All practice questions

Desert varnish is a thin, dark coating found on rock surfaces in arid environments. It is primarily composed of clay minerals, manganese ($Mn$) oxides, and iron ($Fe$) oxides. Two scientists discuss the mechanism behind its formation.

Scientist 1
Desert varnish is formed biochemically by manganese-oxidizing bacteria (such as the genus $\mathit{Metallogenium}$). These bacteria inhabit rock surfaces and utilize soluble divalent manganese ($Mn^{2+}$) from windborne dust as an energy source, oxidizing it to insoluble tetravalent manganese ($Mn^{4+}$) oxides. These oxides, along with clay particles, are cemented to the rock surface by bacterial extracellular polymeric substances (EPS). This biochemical process requires living microbial cells, organic carbon nutrients, and trace liquid water.

Scientist 2
Desert varnish forms through a purely inorganic chemical-physical process. During wet periods, dew or light rain dissolves amorphous silica ($SiO_2$) and trace metals from windborne dust on rock surfaces. As the rock heats and dries, the silica precipitates, forming a silica-rich glaze. This glaze physically traps ambient, pre-oxidized manganese and iron oxides from the dust, cementing them to the rock. This process does not require living organisms, organic nutrients, or biological activity, and can occur in completely sterile environments.

Which of the following experimental procedures would provide the most definitive evidence to resolve the conflict between the two scientists' models?

A group of students studied the decomposition of sodium bicarbonate ($NaHCO_3$) in aqueous solution. They proposed a hypothesis: *The rate of $NaHCO_3$ decomposition increases as the initial concentration of $NaHCO_3$ increases because a higher concentration of reactant particles leads to more frequent collisions.*

To test this, they performed two experiments. In Experiment 1, they measured the volume of carbon dioxide ($CO_2$) gas produced over $10\text{ min}$ at a constant temperature of $25^\circ\text{C}$ using different initial concentrations of $NaHCO_3$ ($0.1\text{ M}$, $0.2\text{ M}$, and $0.3\text{ M}$). In Experiment 2, they repeated the trials at a constant temperature of $50^\circ\text{C}$. The results are shown in the table:

Which of the following statements best describes how the students should modify their hypothesis?

### Coral Bleaching Debate

Scientist 1
Mass coral bleaching is primarily driven by rising sea surface temperatures (SST) due to global climate change. When SST exceeds a threshold of $30^\circ\text{C}$, the symbiotic zooxanthellae algae are expelled from the host coral, leading to bleaching. Although local factors like agricultural runoff can stress corals, they only cause localized damage and cannot trigger mass bleaching events.

Scientist 2
Agricultural fertilizer runoff is the primary cause of mass coral bleaching. Increased nitrogen levels in runoff stimulate excessive algal growth, which disrupts the coral-zooxanthellae relationship. While rising SST ($>30^\circ\text{C}$) exacerbates this disruption, elevated temperatures alone do not cause mass bleaching without high nutrient levels.

Based on the passage, both Scientist 1 and Scientist 2 would agree with which of the following statements?

A student investigates the corrosion of iron in various acidic solutions. The student hypothesizes that the rate of corrosion, measured by the mass loss of an iron nail after $48\text{ hours}$, increases linearly as the pH of the solution decreases from $7.0$ to $2.0$. The student places identical iron nails in solutions of varying pH and records the mass loss in the table below:

Based on these results, which of the following statements best describes how the student should modify their hypothesis?

A student hypothesizes that as the angle of an inclined plane increases, the time it takes for a block to slide down the plane will decrease. The student conducts an experiment and measures the slide times at angles of $15^\circ$, $30^\circ$, and $45^\circ$. If the student's hypothesis is correct, which of the following predictions is most likely true for the slide time of the block at an angle of $60^\circ$?

The Late Devonian mass extinction (approximately 372 million years ago) is characterized by a major loss of marine biodiversity and elevated concentrations of mercury ($Hg$) in sedimentary layers globally. Three hypotheses discuss the triggers and mechanisms of this extinction event.

Hypothesis 1
The extinction was triggered by the eruption of the Viluy Large Igneous Province (LIP). Massive volcanic eruptions released large volumes of carbon dioxide ($CO_2$) and gaseous $Hg$ into the atmosphere. The greenhouse effect from $CO_2$ caused rapid global warming and ocean stratification, leading to widespread marine anoxia (lack of oxygen). Meanwhile, atmospheric deposition of $Hg$ created global spikes in sedimentary mercury, poisoning marine ecosystems.

Hypothesis 2
The extinction was caused by a major asteroid impact. The impact vaporized target rocks, ejecting dust and sulfur compounds into the stratosphere, which blocked sunlight and caused a severe "impact winter" (global cooling). Acid rain from sulfur aerosols accelerated continental weathering, washing deep-seated terrestrial $Hg$ deposits into the oceans. This resulted in elevated sedimentary $Hg$ deposition and poisoned shallow marine habitats.

Hypothesis 3
The extinction was driven by sea-level fluctuations that forced deep, oxygen-depleted, and toxic hydrogen sulfide-rich ($H_2S$) waters onto shallow continental shelves. This toxic upwelling directly suffocated marine life. The high affinity of mercury for organic matter and sulfides caused $Hg$ already present in the ocean to bind rapidly to organic-rich sediments on the shelves, creating an apparent sediment $Hg$ anomaly without requiring any global atmospheric source of mercury.

Match each scientific statement with the combination of viewpoints that supports it.

A student proposes the following hypothesis:

*Hypothesis*: Plants grown under blue light will grow taller than plants grown under green light or red light.

To test this hypothesis, the student grows $3$ identical pea plants under different wavelengths of light for $14$ days, keeping all other variables constant. The heights of the plants at the end of the experiment are shown in the table below:

Based on these results, which of the following statements best describes how the student should modify the hypothesis?

### Banded Iron Formations

Banded Iron Formations (BIFs) are ancient sedimentary rocks consisting of alternating layers of iron-rich minerals (such as magnetite) and silica-rich chert. Two geologists propose different mechanisms for how dissolved ferrous iron ($Fe^{2+}$) in the Precambrian oceans was oxidized to insoluble ferric iron ($Fe^{3+}$) to form these deposits approximately 2.5 billion years ago.

Geologist 1
BIFs were formed through biological activity. Early photosynthetic cyanobacteria in shallow marine waters produced molecular oxygen ($O_2$) as a byproduct of photosynthesis. This free oxygen reacted with dissolved $Fe^{2+}$ in the water, oxidizing it to $Fe^{3+}$, which precipitated out of solution as iron oxides. This process occurred primarily in shallow coastal regions where sunlight was abundant.

Geologist 2
BIFs were formed through abiotic (non-biological) photochemical processes. The oxidation of $Fe^{2+}$ to $Fe^{3+}$ occurred without the involvement of living organisms or free oxygen. Instead, ultraviolet (UV) radiation from the Sun penetrated the Earth's early atmosphere, which lacked a protective ozone layer. This UV light directly catalyzed the photo-oxidation of dissolved $Fe^{2+}$ in the upper ocean layers, leading to the precipitation of iron oxides.

Based on the viewpoints of Geologist 1 and Geologist 2, which of the following is a shared assumption of both geologists regarding the ancient oceans during the period when Banded Iron Formations were deposited?

Geophysicists study the rheology of magma to predict volcanic eruption styles. A volcanologist formulated a hypothesis regarding the combined effects of pressure ($P$) and water content ($W$) on the viscosity ($\eta$) of rhyolitic magma at a constant temperature of $1000^\circ\text{C}$:

*Hypothesis*: Increasing $W$ decreases $\eta$ by disrupting silicate network bonds. However, because higher $P$ compresses the melt and opposes this bond disruption, the viscosity-reducing effect of adding water (defined as the factor by which $\eta$ decreases when $W$ is increased from $0.1\text{ wt}\%$ to $3.0\text{ wt}\%$) will become weaker as $P$ increases.

To test this hypothesis, the volcanologist measured the viscosity of rhyolitic magma samples at $1000^\circ\text{C}$ under different combinations of $P$ and $W$. The results are shown in the table below:

Which of the following statements best describes how the volcanologist should modify the hypothesis in light of these results?

A student proposed the following hypothesis regarding the growth of a bacterial biofilm in a flow chamber:

*Hypothesis*: The biofilm growth rate ($G$, in $\mu\text{m/day}$) is directly proportional to the bulk nutrient concentration ($C$, in $\text{mg/L}$) across all concentrations, and the rate of decrease in $G$ per unit increase in fluid shear stress ($\tau$, in $\text{Pa}$) is constant.

To test this hypothesis, the student conducted two experiments. In Experiment 1, the student varied $C$ while maintaining a constant $\tau$ of $0.10\text{ Pa}$. In Experiment 2, the student varied $\tau$ while maintaining a constant $C$ of $10.0\text{ mg/L}$. The results are shown in the tables below.

**Experiment 1 ($\tau = 0.10\text{ Pa}$)**

**Experiment 2 ($C = 10.0\text{ mg/L}$)**

Based on the results of Experiments 1 and 2, which of the following statements describes how the student should modify the hypothesis?

### Younger Dryas Cooling Debate

The Younger Dryas (approximately $12,900$ to $11,700$ years ago) was a period of abrupt, severe cooling that temporarily reversed the warming trend at the end of the last glacial period. Three scientists discuss competing hypotheses for the trigger of this cooling event.

Scientist 1
The primary trigger of the Younger Dryas was the sudden routing of meltwater from glacial Lake Agassiz into the North Atlantic Ocean. Prior to this event, meltwater drained southward into the Mississippi River. As the Laurentide Ice Sheet retreated, a northern outlet opened, releasing over $9,500\text{ km}^3$ of freshwater. Because freshwater is less dense than saltwater, this release created a surface cap that prevented the sinking of cold, saline water at high latitudes. This shut down the Atlantic Meridional Overturning Circulation (AMOC), stopping the northward transport of tropical heat and causing rapid Northern Hemisphere cooling.

Scientist 2
The cooling was triggered by a cosmic impact—specifically, a fragmented comet or asteroid striking the Laurentide Ice Sheet. This impact caused widespread biomass burning, which injected soot and aerosols into the atmosphere, immediately blocking solar radiation. More importantly, the intense heat of the impact melted a significant portion of the ice sheet, releasing immense volumes of freshwater and icebergs into the North Atlantic. This sudden freshwater influx decreased sea surface salinity, halting the AMOC and plunging the region into a cold state. The presence of nanodiamonds, helium-3, and platinum anomalies in sediments dating to $12,900$ years ago provides physical evidence of this extraterrestrial impact.

Scientist 3
The Younger Dryas was initiated by a combination of internal climate feedbacks driven by a solar activity minimum and volcanic eruptions. Increased volcanic aerosols in the atmosphere reflected incoming solar radiation, while decreased solar irradiance cooled the high Northern Hemisphere. This caused glaciers to expand. As these glaciers advanced and subsequently underwent seasonal retreat, the resulting increased freshwater runoff entered the North Atlantic. This freshwater influx disrupted the AMOC, which amplified the cooling. The event was sustained not by a single cataclysmic trigger, but by long-term orbital forcing and ocean-atmosphere feedbacks.

Based on the passage, which of the following statements represents a point of agreement among all three scientists regarding the Younger Dryas?

### Titan's Atmospheric Methane

Titan, Saturn's largest moon, has a thick atmosphere rich in methane ($CH_4$). Because solar ultraviolet radiation continuously breaks down atmospheric $CH_4$ through photochemical reactions, Titan's atmospheric $CH_4$ must be replenished from an internal reservoir to maintain its observed levels. Two scientists propose differing mechanisms for this replenishment.

Scientist 1
Titan's $CH_4$ is stored as methane clathrate hydrates—compounds in which $CH_4$ molecules are trapped inside cages of water ice—within its outer icy crust. These clathrates were incorporated into Titan during its accretion from the cold solar nebula. Periodically, thermal plumes rising from Titan's rocky core warm the base of the crust, causing the clathrates to dissociate (break apart) and release gaseous $CH_4$. This gas then migrates upward through fractures in the ice shell and enters the atmosphere.

Scientist 2
Titan's $CH_4$ is continuously produced by serpentinization within its rocky core. Liquid water from Titan's subsurface ocean circulates through the olivine-rich rocky core at high temperatures (exceeding $150^\circ\text{C}$). The chemical reaction between water and olivine produces hydrogen gas ($H_2$), which then reacts with carbon dioxide ($CO_2$) via the Sabatier reaction to synthesize $CH_4$. This newly formed $CH_4$ rises through the subsurface ocean and the overlying ice shell to replenish the atmosphere.

Scientist 2's hypothesis relies on which of the following underlying assumptions regarding Titan's internal structure?

Two students discuss the factors that influence the rate of carbon dioxide ($CO_2$) production during yeast fermentation.

Student 1
The fermentation rate depends solely on the type of sugar (glucose versus lactose) metabolized by the yeast. Yeast will ferment glucose much faster than lactose. The temperature of the yeast's environment has no effect on the rate of fermentation.

Student 2
The fermentation rate depends solely on the temperature of the yeast's environment. Higher temperatures increase yeast metabolic activity, leading to a higher fermentation rate. The specific type of sugar provided to the yeast does not affect the rate.

Match each hypothesis or claim on the left with the corresponding experimental outcome on the right that would directly disprove (invalidate) that claim.

A student proposed the following hypothesis regarding the behavior of gases:

*Hypothesis*: Under identical temperature and pressure conditions, gases with a larger molar mass will diffuse through a porous membrane at a faster rate than gases with a smaller molar mass.

To test this hypothesis, the student measured the diffusion time (the time required for a $1.0\text{ L}$ sample of gas to pass completely through a membrane) for four different gases. The results are shown in the table below:

Based on these results, how should the student modify the hypothesis to accurately reflect the relationship between a gas's molar mass and its rate of diffusion?

Researchers investigated the leaching of calcium ions ($Ca^{2+}$) from sandy loam soil under simulated rainfall conditions. Six soil columns, each containing $500\text{ g}$ of soil, were prepared. The simulated rainfall rate was held constant at $50\text{ mL/hr}$ for $10\text{ hours}$. The leachate was collected and analyzed for total dissolved $Ca^{2+}$ concentration (in $\text{mg/L}$). The composition of each column and the pH of the simulated rainfall applied are summarized in the table below:

Which columns must be compared to address each research objective while isolating the single variable of interest? Match each research objective on the left with the correct column comparison on the right.

### The Great Unconformity

The Great Unconformity is a global geological phenomenon where Cambrian-aged sedimentary rocks rest directly on top of much older igneous or metamorphic basement rocks, representing a gap in the rock record of up to $1\text{ billion}$ years. Three geologists propose different hypotheses for the cause of this gap.

Geologist 1
The unconformity was caused by massive glacial erosion during the Neoproterozoic "Snowball Earth" glaciations. Widespread, thick ice sheets covered the continents and scraped away kilometers of the Earth's crust, dumping the sediment into the oceans. This global glacial scour removed pre-Cambrian rock layers, creating the distinct erosional surface before Cambrian sediments were deposited.

Geologist 2
The unconformity was driven by tectonic processes related to the assembly and breakup of the supercontinent Rodinia. The collision of tectonic plates caused massive crustal uplift, exposing vast continental areas. Wind and rain then eroded the uplifted rock over millions of years. This subaerial erosion stripped away the older rock layers prior to Cambrian marine transgressions.

Geologist 3
The unconformity was caused by a severe, globally coordinated drop in sea level. As oceans receded, continental shelves were exposed to the atmosphere. Rain, rivers, and wind eroded the exposed rocks, removing centuries of geological history. Widespread erosion occurred until sea levels rose again during the Cambrian period, depositing new sediment over the eroded surface.

Based on the passage, all three geologists would agree with which of the following statements regarding the creation of the Great Unconformity?

Two students discuss the origin of water on Earth.

Student 1
Earth's water was delivered primarily by icy comets that collided with Earth during its early history. Comets contain water ice with a high deuterium-to-hydrogen ($D/H$) ratio. If comets were the primary source, the $D/H$ ratio of Earth's oceans must be equal to the $D/H$ ratio found in comets.

Student 2
Earth's water originated from volcanic outgassing of water vapor from the mantle. Hydrated minerals deep within the Earth were heated, releasing water that eventually formed the oceans. Since mantle water has a much lower $D/H$ ratio than comet water, the $D/H$ ratio of Earth's oceans must be lower than the $D/H$ ratio of comets.

Match each statement regarding the origin or properties of Earth's water to the student whose viewpoint it represents.

### Passage

To study the biodegradation of a synthetic polymer, polyethylene terephthalate (PET), by a newly isolated bacterium (*Thermobacillus petrolei*), researchers conducted two experiments.

In Experiment 1, five test tubes were prepared with $10\text{ mL}$ of a liquid nutrient medium, $50\text{ mg}$ of PET film, and $1.0\text{ mL}$ of an active *T. petrolei* suspension. The pH and temperature were kept constant. A different concentration of a synthetic surfactant, Surfactant X, was added to Tubes 1–4. Tube 5 did not receive Surfactant X. After 7 days, the remaining mass of PET was measured to evaluate degradation.

In Experiment 2, to determine whether the PET degradation observed in Experiment 1 was due to active bacterial metabolic activity rather than non-biological chemical hydrolysis or passive physical absorption onto the bacterial biomass, the researchers prepared four additional test tubes (Tubes 6–9) under the same environmental conditions as Experiment 1, but modified the tube contents as shown below:

To demonstrate that the degradation of PET film in Experiment 1 was specifically caused by the active metabolism of living *T. petrolei* cells rather than passive physical absorption of the polymer by the bacterial biomass, which of the following tubes from Experiment 2 serves as the most appropriate control when compared to Tube 5?

### Prebiotic Chemistry on Titan

Titan, Saturn's largest moon, has a thick atmosphere rich in nitrogen and methane, and a surface containing water ice and lakes of liquid methane and ethane. Two scientists discuss where prebiotic chemistry (reactions leading to the origin of life) is most likely to occur on Titan.

Scientist 1
Prebiotic chemical pathways on Titan must occur in its surface hydrocarbon lakes. At Titan’s average surface temperature of $-179^\circ\text{C}$, liquid water is completely absent. However, solar ultraviolet radiation photochemically produces complex organic molecules, such as acetylene ($C_2H_2$) and hydrogen cyanide ($HCN$), in the atmosphere. These molecules deposit onto the surface and dissolve in the liquid methane and ethane lakes. In these hydrocarbon solvents, organic molecules can react to form more complex, nitrogen-rich organic polymers. Therefore, these lakes are the primary sites for Titan's prebiotic chemical evolution.

Scientist 2
Prebiotic chemical pathways on Titan must occur in liquid water, which is periodically generated on Titan's surface by meteoroid impacts. When a meteoroid impacts Titan's icy crust, the kinetic energy is converted into heat, melting the ice and creating localized pools of liquid water that can persist for thousands of years before freezing. Atmospheric organic molecules that deposit on the surface dissolve in these impact-generated melt pools. The high reactivity of liquid water enables rapid hydrolysis reactions, converting simple organics into amino acids. Liquid hydrocarbons in Titan's lakes are chemically inert at $-179^\circ\text{C}$ and cannot serve as solvents for prebiotic reactions.

Both Scientist 1 and Scientist 2's arguments rely on which of the following assumptions?

A biology lab group investigated how wind speed affects the rate of water loss in *Phaseolus vulgaris* (common bean) plants. The group initially hypothesized that transpiration rates would increase continuously and linearly across all wind speeds due to the constant removal of the boundary layer of water vapor.

They recorded the transpiration rates at different wind speeds in a wind tunnel and compiled the data in the table below: