An atmospheric scientist conducts an experiment in a temperature-controlled environmental … — ACT

An atmospheric scientist conducts an experiment in a temperature-controlled environmental chamber to investigate the photolysis kinetics of nitryl chloride ( $ClNO_2$ ) under varying moisture conditions. During the experiment, the scientist adjusts the initial atmospheric water vapor concentration ( $H_2O$ mole fraction in ppmv) across five distinct trials while maintaining the total chamber pressure at $760\text{ Torr}$ , ambient temperature at $298\text{ K}$ , and UV actinic flux intensity at $1.5 \times 10^{16}\text{ photons}\cdot\text{cm}^{-2}\cdot\text{s}^{-1}$ . For each trial, the scientist uses cavity ring-down spectroscopy to quantify the first-order photolysis rate constant ( $k_{\text{phot}}$ in $\text{s}^{-1}$ ). Unintended background desorption of trace volatile organic compounds (VOCs) from the chamber walls occurs unpredictably between trials.

Match each experimental component described below to its correct variable classification.

The systematically altered initial concentration of atmospheric water vapor ( $H_2O$ mole fraction)Independent Variable
The measured first-order photolysis rate constant ( $k_{\text{phot}}$ )Dependent Variable
The fixed chamber pressure ( $760\text{ Torr}$ ), temperature ( $298\text{ K}$ ), and UV actinic flux intensityControlled Variable
The unpredictable desorption of trace volatile organic compounds (VOCs) from the chamber wallsConfounding Variable

Answer

The systematically altered water vapor concentration matches with the Independent Variable. The measured photolysis rate constant (

k_{\text{phot}}

) matches with the Dependent Variable. The fixed chamber pressure, temperature, and UV flux match with the Controlled Variable. The unpredictable VOC wall desorption matches with the Confounding Variable.

In experimental design, the independent variable is the parameter systematically varied by the researcher (

H_2O

concentration), while the dependent variable is the quantitative outcome measured in response (

k_{\text{phot}}

). Controlled variables are background conditions purposefully kept constant across all trials (temperature, pressure, and UV flux) so they do not influence the dependent variable. A confounding variable is an extraneous factor that fluctuates outside the researcher's control (unpredictable VOC wall desorption), potentially introducing unwanted variance into the measured results.

Step-by-Step Solution

Identify the factor directly manipulated by the experimenter.

The initial atmospheric water vapor concentration (

H_2O

mole fraction) is adjusted across five trials.

The factor intentionally varied to test its effect is the independent variable.

Identify the response parameter measured to observe the effect.

The first-order photolysis rate constant (

k_{\text{phot}}

) is measured using cavity ring-down spectroscopy.

The outcome measured to determine the effect of the independent variable is the dependent variable.

Identify conditions explicitly kept constant across all trials.

Chamber pressure (

760\text{ Torr}

), temperature (

298\text{ K}

), and UV actinic flux intensity are held fixed.

Parameters kept identical across conditions to isolate the relationship between independent and dependent variables are controlled variables.

Identify extraneous, uncontrolled factors that fluctuate independently.

Trace VOC desorption from chamber walls occurs unpredictably between trials.

An uncontrolled variable that can unintentionally influence the outcome is a confounding variable.

Key Concept

Classification of Variables in Experimental Design

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Key Concept

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