Solar Water Heater Test: Separating Surface Effects from Confounds
Mr. Okafor's physical science class builds solar water heaters from identical 250 mL plastic bottles painted black. Group A leaves its bottle bare; Group B wraps its bottle in a clear plastic sleeve; Group C wraps its bottle in aluminum foil. All three are filled with 250 mL of water and placed on the same concrete pad at 11:40 a.m. Group A starts at 22°C, Group B at 24°C, and Group C at 22°C. After 30 minutes, the temperatures are 38°C, 36°C, and 31°C. A cloud passes over the pad from 12:02 to 12:06 p.m., and Group C's bottle is partly shaded by a fence for the last 10 minutes. The next day the class repeats the test, but Group A uses 400 mL of water and starts at 20°C. A student concludes, 'Black paint is best because Group A reached the highest temperature, and the clear sleeve actually cooled the water.'
1. Calculate each group's temperature change and evaluate the conclusion that black paint is best and the clear sleeve cooled the water.
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Temperature changes:
- Group A: 38°C − 22°C = 16°C
- Group B: 36°C − 24°C = 12°C
- Group C: 31°C − 22°C = 9°C
Group A did warm the most, so the black surface is a reasonable candidate for best absorber. However, the comparison is not clean. Group B started 2°C warmer, so its water was closer to the surroundings and may have gained heat more slowly; its 12°C change cannot be compared directly with A's 16°C change. Group C was shaded by a fence for the last 10 minutes, so its 9°C change is also confounded.
The claim that the clear sleeve 'cooled' the water is unsupported. Group B's water rose 12°C, so it gained thermal energy. A lower final temperature than Group A does not mean cooling; it may mean less absorption or more heat loss. A defensible conclusion requires equal starting temperatures and equal sunlight exposure.
2. Identify the variables the class intended to control and the variables that actually confounded the first test.
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Intended controls: bottle size, water volume (250 mL), starting time, placement on the same pad, and 30-minute heating period. The bottles were also all painted black underneath the coverings, which helps isolate the covering material.
Actual confounds:
- Starting water temperatures differed (22°C, 24°C, 22°C).
- Group C was partly shaded by a fence.
- A cloud reduced sunlight for four minutes during the test.
- The coverings may affect both absorption and heat loss, but the data cannot separate those effects.
Because starting temperature and shading differ among groups, the final temperatures cannot be attributed to the covering alone.
3. Design an improved procedure and explain how it tests the effect of surface covering on solar heating.
Read the worked answer
Fill identical black bottles with the same volume of water and let them sit in the room until all reach the same starting temperature, for example 20°C. Place them side by side on the same unshaded pad, with a bare black bottle as the control. Record temperature every 5 minutes for 30 minutes, rotate bottle positions every 10 minutes to average out uneven sunlight, and repeat the test on another day. Include a plain black control and one covering per test.
This isolates the covering as the variable. Absorbed solar radiation increases the water's internal energy; a clear sleeve may transmit sunlight while reducing heat loss, and foil may reflect radiation. Equal starting temperatures and rotation reduce the chance that position or initial temperature explains the result.