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Difficulty: MediumDesigning Follow-Up Experiments and Test Modifications

In a baseline experiment, a student measured the rate of yeast fermentation by recording the volume of CO2\text{CO}_2 gas produced in a 10%10\% glucose solution at a constant temperature of 30∘C30^\circ\text{C}. The student wants to design a follow-up experiment to determine the effect of pH on the fermentation rate. Arrange the following steps in the correct chronological order to properly execute this modified procedure.

  1. 1Prepare four yeast cultures with identical yeast and glucose concentrations, adjusting each culture to a different pH value (4, 6, 8, or 10).
  2. 2Place all four cultures in a single water bath maintained at a constant temperature of 30∘C30^\circ\text{C}.
  3. 3Allow the cultures to sit in the water bath to reach temperature equilibrium before sealing them with gas-collection tubes.
  4. 4Measure and record the volume of CO2\text{CO}_2 gas produced by each culture after a fixed duration of 20 minutes.

Answer

The correct chronological order is to first prepare the yeast cultures with different pH levels, then place them in the constant-temperature water bath, next allow the cultures to reach temperature equilibrium before sealing, and finally measure the volume of gas produced over the fixed 20-minute interval.
The correct sequence begins with preparing the experimental groups with varying pH (the independent variable) while maintaining constant concentrations. Next, the controlled variable (temperature) is applied by placing them in the water bath. The mixtures must then equilibrate to prevent thermal expansion from affecting measurements. Finally, the dependent variable (gas volume) is recorded over a standardized time interval.

Step-by-Step Solution

1
Vary the independent variable while holding other starting factors constant.
Yeast cultures are prepared with different pH values but identical yeast and glucose levels.
To isolate the effect of pH on fermentation.
2
Apply the temperature control variable to all test groups.
All cultures are incubated at 30∘C30^\circ\text{C}.
To ensure that temperature does not act as a confounding variable.
3
Equilibrate the temperature of the mixtures before sealing the apparatus.
Thermal expansion of gas is prevented from skewing fermentation measurements.
To eliminate physical gas expansion errors from the biological gas production data.
4
Measure the accumulation of the dependent variable over a set time period.
The final volume of gas produced in 20 minutes is recorded.
To calculate and compare the fermentation rates across different pH values.

Key Concept

Designing and sequencing follow-up experiments with controlled variables.
Estimated Time:1m 30s
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Topics

Designing Follow-Up Experiments and Test Modifications

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