All practice questions

An investigator designed an experiment to determine how pH affects the rate of starch hydrolysis by the enzyme amylase. The investigator prepared four test tubes, each containing an identical concentration of starch and amylase at a specific pH. To establish and maintain each pH level, the investigator used different buffer systems, as summarized in the table below:

Given that amylase activity is stimulated by the presence of chloride ($Cl^-$) ions, which of the following statements best identifies the confounding variable in this experiment and its potential impact on the results?

Early Mars Climate Models

Astronomers have proposed two models to explain the geological features on Mars, such as dry river valleys and lake beds, which suggest the past presence of liquid water.

*Model 1 (Warm and Wet)*
Early Mars had a thick atmosphere primarily composed of carbon dioxide ($CO_2$) and water vapor. This thick atmosphere created a strong greenhouse effect that maintained surface temperatures above $0^\circ\text{C}$, allowing liquid water to exist continuously on the surface for millions of years.

*Model 2 (Cold and Icy)*
Early Mars was generally cold, with surface temperatures rarely rising above $0^\circ\text{C}$ due to a thin atmosphere. Liquid water could not exist on the surface for long periods. Instead, liquid water only flowed temporarily during brief warming events triggered by volcanic eruptions or meteor impacts, which temporarily melted surface ice.

Based on the models, which of the following is a point of agreement between Model 1 and Model 2?

Two scientists discuss why the pressure of a sample of nitrogen gas ($N_2$) inside a rigid, sealed container increases when the gas is heated from $20^\circ\text{C}$ to $200^\circ\text{C}$.

Scientist 1
The increase in pressure is due entirely to the increase in the average kinetic energy of the $N_2$ molecules. As the temperature rises, the molecules move faster, colliding with the container walls more frequently and with greater force. The total number of gas molecules remains constant.

Scientist 2
The increase in pressure is due to the thermal dissociation of $N_2$ molecules into individual nitrogen atoms ($N_2 \rightarrow 2N$). As the temperature rises, more molecules split, which increases the total number of gas particles in the container. The average kinetic energy of the particles remains constant.

Which of the following experiments would best determine which scientist's viewpoint is correct?

A student proposed the following hypothesis regarding the corrosion of iron:

*Hypothesis*: The mass of rust that forms on an iron nail submerged in a sodium chloride ($NaCl$) solution for 7 days will increase continuously as the concentration of $NaCl$ in the solution increases from 0% to 10%.

To test this hypothesis, the student submerged identical iron nails in 5 different $NaCl$ solutions for 7 days and recorded the mass of the rust that formed on each nail. The results are shown in the table below:

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

Scientist 1
Hot Jupiters—gas giant exoplanets orbiting extremely close to their parent stars (typically $< 0.1 \text{ AU}$)—form *in situ* (in their current locations). This requires a highly dense protoplanetary disk in the stellar vicinity. Because of the high temperatures near the star ($> 1,500 \text{ K}$), only refractory materials (like iron and silicates) can condense. Consequently, a hot Jupiter formed *in situ* must possess a massive solid core composed of at least $80\%$ refractory silicates and metals by mass, surrounded by a thin, compressed hydrogen and helium envelope making up no more than $20\%$ of the planet's total mass. Volatile compounds (such as water ice and methane) cannot exist in these cores.

Scientist 2
Hot Jupiters cannot form *in situ* because the stellar wind and high temperatures close to a young star deplete the gas required for envelope accretion. Instead, these planets form beyond the 'ice line' ($> 3.0 \text{ AU}$) where temperatures are low enough ($< 150 \text{ K}$) for water, ammonia, and methane to freeze into volatile ices. This abundance of solid material allows a core to grow rapidly and accrete a massive gas envelope representing at least $90\%$ of the planet's total mass. Gravitational interactions with the gas disk then cause the planet to migrate inward. Thus, a migrated hot Jupiter must have a core consisting of more than $50\%$ volatile ices, and its gaseous envelope must constitute at least $90\%$ of its total mass.

Consider the following table summarizing data for three newly discovered exoplanets:

Based on the viewpoints of Scientist 1 and Scientist 2, is the following statement true or false?

'The data for Planet Z is consistent with the predictions of Scientist 2 because its core composition satisfies the requirement of containing more than $50\%$ volatile ices.'

A student wants to modify an experiment measuring sugar solubility in water to determine the effect of higher temperatures ($40^\circ\text{C}$, $60^\circ\text{C}$, and $80^\circ\text{C}$) on the mass of dissolved sugar. Arrange the steps of this modified procedure in the correct chronological order from start to finish.

Origin of the Moon

Three models are proposed to explain the origin of Earth's Moon.

Model 1 (Giant Impact)
Approximately 4.5 billion years ago, Earth collided with a Mars-sized planetesimal called Theia. The collision vaporized Earth’s outer crust and mantle, as well as Theia. The resulting debris ring orbitally coalesced to form the Moon. Because the Moon formed primarily from the vaporized silicate mantle materials of both bodies, it has a very small iron core, a low overall density compared to Earth, and an oxygen isotope ratio nearly identical to Earth's mantle. This model asserts that the extreme heat of the impact depleted volatile elements (such as water and sodium) on the Moon.

Model 2 (Co-formation)
The Earth and the Moon formed simultaneously from the same region of the solar nebula’s accretion disk. As gravity drew dust and gas together, two adjacent accretion centers developed: a larger one for Earth and a smaller one for the Moon. Because they formed from the same reservoir of material, their oxygen isotope signatures are identical. However, this model assumes that both bodies should have similarly sized iron cores and overall densities, as the starting material was uniform throughout that region of the disk.

Model 3 (Capture)
The Moon formed in a different region of the solar system, rich in silicates but poor in iron, explaining its low density and small iron core. Later, as the Moon traveled through the inner solar system, Earth’s gravitational field captured it into a permanent orbit. Because the Moon formed in a separate region of the solar nebula, its initial composition—including its oxygen isotope ratios—was distinct from Earth's. The capture mechanism required a thick primeval atmosphere or tidal dissipation to slow the Moon down during its close flyby.

Based on the passage, which of the following statements correctly identifies a point of disagreement between Model 2 and Model 3 regarding the Moon's formation, and the resulting prediction of its oxygen isotope ratios?

### Archean Atmospheric Composition

During the Archean Eon (approximately $3.8$ to $2.5$ billion years ago), the Sun's energy output was only $70\%$ to $75\%$ of its current value. Under these conditions, without a strong atmospheric greenhouse effect, Earth's surface water would have frozen completely. Yet, geological evidence shows that liquid oceans existed. Two scientists discuss the atmospheric conditions that resolved this "Faint Young Sun Paradox."

Scientist 1
The primary greenhouse gas keeping the Archean Earth warm was biogenic methane ($CH_4$), which was maintained at concentrations above $1,000\text{ ppm}$ by widespread methanogenic archaea. Carbon dioxide ($CO_2$) was not abundant enough to prevent global glaciation. Basaltic rock weathering on the early continents was highly efficient, drawing $CO_2$ out of the atmosphere and mineralizing it as carbonates. This weathering feedback restricted Archean atmospheric $CO_2$ pressure to less than $0.01\text{ bar}$. Because atmospheric methane is unstable and rapidly destroyed by solar ultraviolet radiation (photodissociation), a continuous biological source was required. Without these methanogenic microbes, Earth would have immediately entered a global ice age.

Scientist 2
Methanogenic microbes had not yet evolved during the Archean, so biogenic methane was absent. Instead, Earth was kept warm by extremely high levels of carbon dioxide ($CO_2$)—reaching partial pressures of $1$ to $2\text{ bar}$—supplemented by volcanic hydrogen ($H_2$). Basaltic weathering was negligible because continental landmasses were small and mostly submerged, preventing the drawdown of $CO_2$. Volcanic outgassing continuously supplied $CO_2$ and $H_2$ to the atmosphere. Furthermore, collision-induced absorption between $N_2$, $CO_2$, and $H_2$ significantly boosted the warming effect of these gases. The Archean climate was thus regulated entirely by abiotic, geochemical cycles.

According to the passage, Scientist 1 and Scientist 2 differ in their views regarding which of the following aspects of the Archean Earth?

Two paleontologists discuss the origin of flight in birds.

* Cursorial Hypothesis: Flight evolved in ground-dwelling ancestors that ran along the ground and flapped their forelimbs to assist in running and jumping over obstacles.
* Arboreal Hypothesis: Flight evolved in tree-dwelling ancestors that leaped between tree branches and glided down to the ground.

* New Evidence: Paleontologists discover a fossil of a primitive bird ancestor. An analysis of the fossil shows that its feet were physically incapable of grasping tree branches, but its hind legs were highly adapted for high-speed running on flat ground.

Which of the following describes how this new evidence affects the two hypotheses?

Three students discuss the mechanism by which a newly discovered plant hormone, *abscisigen*, inhibits seed germination.

* Student 1: Abscisigen directly blocks the synthesis of gibberellins (growth-promoting hormones) in the seed embryo.
* Student 2: Abscisigen prevents water uptake by increasing the solute concentration inside the seed coat, making it hypertonic relative to the surrounding environment.
* Student 3: Abscisigen physically hardens the seed coat by promoting lignin deposition, preventing the embryo's radicle (root) from breaking through.

Match each student's hypothesis with the experimental outcome that would directly invalidate (disprove) that hypothesis.

A student conducted an experiment to measure the rate of water loss from a certain plant species under different relative humidity levels. The experimental design is summarized in the table below:

Suppose the student wants to perform a follow-up experiment to determine how temperature affects the water loss of this same plant species. Which of the following modifications to the experimental design would best allow the student to isolate the effect of temperature?

An environmental scientist investigated the relationship between water temperature and the concentration of dissolved oxygen (DO) at saturation. The scientist hypothesized that as water temperature increases, the concentration of DO at saturation also increases because higher temperatures increase the solubility of gases in water. To test this hypothesis, the scientist measured the DO concentration at saturation in water samples at five different temperatures. The results are presented in the table below.

Based on these results, does the data support the scientist's hypothesis, and how should the hypothesis be revised?

Martian Methane Plumes

In 2018, planetary scientists confirmed seasonal fluctuations in the concentration of atmospheric methane ($CH_4$) on Mars, peaking during the late northern summer. Three hypotheses have been proposed to explain the origin and release mechanism of this methane.

*Hypothesis 1*
Methane is produced continuously by active methanogenic microbes residing in deep, warm subsurface aquifers where liquid water is stable. This biologically produced gas migrates upward and becomes trapped in subsurface clathrates (crystalline water-based solids physically caging gas molecules). During the warmer summer months, the thermal gradient in the upper regolith shifts, destabilizing the shallowest clathrates. This physical destabilization releases pulsed streams of methane gas through micro-fractures into the atmosphere.

*Hypothesis 2*
Methane is generated abiotically through serpentinization—a reaction in which liquid water chemically alters olivine-rich rocks within the Martian crust, producing hydrogen gas ($H_2$) which then reacts with carbon dioxide ($CO_2$) to form $CH_4$. This process occurs continuously at depth. The resulting methane migrates upward and is weakly bound (adsorbed) to the surfaces of clay minerals in the cold, dry shallow regolith. During summer, increased solar ultraviolet (UV) radiation heats the shallow regolith, supplying the thermal energy required to desorb the methane from the clay surfaces, releasing it into the atmosphere.

*Hypothesis 3*
Methane is produced entirely at the surface from exogenous (external) sources. Carbonaceous meteorites and interplanetary dust particles continuously deposit organic macromolecular material onto the Martian surface. This accumulated organic matter, when exposed to the high intensity of solar UV radiation during the summer peak, undergoes photolysis (light-activated chemical breakdown), directly releasing $CH_4$ gas into the thin atmosphere. In this view, no subsurface reservoirs or internal geological/biological processes are involved in generating the methane.

Based on the descriptions of the three hypotheses, which of the following statements best identifies the core claim of Hypothesis 2 regarding the generation and release of Martian methane?

Silica-rich deposits discovered on Mars have led to competing models regarding their origin. Three scientists propose different mechanisms for how these deposits formed:

Scientist 1
The deposits formed through acid-sulfate leaching. Acidic groundwater ($pH < 3$) containing dissolved sulfate ions flowed through subterranean basaltic rocks. The acidic fluid selectively dissolved and removed elements such as magnesium ($Mg$), iron ($Fe$), and calcium ($Ca$), leaving behind a highly concentrated, insoluble silica residue ($SiO_2 > 90\%$). This process occurred under ambient, low-temperature subterranean conditions.

Scientist 2
The deposits resulted from solfataric alteration. High-temperature volcanic gases ($>200^\circ\text{C}$), specifically sulfur dioxide ($SO_2$) and hydrogen chloride ($HCl$), mixed with water vapor and rose through crustal fractures. This acidic steam reacted with the surrounding rock, vaporizing volatile metals and carrying them away, leaving amorphous silica crusts at the surface outlets (fumaroles).

Scientist 3
The deposits precipitated directly from a surface water body. A highly alkaline, silica-saturated lake filled the crater. As the lake water evaporated under cold, dry conditions, the concentration of dissolved silica exceeded saturation limits. This caused the silica to precipitate out of the solution alongside evaporite minerals like gypsum.

Match each specific geological mechanism to the scientist whose model proposes that mechanism.

Astrophysicists model the equilibrium temperature, $T_{eq}$ (in kelvins, $\text{K}$), of a planet orbiting a star using the following equation:

where $L$ is the star's luminosity, $a$ is the planet's albedo (the fraction of star radiation reflected by the planet), $d$ is the average distance from the star to the planet, and $\sigma$ is the Stefan-Boltzmann constant. Based on this model, match each proposed change in the physical parameters of the system (on the left) to its resulting effect on the equilibrium temperature $T_{eq}$ (on the right).

### Earth's Hydrothermal Vents and the Origin of Life

Two models describe the environment where life on Earth may have originated:

Model 1 (Hydrothermal Vent Model)
Life began near deep-sea hydrothermal vents. The hot, mineral-rich water emitted from these vents provided a continuous supply of chemical energy (such as hydrogen sulfide and methane) and metal catalysts necessary to synthesize the first organic molecules in the absence of sunlight.

Model 2 (Warm Little Pond Model)
Life began in shallow, terrestrial tidal pools. Wet-dry cycles driven by evaporation and rain concentrated organic compounds. Sunlight provided the energy source, and ultraviolet radiation catalyzed the chemical reactions needed to form complex polymers like RNA.

Match each of the environmental features or assumptions to the model classification that describes it.

A group of students designed several investigations to study how wind speed affects the rate of water evaporation. For each investigation, they set up two trials with different wind speeds. However, each setup introduced a distinct confounding variable or source of error. Match each experimental setup to the primary confounding variable or source of error that threatens its internal validity.

### Solar Coronal Heating

The temperature of the Sun's photosphere is approximately $5800\text{ K}$, yet the solar corona—the outermost layer of the solar atmosphere—reaches temperatures exceeding $10^6\text{ K}$. Two scientists propose different mechanisms to explain this coronal heating problem.

Scientist 1
Coronal heating is primarily driven by Wave Heating (AC heating). Convective motions in the photosphere jostle the footpoints of magnetic field lines, generating magnetohydrodynamic (MHD) waves, specifically Alfvén waves. These waves travel upward along the magnetic field lines into the corona. Because the corona has low density, these waves become non-linear and undergo dissipation (such as phase mixing and resonant absorption), transferring their kinetic and magnetic energy to the coronal plasma. The heating is a steady, continuous process occurring along the entire length of the magnetic loops, and it does not require any change in the overall topology (connection structure) of the magnetic fields.

Scientist 2
Coronal heating is primarily driven by Magnetic Reconnection (DC heating) via "nanoflares." The slow motion of photospheric footpoints causes magnetic loops in the corona to twist, shear, and braid around one another, storing magnetic energy. When the stress exceeds a critical threshold, the magnetic field lines abruptly snap and reconnect into a lower-energy configuration. This reconnection is highly localized and impulsive, releasing energy in brief, explosive bursts called nanoflares. Each nanoflare heats the local plasma to over $10^7\text{ K}$ before it cools. Wave propagation plays no significant role; the primary heating mechanism is the rapid, sporadic release of stored magnetic energy through topological reconfiguration of the magnetic fields.

Based on the viewpoints of Scientist 1 and Scientist 2, match each physical aspect of coronal heating on the left with the correct description of how the two scientists disagree on that aspect on the right.

The Paleocene-Eocene Thermal Maximum (PETM) Carbon Release

During the Paleocene-Eocene Thermal Maximum (PETM), about $56$ million years ago, Earth experienced rapid global warming accompanied by a large negative carbon isotope excursion (CIE), which is a significant decrease in the ratio of carbon-13 ($^{13}\text{C}$) to carbon-12 ($^{12}\text{C}$) in geological samples. Two scientists discuss the primary source of the carbon released during this event.

Scientist 1
The carbon was released from marine methane hydrates (clathrates) stored in continental slope sediments. Initial warming of deep ocean waters, caused by volcanic activity, destabilized these hydrates, rapidly releasing methane ($\text{CH}_4$) gas into the ocean and atmosphere. Methane hydrates have an extremely low carbon isotope signature ($\delta^{13}\text{C}$ of approximately $-60\permil$). Because this source is highly depleted in $^{13}\text{C}$, a relatively small addition of carbon (approximately $1,500$ to $2,500\text{ Pg C}$, where $1\text{ Pg} = 10^{15}\text{ g}$) is sufficient to cause the observed global CIE of about $3.0\permil$ in marine carbonates. Since the release originated in deep ocean sediments, the CIE should be recorded first and most intensely in marine benthic (deep-sea) organisms, with no associated increase in terrestrial combustion markers.

Scientist 2
The carbon was released from the burning and thermal decomposition of terrestrial organic matter, specifically thick peatlands and coal deposits, triggered by massive volcanic intrusions of magma into sedimentary basins. Terrestrial organic carbon has a moderately low carbon isotope signature ($\delta^{13}\text{C}$ of approximately $-25\permil$). Because this source is less depleted in $^{13}\text{C}$ than methane, a much larger mass of carbon (at least $6,000$ to $8,000\text{ Pg C}$) must have been released to produce the global CIE. Because the combustion and release occurred on land, terrestrial records should show the onset of the CIE before marine records. Furthermore, this scenario would lead to widespread global wildfires, leaving a distinct marker of increased charcoal and combustion byproducts, such as polycyclic aromatic hydrocarbons (PAHs), in sediment layers deposited during the CIE.

Researchers analyzed a new high-resolution sediment core spanning the PETM boundary and gathered the following data:

Based on these findings, which of the following statements best describes how the data align with the viewpoints of Scientist 1 and Scientist 2?

### Methane on Mars

Scientists have detected trace amounts of methane ($CH_4$) in the Martian atmosphere. Because methane is rapidly destroyed by ultraviolet (UV) radiation, its presence indicates an active source. Two hypotheses explain the origin and behavior of Martian methane.

Hypothesis 1

Methane is produced biologically by subsurface methanogenic microbes. These microbes reside in deep, liquid-water aquifers insulated by a thick cryosphere. The liquid water is maintained at temperatures around $0^\circ\text{C}$ to $20^\circ\text{C}$ by modest geothermal heat. The microbes combine carbon dioxide ($CO_2$) and hydrogen ($H_2$) to produce energy and release $CH_4$ as a metabolic waste product. The observed seasonal fluctuations in atmospheric methane concentration are due to variations in microbial metabolic rates, which increase during the warmer Martian summer.

Hypothesis 2

Methane is produced abiotically through serpentinization, a geochemical reaction. Deep within the crust, water heated to temperatures between $100^\circ\text{C}$ and $250^\circ\text{C}$ reacts with olivine-rich volcanic rocks to produce $H_2$, which then reacts with dissolved carbon oxides to form $CH_4$. This methane becomes trapped in clathrate hydrates (crystalline water-ice cages) within the cryosphere. The observed seasonal fluctuations are not due to active production, but rather the thermal destabilization of these shallow clathrate hydrates, which release trapped methane into the atmosphere as the ground warms during summer.

Match each parameter of Martian methane production and behavior on the left with the specific point of disagreement between Hypothesis 1 and Hypothesis 2 on the right.