Edit the background information in the worksheet document to match the knowledge of the students. Younger students might not need the detailed thermodynamic equations.
Introduce the role of the Sun as the main heat source of the Earth, along the following discussion and questions.
Q: We experience day and night and the different seasons. What causes the different temperatures when the seasons change?
A: The Sun’s heating.
Q: How much of the Earth’s surface is covered by water and how much by land (approximately)?
A: About 71% is covered by water.
Q: So, between land and oceans, what is the heated the most by insolation?
A: The oceans.
Q: In the deserts, the temperature changes between day and night are very strong. Can you imagine why?
A: The solid surface absorbs heat and gets warm quickly. If something heats up very rapidly, it also cools down at a very high rate. In addition, deserts usually lack the ‘blanket’ of clouds that blocks the heat from warming the Earth.
Q: In comparison, how rapidly does water respond to heating? Imagine a pot of water on a stove.
A: This is a rather slow process.
EXPERIMENTAL SET-UP 1
It would be best if the students worked in groups of two to share responsibilities. They will measure the temperature changes of water and soil heated by a lamp over time. To ensure similar conditions for both substances, the experiment is carried out in two steps.
- Fill one tray with water and the other with soil or sand. The quantities should be the same.
- Place the tray filled with water below the lamp.
- Immerse the tip of the thermometer in the water. Its orientation should be as parallel as possible to the angle of irradiation. This grazing angle helps to reduce the direct heating of the thermometer.
- Let the substances assume room temperature.
- In the meantime, prepare a data table for filling in the measurements. It should allow for 21 lines of data and four columns (Time t (min) | Water ϑ (°C) | Soil ϑ (°C) | Difference Δϑ (°C)).
- The first column lists numbers from 0 to 20 (minutes).
Figure 5: Experimental set-up for the temperature measurement of water. A petri dish is filled with water, and a thermometer is ready to measure the temperature. In order to maintain the thermometer in an upright position, it is attached to the frame with a pin and a paper clip. The lamp is set to illuminate the dish (own work).
Q: What do you think: how do the temperatures change after switching on the lamp?
A: The temperature rises as long as the lamp is on. When it is switched off, the water will cool down again.
Q: After examining the water, you will use soil. Do you expect a different temperature response?
A: The rates of heating and cooling will be different.
Figure 6: While illuminating the water, the temperature is read from the thermometer every minute (own work).
Experimental procedure 1
The procedure is the same for both experiments. Begin with water. 1. Take the first temperature measurement before switching on the lamp. 2. Start the stop watch and switch on the lamp (Figure 6). 3. For 10 minutes, take a measurement every minute and note down the value in the corresponding row in the table. 4. After 10 minutes, switch off the lamp and continue measuring the temperature as before. 5. Stop measuring after 20 minutes. You should have 10 temperature readings with the light switched on and 10 with the light switched off. 6. Replace the water tray with the one filled with soil.
The data are analysed by producing a diagram that shows the time elapsed during the experiment vs. the temperatures measured.
While one of the students of any working group continues to fill the data into diagram (Figure 7), the other prepares the second experiment with the soil.
- Prepare a diagram (e.g. Figure 7, upper panel) with two axes. The horizontal axis lists the time elapsed during the experiment, while the vertical axis lists the temperatures. Be prepared for a temperature range between 20 and 35°C.
- While the thermometer assumes room temperature again, enter the data into the diagram. For each measurement, add a small cross at the coordinate that matches the time and the temperature.
- Connect the data points of the diagram.
Figure 7 : Template for the diagram to record the temperature measurements. The upper panel will contain the temporal changes in temperature. The lower panel is reserved for plotting the difference between the soil and water temperatures. Use different colours to represent the data (own work).
EXPERIMENTAL SET-UP 2
Let the students prepare the second experiment in the same way as the first (Figure 8). Instead of water, the students will examine the soil (dirt, sand).
Experimental procedure 2
The procedure is the same as before. 1. Take the first temperature measurement before switching on the lamp. 2. Start the stop watch and switch on the lamp. 3. For 10 minutes, take a measurement every minute and note down the value in corresponding row in the table. 4. After 10 minutes, switch off the lamp and continue measuring the temperature as before. 5. Stop measuring after 20 minutes. You should have 10 temperatures with the light switched on and 10 with the light switched off.
Figure 8: The set-up of the second experiment is the same as for the one with water, but this time the petri dish is filled with dirt, sand, or soil (own work).
Analysis 2 1. As before, add the new data to the diagram. 2. Connect the data points. 3. In the diagram, indicate the ranges, when the lamp was switched on and when it was switched off. 4. For each time step, calculate the temperature difference and add it to the table. 5. Prepare a second diagram that shows the temporal evolution of that difference. 6. Fill that diagram with the corresponding data.
Figure 9: Sample changes in the temperatures of water (blue) and soil (red). The black curve below depicts the difference between the two temperatures (not revealed to the students in the worksheet, own work).
HINTS FOR MODIFYING THE ACTIVITY
Instead of freshwater, one may use saltwater to investigate differences between them.
If you use a petri dish that provides two reservoirs (as shown in the images), you may be able to set up the experiment such that both measurements can be recorded simultaneously. However, It may be difficult to position the lamp such that it irradiates both samples equally. This set-up was not tested. So, it is recommended to try it first before modifying the activity.
Describe and discuss the results with your group members or classmates.
Q: How did the temperatures change while the lamp was switched on and off?
A: The temperature rises while the substances were illuminated, and they cooled down as soon as the lamp was off.
Q: Do you recognise a difference between the two substances?
A: The water heats up less and more slowly.
Remark: It should also cool down more slowly. However, the particular conditions of the experimental set-up may influence the cooling process, e.g. high room temperature.
Q: The temperatures rose differently. What is causing this? What changed between the experiments?
A: Everything stayed the same, except the substance to be probed. The observed difference must be caused by the properties of the substances.
Q: Imagine now the Earth with its oceans and continents. Describe how these components react to solar irradiation.
A: During daytime, the half of the Earth facing the Sun is irradiated, and its land masses and water are heated. During the night, the heat is released. Further, oceans warm up and cool down more slowly than continents.
Q: Can you determine the role the oceans have in the global climatic system? Think of what would happen to global temperatures without oceans.
A: The oceans have a dampening effect on the global warming, as they can store heat that otherwise would lead to higher global temperatures.
Q: What happens to the oceans, if the heating rate rises, while the cooling rate stays the same?
A: The temperature rises.