A team of atmospheric chemists investigated photochemical smog formation by measuring ozon… — ACT

A team of atmospheric chemists investigated photochemical smog formation by measuring ozone ( $O_3$ ) production in a sealed $10\text{ m}^3$ environmental chamber. Across five experimental trials, the researchers systematically varied the ultraviolet (UV) light irradiance level ( $20$ , $40$ , $60$ , $80$ , and $100\text{ W/m}^2$ ). In all trials, chamber temperature ( $25^\circ\text{C}$ ), relative humidity ( $50\%$ ), and initial concentrations of nitrogen dioxide ( $NO_2$ , $0.5\text{ ppm}$ ) and volatile organic compounds ( $VOCs$ , $2.0\text{ ppm}$ ) were held constant. The rate of $O_3$ concentration accumulation was recorded over a 60-minute period.

Match each experimental component to its correct variable classification.

Ultraviolet (UV) light irradiance level ( $20\text{--}100\text{ W/m}^2$ )Independent Variable
Rate of $O_3$ concentration accumulation after 60 minutesDependent Variable
Chamber temperature ( $25^\circ\text{C}$ ) and initial $NO_2$ concentration ( $0.5\text{ ppm}$ )Controlled Variable

Answer

Ultraviolet light irradiance corresponds to the Independent Variable, rate of ozone accumulation corresponds to the Dependent Variable, and chamber temperature along with initial reactant concentrations correspond to Controlled Variables.

In experimental design, the independent variable is the condition deliberately manipulated by the researcher (UV irradiance), the dependent variable is the measured effect resulting from that manipulation (ozone accumulation rate), and controlled variables are background factors held constant across all conditions (chamber temperature and initial reactant concentrations).

Step-by-Step Solution

Identify the factor directly manipulated by the experimenter across trials.

The UV light irradiance level was adjusted between

20\text{ W/m}^2

and

100\text{ W/m}^2

The variable intentionally varied to test its influence is the independent variable.

Identify the factor measured as a result of the experimental manipulation.

The accumulation rate of

O_3

was observed and recorded.

The outcome measured by researchers that responds to changes in the independent variable is the dependent variable.

Identify the environmental parameters kept invariant throughout the experiment.

Temperature (

25^\circ\text{C}

), humidity (

50\%

), and initial

NO_2

VOC

levels were held unchanged across all trials.

Factors kept constant to eliminate confounding influences are controlled variables.

Key Concept

Classification of Independent, Dependent, and Controlled Variables in Controlled Experiments

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