Fifth-Grade Boiling-Point Investigation With Conflicting Results
Mrs. Okafor's fifth-grade class is investigating: 'Does adding salt change the temperature at which water boils?' Each of six groups receives 100 mL of water, table salt, a thermometer, and a hot plate. The groups design their own procedure. Results: Group 1 started with hot tap water and added 1 spoon of salt; it boiled in 2 minutes. Group 2 used cold water and 3 spoons; it took 7 minutes. Group 3 added salt after boiling began and said salt had no effect. Group 4 stirred continuously and recorded 102 degrees Celsius. Group 5 did not measure the salt and recorded 100 degrees Celsius. Group 6's thermometer touched the bottom of the pot and read 105 degrees Celsius. The class concludes salt makes water boil faster. Mrs. Okafor has one class period to debrief and must address safety and the upcoming science fair.
1. Identify the uncontrolled variables in the investigation and explain how each one could affect the class conclusion.
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The investigation has several uncontrolled variables, so the comparison is not valid. Starting water temperature differs: Group 1 used hot tap water, so its 2-minute time mainly reflects a higher starting temperature, not salt. Amount of salt differs: Group 2 used 3 spoons while Group 1 used 1 spoon, so any difference in time or boiling temperature is confounded with concentration. Timing of salt addition differs: Group 3 added salt after boiling began, which cannot test whether salt changes the boiling point because the water was already boiling. Stirring differs: Group 4 stirred continuously, which can affect temperature readings and heating rate. Salt measurement is missing in Group 5, so the independent variable is unknown. Thermometer placement differs in Group 6; a thermometer touching the bottom reads the hot plate temperature, not the water temperature. The conclusion that salt makes water boil faster is therefore unsupported. The evidence is consistent with several explanations, including different starting temperatures and different amounts of salt. The class needs to separate the effect of salt from the effect of heating conditions.
2. Design a revised procedure and data table that would allow the class to test the question more fairly. Include the variables to control and the measurements to record.
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Revised question: 'How does the amount of table salt affect the boiling point of water?' Use the same volume of water (100 mL) at the same starting temperature (room-temperature bottled water), the same hot plate setting, the same pot, and the same thermometer placement in the water without touching the bottom. The independent variable is salt amount: 0 g, 5 g, and 10 g, each tested twice. The dependent variable is the highest temperature reached once the water is boiling steadily, recorded in degrees Celsius. Controls: same starting volume, same starting temperature, same stirring procedure (no stirring except a single swirl before heating), same timing method, same thermometer, and same hot plate. Safety: goggles, teacher-approved hot plate, no tasting, keep water below a rolling boil that splashes, and keep electrical cords away from water.
A data table should have columns for salt amount, trial, starting temperature, time to boil, boiling temperature, and observations. Students should calculate an average boiling temperature for each salt amount and compare. If the temperatures differ only slightly, students should consider thermometer precision and whether the difference is meaningful. The revised design does not test 'faster' unless time to boil is also controlled; it tests boiling temperature, which is the clearer question.
3. How should Mrs. Okafor respond to the class's current conclusion and to the safety issues, while still using the experience to prepare students for the science fair?
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She should not simply tell students they are wrong. She should display the six results and ask: 'What changed between groups besides the salt?' Students can list starting temperature, salt amount, stirring, thermometer placement, and timing. Then she can ask which conclusion the data support. The class should conclude that the investigation was not a fair test because too many variables changed. This is a productive failure, not a wasted lesson.
For safety, she should name the specific hazards: hot water, hot plate, glass or metal containers, electrical cords, and thermometer breakage. Students should never taste the salt water or touch the hot plate. Group 4's continuous stirring and Group 6's thermometer placement should be corrected in the revised procedure. For the science fair, students can turn the flawed investigation into a methods lesson: they write a revised question, identify controls, and explain why their first results were inconclusive. Mrs. Okafor can use a quick exit ticket asking students to name one controlled variable and one changed variable. This assesses understanding of experimental design rather than recall of a boiling-point fact.