All practice questions

An environmental chemist studies the capture of carbon dioxide ($CO_2$) by a newly synthesized metal-organic framework (MOF) at different temperatures. The chemist proposes the following hypothesis:

*Hypothesis*: As the temperature of the system increases, the amount of $CO_2$ gas adsorbed by the MOF at any given pressure will increase, because the increased kinetic energy of the gas molecules will enhance their interaction with the MOF's pore surface.

The chemist measures the mass of $CO_2$ adsorbed per gram of MOF ($m_{ads}$, in $mg/g$) at three different temperatures across a range of pressures, as shown in the table below:

Based on the experimental results, which of the following describes how the hypothesis should be modified?

Martian Methane Origins

Methane ($\text{CH}_4$) has been detected in the Martian atmosphere, sparking debate over its origin. Because methane is rapidly destroyed by solar ultraviolet radiation, its presence requires an active source. Two scientists present hypotheses regarding the source of Martian methane.

Scientist 1
Martian methane is biogenic, produced by subsurface methanogenic microbes. These microbes reside deep underground where liquid water is stable, utilizing carbon dioxide ($\text{CO}_2$) and hydrogen ($\text{H}_2$) to produce $\text{CH}_4$ as a metabolic byproduct. Scientist 1 points out that the detected methane exhibits seasonal fluctuations, peaking during the Martian summer when warmer temperatures increase microbial metabolic activity. Furthermore, carbon isotope analysis shows a depletion of carbon-13 ($^{13}\text{C}$) relative to carbon-12 ($^{12}\text{C}$) in atmospheric methane samples, which is a classic signature of biological processing.

Scientist 2
Martian methane is abiogenic, produced by serpentinization, a geological process in which water reacts with olivine-rich rocks in the presence of carbon dioxide. This reaction occurs deep within the Martian crust where geothermal heat warms the rocks. Scientist 2 argues that the seasonal fluctuations are caused by the seasonal release of methane trapped in clathrate hydrates (water ice cages) as the surface temperature warms, rather than biological activity. According to Scientist 2, abiotic reactions can produce similar carbon isotope signatures in environments with highly localized hydrogeochemical pathways, meaning the carbon-13 depletion is not exclusive to biological sources.

Based on the viewpoints of the two scientists, which of the following assumptions is implicit in Scientist 1's argument but disputed by Scientist 2?

### Venusian Phosphine Debate

In 2020, researchers reported the detection of phosphine ($PH_3$) in the temperate cloud decks ($50–60\text{ km}$ above the surface) of Venus. Because $PH_3$ is rapidly destroyed by photolysis and oxidation in Venus's highly acidic atmosphere, any detectable level of $PH_3$ suggests a continuous source of production. Three hypotheses were proposed to explain the source of the detected $PH_3$.

Hypothesis 1 (Biotic Source)
The detected $PH_3$ is produced by anaerobic microbial life residing in the temperate cloud decks. In this environment, temperatures and pressures are relatively mild. Terrestrial anaerobic bacteria are known to produce $PH_3$ from phosphate minerals, and similar biochemical pathways must be active on Venus. Because thermodynamic calculations show that the abiotic production of $PH_3$ under Venus's atmospheric conditions is highly unfavorable, non-biological reactions cannot explain the observed concentration. Thus, biological activity is the only viable mechanism.

Hypothesis 2 (Volcanic Source)
Active volcanism on Venus is responsible for the phosphine. Eruptions eject phosphorus-bearing minerals, such as phosphides ($Fe_3P$), from the deep mantle into the lower atmosphere. As these minerals rise into the acidic cloud deck, they react with sulfuric acid ($H_2SO_4$) to form $PH_3$ gas. This abiotic mechanism does not require biological activity and can account for the observed $PH_3$ concentration, provided Venus is volcanically active. Abiotic photochemical models, however, are insufficient to produce $PH_3$.

Hypothesis 3 (Photochemical Source)
Atmospheric photochemistry driven by solar ultraviolet (UV) radiation synthesizes $PH_3$. Solar UV radiation initiates reactions in the upper atmosphere that reduce oxidized phosphorus compounds (like orthophosphoric acid) in the presence of trace hydrogen sources. While thermodynamic models suggest abiotic pathways are unfavorable in the bulk atmosphere, localized photochemical reactions near the cloud tops can generate the observed $PH_3$ abiotically.

Based on the descriptions of the three hypotheses, which hypothesis or group of hypotheses agrees with each statement regarding the production and behavior of Venusian phosphine? Match each statement on the left with the correct hypothesis or group of hypotheses on the right.

Two models are proposed to explain the presence of water vapor in the atmosphere of Exoplanet Kepler-186f:

* Model 1 (Internal Volcanism): Water vapor is released into the atmosphere primarily through volcanic eruptions from the planet's interior.
* Model 2 (Comet Impacts): Water vapor is delivered to the atmosphere through frequent collisions with icy comets.

Match each of the following new astronomical observations with the statement that best describes its relationship to the models.

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

*Hypothesis*: As the surrounding temperature increases, the chirping rate of crickets will decrease because lower temperatures stimulate metabolic activity.

The student measured the chirping rate of a group of crickets at different temperatures and recorded the results in the table below:

Based on the data, how should the student modify the hypothesis?

Two scientists present competing hypotheses regarding the origin of Earth's water.

Scientist 1
Earth's water originated primarily from volcanic outgassing. As the early Earth cooled, water vapor released from molten rock in the mantle condensed to form the oceans. This water was present within the planet's building blocks during its initial formation.

Scientist 2
Earth's water was delivered primarily by icy comets and carbonaceous meteorites that bombarded the planet after its crust had solidified. The isotopic ratio of hydrogen in Earth's oceans matches that of outer solar system comets, suggesting an extraterrestrial origin.

According to Scientist 1, Earth's water was originally introduced to the surface through which of the following processes?

### Passage

To study the biodegradation of polyethylene terephthalate (PET) by a newly engineered strain of *Pseudomonas putida*, researchers set up several liquid culture vessels containing a baseline mineral medium ($M9$), a fixed concentration of PET film ($0.5\%\text{ w/v}$), and varying combinations of additional factors. The strain normally requires a co-substrate, such as succinate, to support growth during PET degradation. The researchers tested the effects of adding $10\text{ mM}$ succinate, adding $50\text{ }\mu\text{M}$ of cadmium ($Cd^{2+}$, a heavy metal inhibitor), and incubating at different temperatures. Each vessel was inoculated with $10^6\text{ cells/mL}$ of *P. putida* and incubated for 7 days.

The setups for the groups are described in the table below:

The researchers measured the dry weight loss percentage of the PET film after 7 days to evaluate PET-degrading activity.

To isolate and determine the specific inhibitory effect of cadmium ($Cd^{2+}$) on PET biodegradation at the standard growth temperature of $30\text{ °C}$, the researchers must compare the PET weight loss of Group 3 to the PET weight loss of which of the following groups?

Passage

The origin of Earth's volatile elements, particularly water, remains a central question in planetary science. Two scientists present competing hypotheses regarding the source and evolution of Earth's water.

Scientist 1
Earth accreted inside the "frost line"—the radial distance in the solar nebula where temperatures were cool enough for volatile compounds like water ice to condense. Consequently, the primordial materials that formed the proto-Earth were completely dry. Earth’s water was delivered during the late veneer phase, after core differentiation was complete, by carbonaceous chondrite meteorites originating from the outer solar system. The deuterium-to-hydrogen ($D/H$) ratio of Earth's oceans has remained constant at approximately $1.5 \times 10^{-4}$ since this delivery. Because this value matches the $D/H$ ratio of carbonaceous chondrites, it serves as a pristine chemical signature of the late-accreting outer solar system material.

Scientist 2
Earth's water is primordial and was accreted directly from enstatite chondrite-like planetesimals in the inner solar system. Although temperatures were too high for water ice to condense, hydrogen was incorporated directly into the iron and silicate mineral lattices of the accreting planetesimals. The $D/H$ ratio of Earth's surface water has not remained constant. Initially, Earth's primordial water had a $D/H$ ratio of $1.0 \times 10^{-4}$, identical to enstatite chondrites. Over billions of years, solar ultraviolet radiation photolyzed atmospheric water vapor, and the lighter protium ($^1H$) isotope preferentially escaped Earth’s gravity compared to the heavier deuterium ($^2H$). This selective escape of protium acted as the primary driver that gradually elevated the surface $D/H$ ratio to its current value of $1.5 \times 10^{-4}$.

Based on the passage, Scientist 1 and Scientist 2 differ in their views regarding which of the following?

Martian Methane Spikes

In 2019, planetary probes detected sudden, seasonal spikes in the concentration of atmospheric methane ($\text{CH}_4$) on Mars, which peaked during the Martian summer. Two scientists propose different explanations for these observations.

Scientist 1
The methane spikes are biogenic in origin, produced by subterranean methanogenic microbes. During the Martian summer, warmer surface temperatures melt subsurface permafrost, allowing the microbes to increase metabolic activity and release accumulated methane gas into the atmosphere. Because biological metabolic processes are highly selective, methanogens preferentially utilize carbon-12 ($^{12}\text{C}$) over carbon-13 ($^{13}\text{C}$), producing methane that is highly enriched in $^{12}\text{C}$ relative to $^{13}\text{C}$. Furthermore, biological methanogenesis produces almost exclusively methane, with negligible amounts of heavier hydrocarbons like ethane ($\text{C}_2\text{H}_6$) or propane ($\text{C}_3\text{H}_8$).

Scientist 2
The methane spikes are abiogenic (geochemical) in origin, resulting from serpentinization—a reaction between water, carbon dioxide ($\text{CO}_2$), and olivine minerals deep within the Martian crust. The gas is trapped in subsurface ice structures called clathrates. During the Martian summer, increased solar radiation warms the shallow crust, melting the clathrates and releasing the trapped gas. Serpentinization is a high-temperature geochemical process that does not preferentially select light carbon isotopes, resulting in methane with standard planetary ratios of $^{12}\text{C}$ to $^{13}\text{C}$. Additionally, serpentinization naturally produces significant quantities of ethane and propane alongside methane.

Which of the following new experiments or measurements would provide the best evidence to resolve the conflict between the two scientists' viewpoints?

A student investigates the rate of an enzyme-catalyzed reaction. The student proposed the following hypothesis:

*Hypothesis*: The rate of the reaction is directly proportional to the enzyme concentration at all temperatures, meaning that doubling the enzyme concentration will double the reaction rate regardless of the temperature.

To test this hypothesis, the student measured the reaction rate (in $\mu\text{mol/min}$) at enzyme concentrations of $1.0\text{ g/L}$ and $2.0\text{ g/L}$ across four different temperatures. The results are shown in the table below:

Which of the following statements best explains whether the data support the student's hypothesis, and how the hypothesis should be modified?

Two students discuss the factors that determine the terminal velocity of a falling object in Earth's atmosphere.

Student 1:
Terminal velocity is determined solely by the mass of the falling object. A heavier object experiences a stronger gravitational force, allowing it to accelerate to a higher speed before air resistance balances gravity. Therefore, an object's mass is the only factor that dictates its terminal velocity.

Student 2:
Terminal velocity is determined solely by the surface area of the falling object facing the direction of fall. An object with a larger surface area collides with more air molecules, increasing air resistance. Therefore, the shape and surface area of the object are the only factors that dictate its terminal velocity.

According to the passage, Student 1 and Student 2 differ in their views regarding which of the following factors determines the terminal velocity of a falling object?

Two students discuss the source of the heat that powers the high-speed winds in Planet Y's atmosphere.

*Student 1*
Planet Y's winds are driven entirely by geothermal heat rising from the planet's hot interior. The planet is covered by a dense layer of dust that reflects 100% of incoming sunlight back into space, meaning solar energy does not heat the atmosphere at all.

*Student 2*
Planet Y's winds are powered entirely by solar radiation. Although the dust layer reflects most sunlight, the top of the dust layer absorbs enough solar energy to create large temperature differences in the upper atmosphere, driving the winds. Geothermal heat from the core is too weak to reach the atmosphere.

Match each atmospheric factor on the left with the correct description of the students' disagreement regarding that factor on the right.

To investigate the effect of wind speed on the rate of transpiration, students placed four identical oak saplings in potometers. Each sapling was exposed to a fan at a different distance to vary the wind speed, with Trial 4 serving as the control group (no fan). The trials were conducted sequentially inside a greenhouse, and the ambient temperature, relative humidity, and transpiration rate were recorded for each 1-hour trial. The results are shown in the table below.

Based on these results, the students concluded that a wind speed corresponding to a distance of $1.5\text{ m}$ from the fan maximizes the rate of transpiration in oak saplings. Which of the following statements identifies the primary flaw in this conclusion?

### Origin of Earth's Oceans

Scientist 1
Earth's water was delivered during the planet's formation by carbonaceous chondrite meteorites originating from the asteroid belt. These meteorites contain up to 20% water by weight bound within their mineral structures. Because the planetesimals that formed in Earth's orbital zone were too hot to retain volatile water, water had to be delivered by chondrites originating beyond the "snow line" (the boundary beyond which water ice could condense). Thus, the deuterium-to-hydrogen ($D/H$) ratio of Earth's oceans reflects the $D/H$ ratio of these chondrites.

Scientist 2
Earth's oceans were formed much later, during the Late Heavy Bombardment (approximately 3.9 billion years ago), by water-rich comets originating from the outer Solar System. Comets consist of up to 80% water ice and would have deposited vast quantities of water upon impacting the young Earth. The volatile elements on early Earth were completely vaporized and lost to space during the giant impact that formed the Moon. Therefore, Earth's current water inventory must have been delivered post-impact by these outer-system comets.

Scientist 2's argument regarding the cometary origin of Earth's oceans relies on which of the following underlying assumptions?

Warm Jupiters are giant exoplanets with orbits between $0.1\text{ AU}$ and $1.0\text{ AU}$. Astronomers propose three models to explain their origin:

* Model 1 (In-Situ Formation): Warm Jupiters form at their current orbital distances. Because this region contains little mass in typical protoplanetary disks, Model 1 assumes that the local disk surface density must have been at least 100 times greater than the minimum mass solar nebula. Under this hypothesis, dust grains coagulated rapidly to form a $10\ M_\oplus$ core, which then triggered runaway gas accretion from the local gas reservoir. This entire process must be completed within 1–2 million years, before the stellar wind disperses the gas.
* Model 2 (Disk Migration): Warm Jupiters form beyond the 'ice line' ($>3\text{ AU}$), where water ice can condense, providing abundant solid material to build a massive core. The planet then migrates inward because of tidal torque from the gas disk (Type II migration). This migration is driven by the exchange of angular momentum between the planet and the gas disk. Model 2 assumes that migration ceases when the planet reaches the inner edge of the gas disk or when the gas disk is photodissipated by the host star.
* Model 3 (High-Eccentricity Tidal Migration): Like Model 2, Model 3 assumes Jovian planets must form beyond the ice line ($>3\text{ AU}$) to acquire enough solid ice and dust for core growth. However, after formation, the planet is perturbed into a highly eccentric orbit ($e > 0.9$) by the gravitational influence of a distant companion star or planet. During periastron (closest approach to the host star), the intense tidal forces stretch and compress the planet, dissipating orbital energy as heat within the planet. This process, known as tidal circularization, slowly shrinks and circularizes the orbit over hundreds of millions of years. Model 3 assumes that the gas disk is completely gone before the gravitational perturbations trigger this high-eccentricity phase.

Based on the models described, is the following statement true or false?

'Model 2 and Model 3 both hypothesize that the initial formation of a gas giant's core requires a region of the protoplanetary disk where temperatures are low enough for water ice to condense, whereas Model 1 assumes that core formation can occur in much warmer regions closer to the star provided there is an exceptionally high density of dust.'

Three students propose hypotheses to explain why a copper coin turns green over time:

* Student 1 believes that the green color is copper carbonate formed when copper reacts with carbon dioxide and water vapor in the air.
* Student 2 believes that the green color is copper chloride formed when copper reacts with airborne chlorine from coastal salt spray.
* Student 3 believes that the green color is copper oxide formed when copper reacts only with gaseous oxygen in dry air.

Match each chemical requirement for the coin turning green to the student who proposes it.

### The Late Ordovician Mass Extinction

The Late Ordovician Mass Extinction (LOME), which occurred approximately 445 million years ago, resulted in the loss of about 85% of marine species. Two scientists discuss the potential triggers and environmental mechanisms responsible for this event.

Scientist 1
The LOME was primarily caused by a sudden, intense period of global cooling initiated by the growth of the Gondwanan ice sheet. This glaciation locked up water, causing global sea levels to drop by over $100\text{ meters}$, which eliminated shallow epicontinental sea habitats. The subsequent rapid deglaciation released vast amounts of freshwater, creating a stratified ocean. This stratification slowed thermohaline circulation and led to widespread marine anoxia (oxygen depletion) in the warming oceans, driving the second pulse of extinction. Throughout both pulses, atmospheric carbon dioxide ($CO_2$) levels decreased significantly due to the rapid silicate weathering of the rising Appalachian Mountains, which drew down $CO_2$ and drove the cooling.

Scientist 2
The LOME was triggered by large-scale volcanism in the Altai-Sayan region, which released massive quantities of greenhouse gases, primarily $CO_2$ and sulfur dioxide ($SO_2$), into the atmosphere. The immediate result was intense global warming and severe ocean acidification, which devastated marine calcifiers. As volcanic activity subsided, the rapid chemical weathering of the newly exposed volcanic rocks caused a sharp drawdown of atmospheric $CO_2$, leading to a brief, secondary cooling phase and minor glaciation. The primary driver of the marine extinction, however, was widespread ocean anoxia. This anoxia persisted from the initial warming phase through the cooling phase because elevated temperatures and continental runoff fertilized massive algal blooms, whose decomposition depleted marine oxygen.

Match each environmental variable on the left to the statement on the right that best describes the specific point of disagreement between Scientist 1 and Scientist 2.

Hydrangea plants can produce flowers that are either pink or blue. Two students propose different hypotheses to explain the primary factor that determines this flower color.

Student 1
The color change of hydrangea flowers is determined directly by the pH of the surrounding soil. When the soil is acidic (pH less than $6.0$), the hydrogen ion concentration directly alters the chemical structure of the pigment anthocyanin within the flower petals, causing it to reflect blue light. In alkaline soil (pH greater than $7.0$), the pigment's structure remains unchanged, and the flowers appear pink.

Student 2
The color change of hydrangea flowers is determined directly by the absorption of aluminum ions ($\text{Al}^{3+}$) from the soil, which form a chemical complex with the pigment anthocyanin to produce the blue color. Soil pH is only an indirect factor: acidic soil makes aluminum ions soluble and available for the plant to absorb, while alkaline soil binds aluminum ions in insoluble compounds, preventing absorption and resulting in pink flowers.

Based on Student 2's hypothesis, which of the following is the direct cause of the blue color in hydrangea flowers?

Three models are proposed to explain the thermal energy source and fracturing mechanism responsible for the cryovolcanic plumes observed at the south pole of Saturn's moon, Enceladus.

Model 1
The parallel fractures (tiger stripes) are open conduits connected to a localized subsurface reservoir of liquid water. Saturn's gravitational pull exerts varying tidal forces on Enceladus along its eccentric orbit. This tidal flexing causes the walls of the fractures to rub against one another. Frictional heating along these sliding faults melts the surrounding ice, generating the heat that keeps the vents open and drives the vapor plumes.

Model 2
The thermal energy source is radiogenic decay within the silicate core, which maintains a global subsurface ocean. As the moon slowly cools, the outer ice shell thickens. Because ice is less dense than liquid water, this freezing process expands the shell, generating intense hydrostatic pressure within the underlying ocean. Once the pressure exceeds the tensile strength of the ice shell, fracturing occurs, violently venting pressurized water into space.

Model 3
Cold water from the subsurface ocean migrates downward, circulating through a porous, fractured silicate core. An exothermic chemical reaction known as serpentinization occurs between the water and olivine-rich rocks in the core, raising the water temperature. This reaction also releases gases, primarily $H_2$. The resulting warm, buoyant, gas-rich fluids rise rapidly, melting conduits through the overlying ice shell to erupt as plumes.

Based on the models provided, match each key hypothesis regarding the primary energy source or fracturing mechanism on Enceladus to the corresponding model.

### The Cambrian Explosion Debate

The Cambrian Explosion, which occurred approximately $541$ million years ago, is characterized by the rapid appearance of most major animal phyla in the fossil record. Prior to this event, multicellular life consisted primarily of simple, soft-bodied organisms. Scientists debate the primary trigger of this evolutionary event.

**Hypothesis $1$ (Environmental)**
The primary driver of the Cambrian Explosion was a critical rise in oceanic oxygen levels. Prior to the Cambrian, low oxygen levels restricted animals to small sizes and simple, low-metabolism structures. Once oxygen crossed a threshold, it enabled the metabolic demands of larger body sizes, active movement, and the synthesis of collagen, which is necessary for constructing hard skeletons. The sudden accumulation of calcium carbonate and silica shells in the fossil record is a direct consequence of this oxygenation event.

**Hypothesis $2$ (Ecological)**
The explosion was triggered by an ecological cascade initiated by the evolution of active predation. The appearance of the first macroscopic predators created intense selective pressure, driving prey species to evolve protective adaptations, such as hard exoskeletons and complex sensory systems. This predator-prey arms race forced rapid morphological diversification. Increased oxygen levels were a necessary prerequisite, but they did not actively trigger the explosion; the pressure of predation was the active mechanism that drove the sudden diversification.

**Hypothesis $3$ (Genetic)**
The fundamental trigger was the acquisition of a critical threshold of developmental genetic machinery, specifically the duplication and modification of the $Hox$ gene cluster. $Hox$ genes govern the body patterning of bilateral animals. Before the Cambrian, organisms lacked the genetic flexibility to form complex body plans. Once these regulatory networks evolved, they allowed for rapid morphological experimentation and the development of specialized tissues, including mineralized skeletons. Environmental changes and ecological interactions merely filled the niches created by this genetic breakthrough.

Based on the three hypotheses, all of the authors would agree with which of the following statements regarding the organisms that emerged during the Cambrian Explosion?