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Difficulty: MediumIdentifying Independent, Dependent, and Controlled Variables

An environmental scientist investigated the rate of nitrate leaching from agricultural soil amended with varying concentrations of biochar. Six identical soil columns were prepared, each containing 5 kg of topsoil. Biochar was added to the columns at concentrations of 0%, 2%, 4%, 6%, 8%, and 10% by mass. All columns were maintained at 22°C and received 50 mL of synthetic rainwater every 24 hours. The concentration of nitrate (NO3−NO_3^-) in the leachate collected from the bottom of each column was measured daily for 30 days.

In this investigation, which of the following represents the independent variable?

  1. The percentage of biochar added to each soil column by massAnswer
  2. B
    The concentration of nitrate (NO3−NO_3^-) in the collected leachate
  3. C
    The volume of synthetic rainwater added to each column every 24 hours
  4. D
    The total mass of topsoil placed in each column

Answer

The independent variable is the percentage of biochar added to each soil column by mass.
The correct answer specifies the percentage of biochar added to each soil column by mass because it is the specific factor altered systematically by the scientist to evaluate its impact on nitrate leaching.

Step-by-Step Solution

1
Identify the condition or factor systematically manipulated by the experimenter across test groups.
The researcher purposefully varied the biochar concentration across six specific levels (0%, 2%, 4%, 6%, 8%, and 10%).
The variable directly controlled and varied by the researcher to observe its effect is the independent variable.
2
Distinguish the independent variable from the dependent variable and controlled variables in the setup.
The biochar concentration is the independent variable, the nitrate leachate concentration is the dependent variable (measured response), and soil mass, temperature, and rainwater volume are controlled variables.
Controlled variables are kept constant across all trials to isolate the effect of the independent variable on the dependent variable.

Key Concept

Independent, Dependent, and Controlled Variables in Experimental Design
Estimated Time:1m 0s
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Topics

Identifying Independent, Dependent, and Controlled Variables

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