Experimental Design and Scientific Method

A team of marine biologists conducted an investigation to determine how varying concentrations of dissolved iron ($\text{Fe}^{2+}$) in seawater affect the growth rate of the phytoplankton species *Emiliania huxleyi*. Four 10 L culture vessels were filled with filtered seawater and kept at a constant temperature of $18^\circ\text{C}$ under identical 12-hour light/14-hour dark photoperiods. Each vessel received a different concentration of $\text{Fe}^{2+}$ ($0.1\ \mu\text{M}$, $0.5\ \mu\text{M}$, $1.0\ \mu\text{M}$, and $2.0\ \mu\text{M}$). After 14 days, the final cell density ($\text{cells/mL}$) of *Emiliania huxleyi* in each vessel was recorded.

Which of the following represents the independent variable in this experiment?

Biochemists investigated whether adding proline, an amino acid, protects the marine microalga *Nannochloropsis oculata* from high-salinity stress. They prepared 4 culture flasks under identical light and temperature conditions for 72 hours:

• Flask 1: Standard nutrient medium ($35\text{ ppt NaCl}$) + *N. oculata*
• Flask 2: Elevated salinity medium ($70\text{ ppt NaCl}$) + *N. oculata*
• Flask 3: Elevated salinity medium ($70\text{ ppt NaCl}$) + $10\text{ mM}$ proline + *N. oculata*
• Flask 4: Elevated salinity medium ($70\text{ ppt NaCl}$) without *N. oculata* (cell-free)

Match each experimental flask setup to its primary methodological purpose in the experimental design.

Botanists conducted an experiment to evaluate how varying concentrations of ground-level ozone ($O_3$) affect the stomatal conductance of soybean (*Glycine max*) leaves over a 14-day period. Four identical growth chambers were maintained at constant temperature ($25^\circ\text{C}$), relative humidity ($65\%$), and photosynthetic photon flux density. The chambers received the following gaseous treatments:

- Chamber 1: Charcoal-filtered air containing $0\text{ ppb}$ of $O_3$.
- Chamber 2: Purified air containing $30\text{ ppb}$ of $O_3$.
- Chamber 3: Purified air containing $60\text{ ppb}$ of $O_3$.
- Chamber 4: Purified air containing $90\text{ ppb}$ of $O_3$.

Which chamber served as the negative control group in this experiment, and what baseline condition did it establish?

Atmospheric scientists conducted an experiment to evaluate how varying the ambient relative humidity ($\text{RH}$, measured as a percentage) influences the crystallization time (in seconds) of acoustically levitated ammonium sulfate ($(\text{NH}_4)_2\text{SO}_4$) microdroplets. Throughout all trials, the ambient temperature was held constant at $20.0^\circ\text{C}$ and the initial droplet radius was held constant at $5.0\,\mu\text{m}$.

Match each experimental component from the investigation to its corresponding variable classification.

A materials scientist investigated how the average pore diameter of polyurethane acoustic foam panels affects their sound absorption performance. Four foam panels of identical thickness ($50\text{ mm}$) were tested in a sound chamber at a constant ambient temperature ($22^\circ\text{C}$) and fixed incident sound frequency ($1,000\text{ Hz}$). The experimental conditions and measured outcomes are summarized in the table below:

Which of the following correctly identifies the independent variable and the dependent variable in this investigation?

A biochemist investigated how varying the pH of a reaction mixture affects the rate of hydrogen peroxide ($\text{H}_2\text{O}_2$) decomposition by the enzyme catalase. In each trial, the catalase concentration, total reaction volume, and incubation temperature ($25^\circ\text{C}$) were held constant, while the volume of oxygen gas ($\text{O}_2$) produced per minute was measured across solutions ranging from $\text{pH } 4.0$ to $\text{pH } 10.0$.

Match each experimental component from the study to its correct variable classification.

A team of biomedical researchers investigated the inhibitory effect of a plant-derived polyphenol, compound CP, on bacterial biofilm formation by *Pseudomonas aeruginosa* on silicone catheter surfaces. The researchers prepared four different culture setups in microtiter plate wells and measured biofilm mass after incubation at $37^\circ\text{C}$ for 24 hours:

- Setup I: Silicone catheter disc + nutrient broth + *P. aeruginosa* + $0\text{ }\mu\text{g/mL}$ compound CP (vehicle solvent only)
- Setup II: Silicone catheter disc + nutrient broth + *P. aeruginosa* + $50\text{ }\mu\text{g/mL}$ compound CP
- Setup III: Silicone catheter disc + nutrient broth (no bacteria added) + $0\text{ }\mu\text{g/mL}$ compound CP
- Setup IV: Nutrient broth only (no catheter disc, no bacteria added)

Based on the experimental design, match each culture setup (Setups I–IV) to its specific experimental role or baseline function.

A team of environmental engineers conducted an experiment to evaluate how varying concentrations of copper sulfate ($\text{CuSO}_4$) in water affect the photosynthetic rate of *Chlorella vulgaris* algae. Four identical $500\text{ mL}$ glass bioreactors were prepared with equal initial algal cell densities. Each bioreactor received a different concentration of $\text{CuSO}_4$ ($0.0\text{ mg/L}$, $0.5\text{ mg/L}$, $1.0\text{ mg/L}$, and $1.5\text{ mg/L}$). All bioreactors were maintained at a constant temperature of $22^\circ\text{C}$ and exposed to continuous artificial light of $150\text{ }\mu\text{mol}\cdot\text{m}^{-2}\cdot\text{s}^{-1}$ for 72 hours. At the end of the trial, the total dissolved oxygen produced ($\text{mg/L}$) was measured to quantify the photosynthetic rate of the algae.

Which of the following identifies the dependent variable in this experiment?

A student group investigated the enzymatic degradation of cellobiose by cellulase under varying concentrations of a potential heavy-metal inhibitor, cadmium chloride ($CdCl_2$). The cellulase enzyme was dissolved in a $0.05\text{ M}$ citrate buffer solution at $\text{pH } 4.8$.

Four reaction vessels were prepared as shown in the table below:

All vessels were incubated at $37^\circ\text{C}$ for 30 minutes, and the glucose concentration resulting from cellobiose breakdown was measured.

Based on the experimental design, which vessel served as the baseline control to measure uninhibited enzymatic activity?

A student performs an experiment to determine the specific heat capacity of an unknown solid alloy block using a water calorimeter. The apparatus includes a hot plate with a boiling water bath, an insulated calorimeter cup containing cool distilled water, a temperature probe, and a manual stirrer.

Based on standard experimental procedure, arrange the following steps in the correct chronological sequence from first to last:

A researcher conducted an investigation to determine how varying the strength of an external magnetic field influences the thermal conductivity of a magnetite-based ferrofluid. In each trial, a ferrofluid sample of fixed volume ($50\text{ mL}$) and fixed magnetite concentration ($5.0\%\text{ v/v}$) was placed inside a thermal cell maintained at $25^\circ\text{C}$. The applied magnetic field strength was varied from $0\text{ mT}$ to $200\text{ mT}$ in increments of $50\text{ mT}$, and the resulting thermal conductivity of the ferrofluid was measured.

Match each experimental component on the left with its correct variable classification on the right.

A group of atmospheric scientists investigated the effect of methane ($\text{CH}_4$) 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 ($\text{N}_2$), carbon dioxide ($\text{CO}_2$), and varying amounts of methane ($\text{CH}_4$). All chambers were maintained at a constant temperature of $298\text{ K}$ and exposed to an ultraviolet ($\text{UV}$) light intensity of $50\text{ W/m}^2$ for $12\text{ hours}$. 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?

A team of volcanologists conducted an investigation to measure how varying the silica ($\text{SiO}_2$) mass percentage in synthetic basaltic lava affects its flow viscosity. Each lava sample was melted in a specialized induction furnace maintained at a constant temperature of $1,200^\circ\text{C}$ under standard atmospheric pressure ($1\text{ atm}$). Match each experimental factor on the left with its corresponding variable role on the right.

Experiment 1
Students constructed a continuous-flow soil leaching apparatus to analyze solute transport through saturated soil columns under controlled hydraulic conditions.

The apparatus consists of four primary components:
1. A Vacuum Degasser Unit attached to the influent water supply line before fluid enters the soil column.
2. A Mariotte Bottle Reservoir mounted above the column to supply the influent liquid.
3. An In-Line Electrical Conductivity (EC) Sensor installed directly at the outflow port at the column base.
4. An Automated Fraction Collector positioned beneath the outflow port.

Match each component of the experimental apparatus with its primary procedural function.

A marine biologist conducted an experiment to investigate how varying water salinity affects the bioluminescence of the dinoflagellate *Pyrocystis fusiformis*. Four identical $500\text{ mL}$ cultures were maintained at $22^\circ\text{C}$ with a cell density of $1,000\text{ cells/mL}$. Each culture was exposed to a different salinity level ($20\text{ PSU}$, $25\text{ PSU}$, $30\text{ PSU}$, and $35\text{ PSU}$) and subjected to identical mechanical agitation to induce bioluminescence, after which the peak light intensity was measured.

Match each experimental component listed on the left to its corresponding variable classification on the right.

A materials science research group conducted an experiment to analyze the photo-degradation of polylactic acid (PLA) films. Four identical $0.5\text{ mm}$ thick PLA samples were placed into separate sealed test chambers, each maintained at a constant temperature of $25^\circ\text{C}$ and relative humidity of $50\%$. Each sample was exposed to a different UV-C radiation intensity ($2.0$, $4.0$, $6.0$, and $8.0\text{ W/m}^2$) for 48 hours, after which the percentage reduction in sample mass was measured.

Which of the following correctly identifies the independent variable and the dependent variable in this experiment?

Experiment 1

Students set up a capillary potometer to measure the rate of water uptake (transpiration) by a leafy plant shoot under different light intensities. The apparatus consists of a plant stem fitted tightly into a water-filled glass tube connected horizontally to a calibrated capillary tube containing a single air bubble. Attached above the capillary tube is a water reservoir controlled by a stopcock valve.

Based on the described experimental setup, which of the following best explains the purpose of including the water reservoir and stopcock valve in this apparatus?

Experiment 1
Students conducted a gravimetric analysis to determine the phosphate concentration in a water sample by precipitating magnesium ammonium phosphate hexahydrate ($\text{MgNH}_4\text{PO}_4 \cdot 6\text{H}_2\text{O}$). The apparatus included a Büchner funnel, a heavy-walled filter flask attached to a vacuum aspirator line, filter paper, a wash bottle filled with ice-cold dilute ammonia solution, and an oven.

To ensure quantitative transfer, efficient impurity removal, and accurate final mass measurement of the precipitate without losing product, in what chronological order should the students perform the following steps of the filtration and drying procedure?

A soil scientist investigated the effect of varying soil copper ($\text{Cu}^{2+}$) concentrations on the rate of nitrate uptake in spinach roots. Five groups of ten plants each were grown in identical hydroponic solutions containing $\text{Cu}^{2+}$ concentrations of $0\text{ }\mu\text{M}$, $5\text{ }\mu\text{M}$, $10\text{ }\mu\text{M}$, $25\text{ }\mu\text{M}$, and $50\text{ }\mu\text{M}$, respectively. Throughout the 14-day experiment, all plant groups were maintained at $22^\circ\text{C}$ under a constant 16-hour light / 8-hour dark cycle. At the conclusion of the experiment, the researcher recorded the total nitrate concentration ($\mu\text{mol/g}$) remaining in the nutrient solutions.

In this investigation, which factor serves as the independent variable?

A group of mechanical engineers conducted an experiment to evaluate how the frequency of ambient mechanical vibration affects the peak electrical voltage output of a piezoelectric cantilever beam. In each trial, a cantilever beam of identical length ($10\text{ cm}$), thickness ($1\text{ mm}$), and material composition (lead zirconate titanate) was mounted on a shaker table maintained at a constant displacement amplitude of $0.5\text{ mm}$ and a room temperature of $22^\circ\text{C}$. The vibration frequency was varied across $\text{50 Hz}$, $\text{100 Hz}$, $\text{150 Hz}$, and $\text{200 Hz}$, and the peak output voltage generated across a $100\text{ }\Omega$ load resistor was recorded.

Match each experimental component listed on the left with its corresponding variable classification on the right.