Experimental Design and Scientific Method

Experiment 1
Students investigated the rate of carbon dioxide ($CO_2$) evolution during the reaction between calcium carbonate ($CaCO_3$) and hydrochloric acid ($HCl$). The apparatus setup consisted of several components designed to ensure accurate measurement of dry gas at a stable temperature.

Based on the described experimental setup, match each apparatus component on the left with its specific procedural function on the right.

Experiment 1
Students measured the heat of combustion of three liquid fuel samples using a bomb calorimeter apparatus. A small sample of fuel was placed in a sealed steel reaction vessel (the bomb) filled with pure oxygen. The bomb was submerged in an insulated container filled with $2,000\text{ mL}$ of distilled water. An electric ignition wire ignited the fuel. A motorized stirrer continuously agitated the surrounding water bath, while a digital thermal sensor recorded the temperature of the water every 5 seconds until a maximum temperature was observed.

What was the primary purpose of continuously operating the motorized stirrer during this experimental procedure?

A team of atmospheric scientists investigated how varying the relative humidity inside a sealed reaction chamber affects the rate of ozone ($\text{O}_3$) depletion by aerosolized sea salt particles. In five separate trials, the relative humidity was set to 20%, 35%, 50%, 65%, and 80%, while the initial ozone concentration, temperature, and mass of sea salt particles were held constant. Based on the description of the experiment, which parameter was the dependent variable?

A team of materials engineers conducted an experiment to evaluate how different zirconium dioxide ($\text{ZrO}_2$) additive concentrations in ceramic heat-shield tiles affect peak thermal conductivity under simulated re-entry conditions. In each trial, ceramic tiles of identical dimensions were exposed to a $1200^\circ\text{C}$ plasma jet at a constant chamber atmospheric pressure of $1.0\text{ atm}$ for exactly 300 seconds. Match each experimental component to its correct variable classification.

Experiment 1

Students investigated the rate of enzymatic hydrolysis of $o$-nitrophenyl-$\beta$-D-galactopyranoside ($ONPG$) catalyzed by the enzyme lactase. The reaction yields $o$-nitrophenol, a product that absorbs light strongly at a wavelength of $420\text{ nm}$. To accurately measure the rate of product formation using a spectrophotometer without error from background absorbance or premature reaction, which of the following represents the correct chronological sequence of procedural steps the students must perform?

A biochemist conducted an experiment to investigate how substrate availability influences cellular respiration rates in yeast. Five identical incubation flasks were prepared, each containing $50\text{ mL}$ of yeast suspension in a buffer solution maintained at a constant temperature of $30^\circ\text{C}$ and a $\text{pH}$ of $6.5$. Different masses of glucose—$0.5\text{ g}$, $1.0\text{ g}$, $1.5\text{ g}$, $2.0\text{ g}$, and $2.5\text{ g}$—were added to Flasks 1 through 5, respectively. The volume of carbon dioxide ($\text{CO}_2$) gas generated by each flask was measured after $10\text{ minutes}$.

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

A marine biologist conducted an experiment to investigate how varying water salinity affects the oxygen consumption rate of shore crabs (*Carcinus maenas*). Five identical tanks were prepared with water salinities of $10\text{ ppt}$, $15\text{ ppt}$, $20\text{ ppt}$, $25\text{ ppt}$, and $30\text{ ppt}$. All tanks were held at a constant water temperature of $18^\circ\text{C}$, and crabs of equal body mass were used in each trial.

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

A team of acoustic engineers conducted an experiment to evaluate how the sound dampening efficiency of polyurethane foam panels is impacted by the concentration of embedded micro-perforated ceramic beads. Five panels of identical dimensions ($50\text{ cm} \times 50\text{ cm} \times 5\text{ cm}$) were manufactured with varying concentrations of ceramic beads ($0\%$, $5\%$, $10\%$, $15\%$, and $20\%$ by mass). Each panel was placed between a sound generator emitting a constant $1\text{-kHz}$ sine wave tone at $90\text{ dB}$ and a decibel sensor positioned $1.0\text{ m}$ away in an anechoic chamber held at $20^\circ\text{C}$. The decibel sensor measured the acoustic absorption coefficient ($\alpha$) of each panel. In this experiment, which factor served as the independent variable?

An environmental engineer conducted an experiment to evaluate wastewater purification efficiency. She prepared three identical filtration columns containing different concentrations of activated charcoal ($5\text{ g/L}$, $10\text{ g/L}$, and $15\text{ g/L}$). For all trials, she maintained a fluid temperature of $22^\circ\text{C}$, a flow rate of $10\text{ mL/min}$, and an initial methylene blue dye concentration of $50\text{ mg/L}$. She recorded the time required to achieve $99\%$ decolorization of the solution in each column.

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

Experiment 1
Students constructed a Soxhlet extraction apparatus to isolate lipids from an algal biomass sample using an organic solvent. The apparatus consists of a boiling flask on a heating mantle, a thimble chamber containing the solid algal sample, an outer vapor tube, a side-arm siphon tube, and a water-cooled reflux condenser mounted at the top.

During operation, solvent boiled in the flask rises as vapor, condenses, and continuously washes over the solid sample inside the extraction chamber until a siphon cycle is triggered.

Match each apparatus component below with its primary procedural function in the extraction process.

Study 1
Students determined the soil organic matter (SOM) percentage of a soil sample using a loss-on-ignition (LOI) procedure. The apparatus consisted of an analytical balance, porcelain crucibles, a drying oven ($105^\circ\text{C}$), a high-temperature muffle furnace ($550^\circ\text{C}$), and a desiccator containing active silica gel desiccant.

Based on standard laboratory procedures for analyzing soil composition and controlling moisture error, in what chronological sequence should the students perform the steps below?

Experiment 1
Students set up an electrolysis apparatus to produce hydrogen gas ($H_2$) and chlorine gas ($Cl_2$) from an aqueous sodium chloride ($NaCl$) solution. The apparatus consisted of a U-shaped glass tube separated into two distinct chambers by a porous ceramic diaphragm placed at the bottom bend. Platinum electrodes connected to a direct-current power source were immersed in each chamber. Chamber 1 contained the cathode, where $H_2$ gas and hydroxide ions ($OH^-$) were generated. Chamber 2 contained the anode, where $Cl_2$ gas was generated.

Based on the described experimental setup, what is the primary procedural function of the porous ceramic diaphragm between Chamber 1 and Chamber 2?

Study 1
Students performed a column chromatography experiment to isolate chlorophyll $a$ from a spinach leaf extract using a silica gel column and a hexane-acetone solvent mixture.

Based on standard laboratory protocol for column chromatography, place the following procedural steps in the correct chronological order from first to last.

Experiment 1
Students constructed a constant-volume gas thermometry apparatus to calibrate a high-precision temperature sensor by measuring pressure changes in a gas sample across various temperatures. The experimental assembly includes a constant-volume gas bulb, a variable-temperature oil bath, a mercury manometer, and a vacuum desiccant trap.

Match each apparatus component to its primary procedural function in the experiment.

Experiment 1
Students assembled a constant-head permeameter apparatus to measure the hydraulic conductivity ($K$) of a compacted soil core. The apparatus consisted of a cylindrical soil container fitted with porous end plates to retain the soil while allowing water flow. Water was continuously supplied to an upper overflow reservoir to maintain a constant head difference ($\Delta h$) between the inlet at the top and an overflow outlet at the bottom. Two manometer tubes were tapped directly into the side of the soil container at a fixed vertical distance ($L$) apart to monitor pressure head within the soil core. Water discharging from the bottom outlet was collected in a graduated cylinder over a measured time interval ($t$).

What was the primary function of tapping the two manometer tubes directly into the sides of the soil container rather than relying on the water levels in the external upper and lower supply reservoirs?

Experiment 1
Students performed a gravimetric analysis to determine the concentration of sulfate ions ($\text{SO}_4^{2-}$) in a water sample using a precipitation and vacuum filtration apparatus.

Arrange the following experimental steps in the correct procedural order from first to last to ensure complete precipitation, crystal growth, isolation, and mass determination of the resulting barium sulfate ($\text{BaSO}_4$).

Experiment 1
Students assembled a continuous-flow photocatalytic reactor system to evaluate the degradation kinetics of an aqueous organic pollutant under ultraviolet ($\text{UV}$) light. The system consists of four primary components: a peristaltic metering pump, a quartz reaction column coated internally with titanium dioxide ($\text{TiO}_2$), a degassing bubbler module supplying compressed air, and an inline UV-Vis flow-through spectrophotometer.

Match each apparatus component used in Experiment 1 to its primary function within the experimental setup.

Study 1
Students constructed a horizontal choice-chamber apparatus to investigate the light wavelength preferences of *Daphnia magna* (water fleas). The acrylic tube apparatus was divided into two equal volume sections—Chamber A (fitted with a 450 nm blue optical filter) and Chamber B (fitted with a 650 nm red optical filter)—separated by a central removable barrier. An overhead white light source provided uniform light intensity across both chambers.

For each trial, 40 *Daphnia* were placed into the central barrier compartment and kept in complete darkness for 5 minutes. The central barrier was then raised simultaneously with switching on the overhead light source. After 10 minutes, the barrier was lowered to trap the organisms in their respective chambers, and the number of *Daphnia* in each chamber was recorded.

What was the primary procedural purpose of keeping the *Daphnia magna* in complete darkness for 5 minutes prior to raising the central barrier and activating the light source?

Students performed an experiment to study the effects of different metal oxide catalysts on the rate of decomposition of hydrogen peroxide ($H_2O_2$) into water ($H_2O$) and oxygen gas ($O_2$). In each of 4 trials, the students added $5\text{ mL}$ of a $3\%$ $H_2O_2$ solution to a test tube at $25^\circ\text{C}$ containing either a specific catalyst or no catalyst. They measured the total volume of $O_2$ gas produced in milliliters ($\text{mL}$) over a period of 5 minutes. The experimental setups are summarized in the table below:

To determine the baseline rate of hydrogen peroxide decomposition in the absence of any catalyst, the students should refer to the results of which trial?

A student group designed four different scientific experiments but realized that each procedure contained a critical confounding variable or source of error that could compromise the validity of the results.

Match each experimental procedure on the left with its primary confounding variable or source of error on the right.