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Difficulty: HardIdentifying Sources of Error and Confounding Variables

A group of students designed several investigations to study how wind speed affects the rate of water evaporation. For each investigation, they set up two trials with different wind speeds. However, each setup introduced a distinct confounding variable or source of error. Match each experimental setup to the primary confounding variable or source of error that threatens its internal validity.

  • Two identical 150 mL150\text{ mL} beakers, each containing 100 mL100\text{ mL} of water, are exposed to different wind speeds. One beaker is positioned directly beneath a laboratory ventilation duct that blows warm air, while the other is placed in a cooler corner of the room.Differences in thermal energy input
  • The evaporation rate under high wind speed is measured using water in a wide-mouthed Petri dish (diameter 10 cm10\text{ cm}), while the rate under low wind speed is measured using water in a narrow beaker (diameter 4 cm4\text{ cm}).Differences in exposed liquid-gas interface area
  • The high-wind trial is performed using a 1.0 M1.0\text{ M} sodium chloride (NaCl\text{NaCl}) aqueous solution, while the low-wind trial is performed using pure, deionized water.Differences in solute concentration affecting vapor pressure
  • Evaporation rates are compared by measuring the volume of water lost after a 60-minute60\text{-minute} exposure for the high-wind trial, and after a 120-minute120\text{-minute} exposure for the low-wind trial.Differences in total duration of evaporation

Answer

The experimental setups match their confounding variables as follows: Setup 1 matches with differences in thermal energy input; Setup 2 matches with differences in exposed liquid-gas interface area; Setup 3 matches with differences in solute concentration; Setup 4 matches with differences in total duration of evaporation.
Each experimental setup introduces a distinct uncontrolled variable: temperature variation corresponds to differences in thermal energy input; diameter differences correspond to exposed liquid-gas interface area; the presence of sodium chloride corresponds to solute concentration; and unequal trial lengths correspond to duration of evaporation.

Step-by-Step Solution

1
Analyze Setup 1, which places one beaker under warm ventilation air and the other in a cooler corner.
This setup introduces temperature differences.
Since temperature directly affects kinetic energy and evaporation rate, this represents differences in thermal energy input.
2
Analyze Setup 2, which uses a 10 cm10\text{ cm} Petri dish and a 4 cm4\text{ cm} beaker.
This setup introduces variations in the surface area of the water exposed to air.
Water evaporates only from its surface, so changing the diameter alters the exposed liquid-gas interface area.
3
Analyze Setup 3, which compares a sodium chloride (NaCl\text{NaCl}) solution to deionized water.
This introduces solute concentration variations.
Solutes lower the chemical potential of the solvent and lower the vapor pressure, affecting evaporation independent of wind.
4
Analyze Setup 4, which measures evaporation over 60 minutes60\text{ minutes} versus 120 minutes120\text{ minutes}.
This setup varies the duration of the trial.
Unequal time intervals prevent a direct comparison of total volume lost unless normalized, representing differences in total duration of evaporation.

Key Concept

An experimental design must control all variables except the independent variable (wind speed). Any uncontrolled variable that can affect the dependent variable (evaporation rate) is a confounding factor that introduces potential error.
Estimated Time:2m 30s
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Identifying Sources of Error and Confounding Variables

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