All practice questions

Astronomers detected a spectral line at $1.12\text{ mm}$ in the atmosphere of Venus, which they attribute to phosphine ($PH_3$) at a concentration of approximately $20\text{ ppb}$ (parts per billion). Three hypotheses were proposed to explain this observation.

*Hypothesis 1*
Phosphine is produced by biological activity. Anaerobic microorganisms living in the Venusian cloud decks generate $PH_3$ as a metabolic byproduct, similar to certain microbial life forms on Earth.

*Hypothesis 2*
Phosphine is produced by abiotic volcanism. Active volcanoes on Venus eject phosphide minerals from the mantle. These minerals are carried into the atmosphere, where they react with sulfuric acid droplets to produce $PH_3$ gas.

*Hypothesis 3*
The detection is a data processing artifact. The spectral line at $1.12\text{ mm}$ was actually caused by sulfur dioxide ($SO_2$), which is highly abundant in the Venusian atmosphere. The line was mistaken for $PH_3$ due to noise-reduction filtering of the radio telescope data.

Suppose a new study finds that $PH_3$ is rapidly destroyed in the Venusian atmosphere by ultraviolet radiation and acid reactions, requiring a continuous replenishment rate of $1.5 \\times 10^7\text{ kg/year}$ to maintain a concentration of $20\text{ ppb}$. The study also calculates that Venusian volcanic activity can release at most $2.0 \\times 10^3\text{ kg/year}$ of phosphorus-containing gases. Based on the hypotheses presented, how does this new evidence affect the validity of Hypothesis 2?

### Amphibian Population Declines

Amphibian populations worldwide have experienced severe declines over the past several decades. Two scientists discuss the primary causes of these declines.

Scientist 1
Global amphibian declines are driven primarily by the spread of the chytrid fungus (*Batrachochytrium dendrobatidis*). This pathogen infects the skin of amphibians, disrupting their osmotic regulation and causing death. While climate change and habitat loss may stress populations, the direct causal agent of these mass mortality events is the fungal pathogen. Outbreaks occur even in pristine, undisturbed habitats, demonstrating that environmental contamination is not a prerequisite for population collapse.

Scientist 2
The primary driver of global amphibian declines is agricultural runoff containing chemical pesticides, particularly atrazine. These contaminants act as endocrine disruptors, weakening the amphibians' immune systems and making them highly susceptible to opportunistic infections, including the chytrid fungus. The fungus itself has coexisted with amphibians for decades without causing massive declines. Only when chemical contamination compromises the host's physiological defenses do lethal disease outbreaks occur. Therefore, pesticide regulation, not pathogen eradication, is the key to conservation.

Scientist 1 and Scientist 2 differ in their views regarding which of the following questions?

### Models of Atomic Structure

Historically, scientists proposed different models to explain the internal structure of the atom.

Model 1 (Uniform Distribution Model)
This model proposes that an atom consists of a large, spherical cloud of positive electric charge. Negatively charged electrons are embedded evenly throughout this positive cloud. The mass of the atom is distributed uniformly across its entire volume, and there is no centralized core or empty space within the atom.

Model 2 (Centralized Nucleus Model)
This model proposes that nearly all of an atom's mass and all of its positive charge are concentrated in a tiny, extremely dense region at the center of the atom called the nucleus. Negatively charged electrons orbit this nucleus at relatively large distances. The rest of the atom is empty space, through which electrons move.

Based on Model 1 and Model 2, how do the two models differ regarding the distribution of positive charge within an atom?

Two students discuss the energy source of a newly discovered bacterium, *Bacterium X*, which lives near deep-sea hydrothermal vents.

Student 1 Hypothesis
*Bacterium X* is a photoautotroph. It utilizes the faint geothermal light emitted by the hot vent fluids to perform photosynthesis.

Student 2 Hypothesis
*Bacterium X* is a chemoautotroph. It obtains energy by oxidizing hydrogen sulfide ($H_2S$) dissolved in the vent fluids and does not require light.

Researchers subsequently cultured *Bacterium X* in a completely dark laboratory environment rich in hydrogen sulfide and found that the bacteria grew and reproduced at normal rates. Which of the following statements best describes how this new finding affects the students' hypotheses?

Deep-Sea Bioluminescence

A newly discovered species of deep-sea jellyfish, *Aurelia noctiluca*, emits a bright blue-green light when disturbed. Two scientists discuss the biological mechanism responsible for this bioluminescence.

Scientist 1
*A. noctiluca* produces light through an endogenous (internal) chemical reaction. The jellyfish genetically synthesizes its own light-emitting substrate (luciferin) and enzyme (luciferase). When mechanical stress is applied, internal calcium ions trigger the reaction, producing light. This process is independent of any foreign organisms. Consequently, the jellyfish will display bioluminescence throughout its entire lifecycle even in a completely sterile environment.

Scientist 2
*A. noctiluca* does not possess the genes to produce bioluminescence. Instead, the light is produced by symbiotic, bioluminescent bacteria (*Vibrio* species) colonizing specialized organs on the jellyfish's outer bell. The jellyfish provides nutrients, and in return, the bacteria emit light once they reach a threshold population density. The jellyfish cannot bioluminesce unless it acquires these bacteria from the surrounding ocean water during its early developmental stages.

Which of the following experimental procedures would provide the most direct evidence to determine which scientist's hypothesis is correct?

Origin of Crater X

Two astronomers debate the geological origin of Crater X on Mars.

Astronomer 1
Crater X was formed by a volcanic eruption. Volcanic eruptions on Mars produce basaltic rocks containing high levels of sulfur, but they never produce shock-metamorphosed quartz.

Astronomer 2
Crater X was formed by a meteoroid impact. Impact events generate extreme pressures that produce shock-metamorphosed quartz, but they do not typically leave high concentrations of sulfur.

Suppose a rover collects rock samples from Crater X and detects high concentrations of shock-metamorphosed quartz but negligible levels of sulfur. How does this new finding impact the astronomers' viewpoints?

An engineering group uses a mathematical model to estimate the theoretical power output, $P$ (in watts, $\text{W}$), of a wind turbine. The model is given by the following equation:

where $\rho$ represents the air density (in $\text{kg/m}^3$), $r$ represents the turbine blade length (in meters, $\text{m}$), and $v$ represents the wind speed (in $\text{m/s}$). Match each proposed modification of the turbine's operating parameters on the left to its corresponding effect on the theoretical power output ($P$) on the right.

An investigator conducted two experiments to study the factors affecting the rate of a chemical reaction between zinc metal ($\text{Zn}$) and hydrochloric acid ($\text{HCl}$).

In Experiment 1, the investigator added $1.0\text{ g}$ of zinc powder to $50\text{ mL}$ of $1.0\text{ M}$ $\text{HCl}$ solution in a beaker. The reaction was carried out at different temperatures ($20^\circ\text{C}$, $30^\circ\text{C}$, $40^\circ\text{C}$, and $50^\circ\text{C}$), and the time required for the zinc to completely dissolve was recorded for each trial.

In Experiment 2, the investigator added $1.0\text{ g}$ of zinc powder to $50\text{ mL}$ of $\text{HCl}$ solution of different concentrations ($0.5\text{ M}$, $1.0\text{ M}$, $1.5\text{ M}$, and $2.0\text{ M}$) in a beaker. The reaction was carried out at a constant temperature of $20^\circ\text{C}$, and the time required for the zinc to completely dissolve was recorded for each trial.

Based on the descriptions of these experimental designs, is the statement that 'the temperature of the reaction was varied in Experiment 1 but was kept constant in Experiment 2' true or false?

A student proposed the following hypothesis regarding electrical circuits:

*Hypothesis*: As the cross-sectional area of a metal wire increases, its electrical resistance will increase because a larger wire contains more atoms that collide with and resist the flow of electrons.

To test this hypothesis, the student measured the electrical resistance of four copper wires, each $10\text{ m}$ long but with different diameters, at a constant temperature of $20^\circ\text{C}$. The results are shown in the table below:

Based on these results, how should the student modify the hypothesis?

Onset of the Sturtian Glaciation

The Sturtian glaciation (approximately 717 million years ago) was one of the most severe ice ages in Earth's history, covering almost the entire planet in ice. Two hypotheses are proposed to explain the trigger for this "Snowball Earth" event.

Hypothesis 1
The glaciation was initiated by the rapid chemical weathering of the Franklin Large Igneous Province (LIP), a massive basaltic province erupted near the equator. The warm, wet equatorial climate accelerated the chemical weathering of the newly exposed basaltic rocks. This weathering consumed vast quantities of atmospheric carbon dioxide ($CO_2$) through the reaction of silicate minerals with acid rain. This process drew down $CO_2$ levels over several hundred thousand years to a critical threshold, triggering runaway ice-albedo cooling.

Hypothesis 2
The glaciation was initiated by the sulfur-rich explosive eruptions of the Franklin LIP. Because these eruptions occurred near the equator, intense equatorial convection carried massive quantities of sulfur dioxide ($SO_2$) gas directly into the stratosphere. There, the $SO_2$ reacted to form highly reflective sulfate aerosols. These aerosols remained suspended in the stratosphere for years, reflecting solar radiation back into space. This rapid, severe reduction in solar radiation caused global temperatures to plunge, initiating the glaciation within years, long before basalt weathering could significantly affect the atmosphere.

Based on the proposed hypotheses, proponents of Hypothesis 2 would most likely agree with which of the following statements regarding the timing and mechanism of the glaciation's onset?

To study thermal expansion in solids, a researcher proposed that the coefficient of linear expansion ($\alpha$) of a metal rod depends on its starting length. Specifically, the researcher hypothesized that longer rods of the same metal would exhibit a larger value of $\alpha$ when subjected to the same temperature change.

To test this hypothesis, the researcher measured the physical expansion of three copper rods under identical heating conditions. The results of the measurements and the calculated values of $\alpha$ are recorded in the table below.

Based on the results of the experiment, does the data support the researcher's hypothesis, and how should the hypothesis be modified?

Enceladus, a small icy moon of Saturn, is known to harbor a global liquid water ocean beneath its icy crust. Scientists debate the primary source of heat required to maintain this liquid ocean.

Hypothesis 1
The ocean is maintained by tidal heating concentrated in Enceladus’s porous silicate core. As Enceladus follows its eccentric orbit around Saturn, the varying gravitational pull deforms the moon. This deformation causes friction within the porous rock of the core, heating the water that circulates through it via hydrothermal convection. Tidal dissipation within the ice shell itself is negligible because the ice is too rigid to generate sufficient heat.

Hypothesis 2
The heat is primarily generated by serpentinization, an exothermic chemical reaction. Cold, slightly acidic seawater from the ocean percolates deep into the silicate core, reacting with olivine ($\text{Mg}_2\text{SiO}_4$) to form serpentine minerals, magnetite, and hydrogen gas ($\text{H}_2$). This reaction continuously releases thermal energy. Gravitational tidal forces merely keep the fractures in the core open to allow seawater circulation but do not contribute directly to the heat generation.

Hypothesis 3
The liquid ocean is a transient feature sustained by the radioactive decay of long-lived isotopes ($^{40}\text{K}$, $^{232}\text{Th}$, $^{235}\text{U}$, and $^{238}\text{U}$) inside the core. While this radioactive heat generation has declined over geological time, it remains sufficient to prevent complete freezing because Enceladus's icy crust contains a thick, highly insulating layer of clathrate hydrates. This insulating layer prevents thermal energy from escaping into space at a rate faster than it is produced by isotopic decay.

Both Hypothesis 1 and Hypothesis 2 address the role of Saturn's gravitational tidal forces on Enceladus. Which of the following statements best describes how these two hypotheses differ in their core claims regarding the effect of these tidal forces?

Students conducted two experiments to study the growth of *Arabidopsis thaliana* plants over a 30-day period.

Experiment 1
Five groups of 10 plants were grown in identical pots containing the same soil type. Each group received a different concentration of Fertilizer X (0%, 1%, 2%, 3%, or 4% by volume) dissolved in water. All plants were kept under continuous white light at a constant temperature of 22°C and watered daily with 50 mL of their respective solution. The average height of the plants in each group was measured at the end of 30 days.

Experiment 2
Five groups of 10 plants were grown under the same conditions as in Experiment 1, except that all plants received a 2% solution of Fertilizer X, and each group was exposed to a different color of light (blue, green, red, yellow, or white).

Based on the descriptions of the two experiments, which of the following variables was kept constant in Experiment 1 but was varied in Experiment 2?

The following passage contains an underlined portion. Choose the option that best replaces this portion to ensure the sentence is grammatically correct.

Discovered in 1977, deep-sea hydrothermal vents support extraordinary ecosystems that thrive in complete darkness. Because these organisms cannot access sunlight, they must find alternative energy sources; consequently, they rely on chemosynthetic bacteria. These specialized microbes, which live in symbiosis with tube worms, convert toxic chemicals into usable organic <u>matter, they form</u> the foundation of a unique food web.

Which of the following choices best replaces the underlined portion of the passage?

### Hydrothermal Vents and Prebiotic Synthesis

Deep-sea hydrothermal vents are considered potential sites for the origin of life on Earth. Two scientists debate the chemical and physical conditions under which the first organic molecules were synthesized.

Scientist 1
Organic molecules were synthesized at alkaline hydrothermal vents (such as the Lost City field) where warm fluid ($40^\circ\text{C}$ to $90^\circ\text{C}$) rich in dissolved $H_2$ and $CH_4$ mixes with acidic, $CO_2$-rich ocean water. The pH gradient between the alkaline fluid ($\text{pH} \approx 9\text{--}11$) and the acidic ocean water ($\text{pH} \approx 5\text{--}6$) acted as a natural proton-motive force, driving the reduction of $CO_2$ by $H_2$ to form organic compounds. The catalysts were mineral deposits of iron-sulfur minerals (like mackinawite) within the porous chimneys. High-temperature hydrothermal vents ($>300^\circ\text{C}$) are too hot and would destroy organic molecules, preventing prebiotic synthesis.

Scientist 2
Prebiotic synthesis occurred at high-temperature volcanic hydrothermal vents (black smokers), where acidic fluids ($\text{pH} \approx 2\text{--}3$) at temperatures exceeding $350^\circ\text{C}$ erupt into the ocean. The cooling gradient as the fluid meets the ambient ocean water ($\approx 2^\circ\text{C}$) allows for the rapid stabilization of synthesized compounds. The primary driver of prebiotic synthesis was the high concentration of transition metal sulfides (such as pyrite, $FeS_2$) and volcanic gases like $CO$ and $H_2S$. The energy for synthesis was provided directly by the chemical potential of mineral precipitation (e.g., $FeS + H_2S \rightarrow FeS_2 + H_2$) rather than a pH gradient. Alkaline vents lack the thermal energy and transition metals required to overcome the activation energy barrier for carbon fixation.

Match each of the following claims about prebiotic synthesis to the scientist(s) whose viewpoint supports it.

Three models are proposed to explain the Cretaceous-Paleogene (K-Pg) extinction event. Match each proposed mechanism of extinction to the model that features it based on the descriptions below:

* Model 1 (Asteroid Impact): Proposes that a massive asteroid collision injected dust and sulfur into the atmosphere, causing immediate, widespread cooling and blocking sunlight.
* Model 2 (Deccan Traps Volcanism): Proposes that massive volcanic eruptions released large volumes of carbon dioxide ($CO_2$) over hundreds of thousands of years, causing gradual greenhouse warming and ocean acidification.
* Model 3 (Marine Regression): Proposes that a drop in global sea levels drained shallow interior seaways, destroying coastal habitats and gradually reducing species diversity before the final extinction.

Three students discuss the origin of the distinct pink color of Lake Hillier, a hypersaline lake.

* Student 1 proposes that the pink color is caused by pigments produced by halophilic microalgae and bacteria.
* Student 2 proposes that the pink color is due to dissolved iron- and cobalt-rich minerals leaching from the lake bed.
* Student 3 proposes that the pink color is a physical optical effect of light scattering off suspended microscopic salt crystals.

Match each student's hypothesis with the experimental outcome that would directly invalidate it.

Early Martian Climate

Two scientists discuss the primary greenhouse gas responsible for keeping liquid water on Mars's surface billions of years ago.

Scientist 1
Early Mars was kept warm primarily by a thick atmosphere rich in carbon dioxide ($CO_2$). Volcanic activity released vast amounts of $CO_2$ and water vapor, creating a strong greenhouse effect that maintained temperatures above freezing.

Scientist 2
Early Mars was kept warm primarily by methane ($CH_4$) gas. Although $CO_2$ was present, it was not abundant enough to prevent freezing. Instead, specialized early microbes produced methane, which is a much more potent greenhouse gas than $CO_2$.

New Evidence
A rover analyzes ancient Martian sedimentary rocks and discovers large deposits of carbonate minerals, which form readily in the presence of high atmospheric $CO_2$ levels. However, the rover finds no organic biomarkers or chemical signatures of methane-producing microbes.

Based on this new evidence, which of the following statements best describes the impact of this discovery on the scientists' viewpoints?

A student measures the volume of a sample of gas at different temperatures while keeping the pressure constant. The data are shown in the table below:

Based on this table, which of the following mathematical relationships best describes the relationship between the temperature and the volume of the gas sample?

A student conducts a series of measurements on a sample of gas in a container. Match each experimental variable relationship on the left with its correct proportional trend on the right.