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Difficulty: MediumIdentifying Independent, Dependent, and Controlled Variables

A group of atmospheric scientists investigated the effect of methane (CH4\text{CH}_4) concentration on the rate of photochemical haze formation in a simulated planetary atmosphere. Five identical glass chambers were filled with a gas mixture consisting of nitrogen (N2\text{N}_2), carbon dioxide (CO2\text{CO}_2), and varying amounts of methane (CH4\text{CH}_4). All chambers were maintained at a constant temperature of 298 K298\text{ K} and exposed to an ultraviolet (UV\text{UV}) light intensity of 50 W/m250\text{ W/m}^2 for 12 hours12\text{ hours}. The optical density of the resulting haze in each chamber was measured at the end of the trial.

Based on the experimental design described, which of the following parameters served as a controlled variable?

  1. The temperature of the glass reaction chambers (298 K298\text{ K})Answer
  2. B
    The concentration of methane (CH4\text{CH}_4) in the gas mixture
  3. C
    The optical density of the photochemical haze measured after 12 hours12\text{ hours}
  4. D
    The chemical composition of the haze particles formed

Answer

The temperature of the glass reaction chambers (298 K298\text{ K}) served as a controlled variable because it was kept constant across all trials.
In scientific experiments, controlled variables are parameters kept constant across all trials so that they do not influence the relationship between the independent and dependent variables. The text explicitly states that all five chambers were maintained at a constant temperature of 298 K298\text{ K}, making chamber temperature a controlled variable.

Step-by-Step Solution

1
Identify the experimental manipulation (Independent Variable)
The researcher varied the concentration of methane (CH4\text{CH}_4) across the five chambers.
The variable intentionally changed by the experimenter is the independent variable.
2
Identify the measured outcome (Dependent Variable)
The researcher measured the optical density of the haze at the end of the trial.
The observed or measured response is the dependent variable.
3
Identify parameters held constant (Controlled Variables)
The temperature (298 K298\text{ K}), UV light intensity (50 W/m250\text{ W/m}^2), and duration (12 hours12\text{ hours}) were maintained equally for all chambers.
Controlled variables are conditions kept uniform to ensure a fair test where only the independent variable affects the dependent variable.

Key Concept

Controlled variables are experimental conditions kept constant across all test groups so that changes in the dependent variable can be attributed solely to the independent variable.
Estimated Time:1m 0s
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Topics

Identifying Independent, Dependent, and Controlled Variables

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