" rectangle. Save the remainder of the foil for additional dividers.
3. Lay the vertical part of the T in the center of the short side of the 3" x 12" piece of foil, with the edge of the 3" side against the bottom of the cross part of the T (see Figure 2a).
   {12665_Preparation_Figure_2a}
4. Fold each side of the foil in, covering the vertical part of the T (like a gum wrapper) (see Figure 2b).
   {12665_Preparation_Figure_2b}
5. Fold the bottom extra length of foil up. Part of the foil will extend beyond the top of the T (see Figure 2c).
   {12665_Preparation_Figure_2c}
6. Fold the tab of foil over the top of the T and press the foil down on the other side (see Figure 2d). Only the two arms of the T should be exposed. The vertical center of the T should be covered in foil. Excess overlapping foil may be trimmed.
   {12665_Preparation_Figure_2d}

Procedure

Convection of a Liquid

1. Obtain a 125-mL Erlenmeyer flask. Wearing heat-resistant gloves, fill the flask almost to the top with very hot tap water.
2. Add 2–3 drops of red food coloring to the hot water in the flask. Stir with a stirring rod until the water is a uniform color.
3. Carefully insert the rubber stopper assembly into the flask. The shorter piece of glass tubing should reach the water.
4. Obtain a 1-L tall-form beaker and slowly lower the Erlenmeyer flask into the beaker.
5. Fill the tall-form beaker with cold tap water or ice water by carefully pouring the water down the inside wall of the beaker, not over the flask. Fill the beaker with enough cold water to cover the higher glass tube by about ½ centimeter (see Figure 3). Start the timer or note the time
   {12665_Procedure_Figure_3}
6. Using a thermometer, measure the temperature of the water near the top of the large beaker. Students should record this temperature as the zero-minute in Data Table 1 on the Heat Convection in Fluids Worksheet.
7. Observe any movement of water in both the flask and the beaker. Optional: Place a piece of white paper behind the beaker to better see any movement of liquid. Students should answer Question 1 on the Heat Convection in Fluids Worksheet.
8. Measure the temperature of the water at the top of the beaker every minute for three minutes. Place the thermometer at the same level in the beaker each time. Have students record the data on their worksheets.

Convection of a Gas

1. Obtain a new (unused) birthday candle and press the bottom of the candle into the center of the clay in the weighing dish so the candle stands up straight.
2. Add water to the bottom of the weighing dish, about halfway up the piece of clay (see Figure 4)
   {12665_Procedure_Figure_4}
3. Hold the 12" plastic tube in one hand and, using a butane safety lighter, light the candle.
4. Lower the tube over the candle, standing the tube upright in the weighing dish. The water should prevent any air from entering the tube from the bottom.
5. Time how long the candle remains lit once the tube is in place. (The candle should go out in a few seconds.) Students should record the burning time and appearance of the flame in Data Table 2 on the Heat Convection in Fluids Worksheet.
6. Once the candle goes out, grasp the tube near the top and remove it from the weighing dish. Dry the bottom of the tube, if necessary.
7. Obtain the foil-covered laminated T-shaped divider. Insert the vertical part of the T into the top of the tube (see Figure 5).
   {12665_Procedure_Figure_5}
8. Repeat Convection of a Gas steps 3–5—the candle should keep burning.
9. Allow the candle to burn for about 20 seconds. Students should record observations of the appearance of the flame.
10. Place one finger from each hand an inch above the tube, on opposite sides of the divider, for about two seconds (alternatively, have a student volunteer do this). Describe the felt difference in air temperature on each side of the divider. Have students record the observations (Question 5).
11. Remove the T-shaped divider, but leave the tube in place. Observe any changes in the flame (the candle should quickly go out). Students should note how much time elapses after the divider is removed until the flame goes out and record the time and any observations in Data Table 2.

Student Worksheet PDF

12665_Student1.pdf

Teacher Tips

• This kit contains enough materials to perform the demonstration at least seven times: 15 mL of red food coloring, 24 birthday candles, seven plastic weighing dishes, laminated template and aluminum foil (enough to make six reusable dividers), reusable glass tubing (2½" and 5"), a reusable 5 two-hole rubber stopper, and a reusable 12" plastic tube.
• The greater the difference in temperature of the two regions of water, the more noticeable the convection will be. Hot tap water is usually between 50–60 °C. A hot plate may be used to keep the water hot between demonstrations. Cold tap water is usually around 18–20 °C. A pitcher of ice water may be used to keep the water cold from one class to the next.
• Start each Convection of a Gas demonstration with a new candle.
• Be sure the candle is upright and not leaning. The heat from the flame can weaken the wall of the tube.
• Practice lowering the tube over the candle. If the flame does not go out after a few seconds without the T-shape divider in place, it is possible the tube was lowered too slowly over the candle, allowing a convection current to be established before the bottom of the tube was “sealed.” If the tube is lowered too quickly with the divider, the flame may go out before convection can be established. Lowering the tube in 2–3 seconds usually works well with and without the divider.
• Do not allow the candle to burn for more than one minute. Heat from the flame can cause distortions in the wall of the tube. Remove the cylinder and blow out the candle if the candle continues to burn when the divider is not in place. Allow the tube to cool between demonstrations.
• With the divider in place, the flame height and brightness may vary quite a bit. As the air around the flame gets hotter, the “parcel” of air will rise rapidly, and may temporarily interfere with the colder descending air. The flame will then begin to diminish. The air around the smaller flame will not heat up as much as before, resulting in a slower rate of ascending air that interferes less with the descending oxygenated air. The flame will once again burn brightly.
• Replace the aluminum covering for the laminated divider if it gets torn. The aluminum foil protects the divider from being singed or melting. Replace the divider if damaged.

Sample Data

Answers to Questions

1. During the Convection of a Liquid demonstration, describe any movement of water that was observed once the cold water covered the flask of hot water. Did this movement change over time? If so, how?

   A stream of colored water flowed out of the flask through the longer tube. The flow rate slowed over time.

2. If the flask and beaker setup were left for a longer period of time, would the convection of water eventually stop? Why or why not?

   Convection would eventually stop when thermal equilibrium was reached between the water in the beaker and the water in the flask.

3. During the Convection of a Gas demonstration, describe and compare the appearance of the flame in the tube when the divider was not in place with the appearance of the flame when the divider was in place.

   The flame got smaller and quickly went out when the cylinder was placed over the candle. With the divider in place, the candle continued to burn.

4. Record the observations that were described when two fingers were placed on opposite sides of the divider above the tube.

   One side of the divider felt very warm, the other side felt cooler.

5. Explain how the observations from this activity demonstrate that the divider allowed a convection current to form in the cylinder.

   When the divider was not in place, the rising warm (deoxygenated) air did not allow cooler oxygenated air to flow down into the tube. The burning candle quickly used up the oxygen in the tube and the flame was extinguished. With the divider in place, a difference in temperature was felt on either side, indicating warm air was rising on one side and cooler air was descending on the other side, providing oxygen for the candle which continued to burn.

Discussion

The Convection of a Liquid demonstration illustrates differences in densities of a liquid at two different temperatures. Each cubic centimeter of warm water is less dense than each cubic centimeter of cold water. The warm water rises to the top of the flask and flows out of one tube, while at the same time the cold water from the beaker sinks, flowing into the flask through the other tube. The addition of food coloring to the warm water allows students to observe the movement of the water. As the temperature difference between the two containers decreases, convection begins to slow down, and then eventually stops when thermal equilibrium is reached.

In the Convection of a Gas demonstration, the rising warm (deoxygenated) air prevents the cooler oxygenated air from flowing down into the tube. The burning candle quickly uses up the oxygen in the tube and the flame is extinguished. With the divider in place, the warm air rises up one side of the tube and the divider allows cooler, oxygenated air to flow down the other side. When the divider is removed, convection is once again disrupted and the flame goes out.

These demonstrations provide a visual image for students as they discuss real world convection patterns. Air, for example, is warmed by the sun and rises from the earth as surrounding colder air masses sink. This continually changing temperature differential results in a global pattern of atmospheric movement that influences local weather. All bodies of water are in continual density turnover as night and day warming/cooling cycles occur. The Earth’s internal heat drives convection circulation in the mantle. Many other examples of convection in fluids can be discussed and related to these demonstrations.