All practice questions

Suppose a student wants to modify an experiment measuring the evaporation rate of salt water ($200\text{ mL}$ of $5\%$ saline solution heated by a $100\text{ W}$ heat lamp placed $30\text{ cm}$ above the beaker in a draft-free room) to determine the specific impact of wind speed on the evaporation rate, while ensuring that the thermal energy input and other variables remain controlled. Arrange the following steps in the correct chronological sequence to successfully conduct this follow-up experiment.

Earth’s water content and its source have been a subject of ongoing debate. Two major models address the origin and delivery of water to early Earth.

Model 1 (Endogenous Mantle Source)
This model proposes that Earth accreted 'wet' from planetesimals that formed in the inner Solar System (within 2.5 astronomical units, or $\text{AU}$, from the Sun). These planetesimals contained hydrous silicate minerals that trapped water during their formation. According to Model 1, early Earth's mantle stored large reservoirs of water, which were gradually released to the surface through volcanic outgassing during the Hadean eon. The isotopic composition of hydrogen, specifically the deuterium-to-hydrogen ($D/H$) ratio, of Earth's water is believed to match that of these inner Solar System planetesimals.

Model 2 (Late Veneer Asteroidal Source)
This model proposes that Earth accreted 'dry' due to high temperatures in the inner solar nebula, which prevented water from condensing or remaining bound to inner Solar System planetesimals. Instead, Earth's water was delivered during a 'late veneer' phase—a period of intense bombardment about 100 to 200 million years after Earth's core formation. This delivery occurred via carbonaceous chondrite asteroids originating from the outer Solar System (beyond 2.5 $\text{AU}$). Model 2 assumes that the $D/H$ ratio of Earth's oceans is identical to that of outer Solar System carbonaceous chondrites, which differs significantly from the $D/H$ ratio of primordial inner Solar System materials.

Based on the descriptions of Model 1 and Model 2, is the following statement true or false?

Statement: Model 1 proposes that the deuterium-to-hydrogen ($D/H$) ratio of Earth's oceans is identical to that of carbonaceous chondrite asteroids originating beyond 2.5 $\text{AU}$.

### Origin of Lunar Material

Three hypotheses have been proposed to explain the origin of the Moon, particularly focusing on why the Moon's isotopic composition (such as the ratio of oxygen isotopes $^{17}\text{O}$ and $^{18}\text{O}$) is nearly identical to that of Earth's mantle, while other solar system bodies have distinct isotopic signatures.

*Hypothesis 1*
The Moon formed from a single, high-velocity, grazing collision between the proto-Earth and a Mars-sized planetesimal named Theia. The impact ejected a disk of molten debris into orbit. Because of the grazing angle, the debris disk was composed almost entirely (more than $80\%$) of mantle material from Theia. The Moon then accreted from this disk. The isotopic similarity between Earth and the Moon is an accidental consequence of Theia having formed in a similar region of the solar nebula as Earth, sharing the same isotopic reservoir.

*Hypothesis 2*
A high-energy, high-angular-momentum collision between proto-Earth and Theia completely vaporized both bodies, creating a giant, rapidly rotating, donut-shaped structure of silicate vapor called a *synestia*. The synestia was a single, fully homogenized system where turbulent mixing equalized all isotopic ratios. As the outer regions cooled below the condensation temperature of silicates, molten droplets condensed and accreted to form the Moon, while the inner region contracted to form the Earth.

*Hypothesis 3*
The Moon is the product of approximately 20 successive, smaller impacts by planetesimals ($1\%$-$10\%$ of Earth's mass) rather than a single giant impact. Each collision ejected a mix of proto-Earth mantle and impactor material, forming a debris disk that accreted into a "moonlet." Tidal forces caused each new moonlet to migrate outward and merge with pre-existing moonlets, eventually forming the Moon. The isotopic similarity to Earth is due to the statistical averaging of the varied impactor compositions and the fact that a large fraction of the ejected material in each smaller impact came directly from Earth's mantle.

Based on Hypothesis 1 and Hypothesis 3, which of the following statements represents a core difference in how the two hypotheses explain the isotopic similarity between the Earth and the Moon?

A student measures the electric current, $I$ (in amperes, $\text{A}$), passing through a resistor in a closed circuit with a constant voltage. The current is inversely proportional to the resistance, $R$ (in ohms, $\Omega$). When the resistance is $4.0\text{ }\Omega$, the current is $3.0\text{ A}$. What is the current, in amperes, when the resistance is changed to $6.0\text{ }\Omega$?

### Origins of Prebiotic Organic Molecules

How did organic molecules, the building blocks of life, first accumulate on early Earth? Two researchers propose different hypotheses.

Researcher 1
Organic molecules on early Earth were synthesized endogenously (locally) at deep-sea hydrothermal vents. The reducing fluids rich in dissolved gases such as hydrogen ($H_2$) and carbon dioxide ($CO_2$) reacted in the presence of iron-sulfide mineral catalysts. These chemical reactions occurred at high temperatures ($100^\circ\text{C}$ to $350^\circ\text{C}$) and high pressures, producing amino acids and other complex organic compounds. Early Earth's atmospheric composition was irrelevant to this process because the synthesis occurred deep within the oceans, isolated from the atmosphere.

Researcher 2
Organic molecules on early Earth were delivered exogenously by carbonaceous meteorites and cosmic dust during the Late Heavy Bombardment. Synthesis of these molecules occurred in interstellar space under extremely low temperatures (near $-260^\circ\text{C}$) and low pressures, catalyzed by UV radiation on ice-grain surfaces. Endogenous synthesis at hydrothermal vents was impossible because the high temperatures ($>100^\circ\text{C}$) at these vents would rapidly decompose, rather than build, complex organic molecules like amino acids.

Based on the viewpoints of Researcher 1 and Researcher 2, on which of the following points do the two researchers disagree?

### Origin of Earth's Water

Scientists have proposed two competing models to explain the source and timing of the accumulation of Earth's water.

Model 1 (Late Veneer Delivery)
Earth accreted in a region of the solar nebula that was too hot for volatile compounds, such as water, to condense. Consequently, the proto-Earth was dry. After Earth’s core formed, water-rich carbonaceous chondrites (asteroids) from the outer solar system impacted Earth, delivering water and volatile elements. This model is supported by the concentrations of highly siderophile (iron-loving) elements (HSEs) in Earth's mantle, which are found in chondritic proportions. Since core formation would have stripped primordial HSEs from the mantle, these elements must have arrived via a "late veneer" of asteroid impacts after core formation. The deuterium-to-hydrogen ($D/H$) ratio of Earth's oceans matches that of carbonaceous chondrites ($1.5 \times 10^{-4}$).

Model 2 (Endogenous Wet Accretion)
Earth accreted from material that already contained water-bearing minerals. Primordial dust grains and chondrites in Earth's accretion zone contained adsorbed water or hydrous silicates that survived the high temperatures. As Earth grew, this water was incorporated directly into the mantle and dissolved in the early magma ocean. High-pressure mineral phases, such as ringwoodite in the transition zone, stored vast reservoirs of water. Over time, volcanic activity outgassed water vapor to form the oceans. This model is supported by isotopic analyses showing that deep mantle reservoirs have a $D/H$ ratio of $1.3 \times 10^{-4}$, which is significantly lower than surface oceans but matches enstatite chondrites, the primary isotopic match for Earth’s bulk rock composition.

Based on the models, which of the following statements best describes a major difference between Model 1 and Model 2 regarding the timing of Earth's core formation relative to the arrival of Earth's water?

Early in Earth's history, about $3.8$ billion years ago, the Sun's energy output was approximately $70\%$ of its current value. Under these conditions, liquid surface water should have frozen, yet geological evidence confirms liquid water existed. Two models attempt to resolve this "Faint Young Sun Paradox."

Model 1 (Carbon Dioxide–Methane Greenhouse)
Early Earth's atmosphere contained extremely high levels of carbon dioxide ($CO_2$) and methane ($CH_4$). Volcanic outgassing and the lack of continental weathering maintained $CO_2$ levels up to $100$ times higher than today. Early methanogenic microbes biokinetically produced $CH_4$. Together, these greenhouse gases trapped sufficient outgoing infrared radiation to keep surface temperatures above freezing.

Model 2 (Ammonia Greenhouse)
Volcanic environments reacted outgassed nitrogen compounds with iron catalysts to produce significant quantities of ammonia ($NH_3$). As a potent greenhouse gas, even a few parts per million of $NH_3$ would keep Earth warm. Although solar ultraviolet (UV) radiation photolyzes $NH_3$ into inert nitrogen gas ($N_2$), a thick organic haze in the upper atmosphere shielded the $NH_3$ from UV destruction.

Based on the descriptions of Model 1 and Model 2, match each of the new scientific findings below to the statement that best describes its logical impact on the models.

### Models of Acid-Base Behavior

Two models are proposed to describe the behavior of acids and bases in chemical reactions.

Model 1 (Arrhenius Model)
Acids are substances that dissociate in aqueous (water-based) solutions to produce hydrogen ions ($H^+$). Bases are substances that dissociate in aqueous solutions to produce hydroxide ions ($OH^-$). Under this model, acid-base reactions are limited to aqueous environments.

Model 2 (Brønsted-Lowry Model)
Acids are substances that donate a proton ($H^+$) to another substance in a reaction. Bases are substances that accept a proton ($H^+$) from another substance. Under this model, acid-base reactions do not require an aqueous solution.

Based on these models, which of the following statements describes a major difference between Model 1 and Model 2 regarding the environment in which acid-base reactions can occur?

### The Paleocene-Eocene Thermal Maximum

During the Paleocene-Eocene Thermal Maximum (PETM), a rapid injection of carbon into the atmosphere and oceans caused global warming and a significant negative carbon isotope excursion (CIE)—a sharp decrease in the ratio of carbon-13 ($^{13}\text{C}$) to carbon-12 ($^{12}\text{C}$) in sediment samples. Two hypotheses explain the source of this carbon.

Hypothesis 1 (Methane Clathrate Release)
The carbon was released via the sudden dissociation of methane clathrates (methane ice trapped in deep-sea sediments). Because clathrates contain biogenic methane, they are extremely depleted in $^{13}\text{C}$ ($\delta^{13}\text{C} \approx -60\text{‰}$). Due to this extreme depletion, a relatively small mass of carbon (approximately $1,500\text{ to }2,000\text{ gigatons}$) is sufficient to cause the observed CIE.

Hypothesis 2 (Volcanic Outgassing)
The carbon was released by volcanic activity associated with the North Atlantic Igneous Province. Magma intruded into organic-rich basins, venting thermogenic methane and carbon dioxide ($\text{CO}_2$). This volcanic/thermogenic carbon is moderately depleted in $^{13}\text{C}$ ($\delta^{13}\text{C} \approx -30\text{‰}$). Because it is less depleted than biogenic methane, a much larger mass of carbon (approximately $4,000\text{ to }5,000\text{ gigatons}$) is required to cause the observed CIE.

New Evidence
Researchers recently calculated the total mass of carbon added to the oceans and atmosphere during the PETM onset to be approximately $4,500\text{ gigatons}$.

Based on this new evidence, how does the calculated mass of $4,500\text{ gigatons}$ of carbon affect the two hypotheses?

Triassic-Jurassic Extinction

Two scientists discuss the cause of the mass extinction at the end of the Triassic period:

*Scientist 1*
The extinction was caused by massive volcanic eruptions in the Central Atlantic Magmatic Province. These eruptions released immense amounts of carbon dioxide, leading to rapid global warming and acid rain, which devastated ecosystems.

*Scientist 2*
The extinction was caused by a large asteroid impact. The impact blasted dust and sulfur compounds into the atmosphere, blocking sunlight and causing a sudden, severe global cooling event that killed off most species.

Geologists recently analyzed sediment layers from the Triassic-Jurassic boundary and found high concentrations of volcanic ash and basalt fragments, but no evidence of shocked quartz or iridium (elements commonly associated with asteroid impacts).

Based on this new evidence, how are the scientists' viewpoints affected?

A scientist conducted two experiments to study soil respiration (measured as the rate of carbon dioxide release, $R_s$, in $\text{g CO}_2/\text{m}^2/\text{day}$) from a forest soil sample under different conditions.

Suppose a scientist wants to determine if the positive relationship between SMC and $R_s$ observed at $20^\circ\text{C}$ remains positive at a near-freezing temperature of $2^\circ\text{C}$. Which of the following modifications to the experimental design would best allow the scientist to test this hypothesis?

### Plant Growth and Light Wavelengths

Two students discuss how the color of light affects plant growth. Both students agree that light is necessary for plants to produce food, but they disagree on which color of light is most effective.

Student 1
Plants grow tallest and healthiest when exposed to green light. Leaves appear green because they contain chlorophyll, a pigment that is naturally tuned to green wavelengths. Therefore, chlorophyll absorbs green light more efficiently than any other color, leading to higher rates of photosynthesis and growth.

Student 2
Plants grow tallest and healthiest when exposed to blue and red light. Chlorophyll absorbs blue and red light to power photosynthesis, while reflecting green light. Because green light is reflected rather than absorbed, plants exposed only to green light will exhibit very little growth.

Based on the passage, which of the following statements best represents the core hypothesis of Student 1?

A student proposed the following hypothesis regarding soil drainage:

*Hypothesis*: Soil permeability (the rate at which water flows through soil) is determined by the average particle size of the soil, such that soils with larger average particle sizes will always have higher water flow rates, regardless of the compaction level of the soil.

To test this hypothesis, the student measured the water flow rate, in milliliters per minute ($\text{mL/min}$), through three different soil samples under both uncompacted and compacted conditions. The results are shown in the table below.

Based on these results, do the data support the student's hypothesis?

A team of astrophysicists modeled the equilibrium surface temperature, $T$ (in Kelvin, $\text{K}$), of airless rocky planets orbiting a distant star. According to the model, the temperature is predicted by the following equation:

where:
- $T_0$ is a star-specific constant equal to $400\text{ K}$.
- $a$ is the planet's albedo (reflectivity), ranging from $0.0$ to $1.0$.
- $D$ is the planet's distance from the star in astronomical units ($\text{AU}$).

Based on this model, arrange the four planets (W, X, Y, and Z) shown in the diagram in order of their predicted equilibrium surface temperature, from lowest to highest.

Two students discuss how migratory birds navigate over long distances.

*Student 1* claims that birds navigate primarily by detecting the Earth's magnetic field (magnetoreception). According to this view, birds can navigate successfully regardless of visibility or geographical features.

*Student 2* claims that birds navigate primarily by recognizing visual landmarks, such as coastlines, rivers, and mountain ranges. According to this view, birds require clear visibility and familiar geographical features to remain on course.

A researcher conducted an experiment where a flock of migratory birds was tracked during an overcast night as they flew over a featureless ocean. The birds successfully maintained their standard migration route.

This experimental finding is consistent with the viewpoint(s) of which student(s)?

### Heat Source of Enceladus

Scientists debate the primary heat source maintaining the subsurface liquid water ocean on Saturn's moon, Enceladus.

Hypothesis 1
The liquid ocean is maintained primarily by tidal heating. Gravitational interactions with Saturn and other moons deform Enceladus, generating frictional heat within its silicate core and ice shell. Frictional heating in the core is highly localized and can produce temperatures exceeding $100^\circ\text{C}$.

Hypothesis 2
The liquid ocean is maintained primarily by radiogenic heating. The decay of radioactive isotopes (such as Uranium-235 and Potassium-40) in the moon's rocky core releases steady, uniform thermal energy. This decay is estimated to produce maximum core-mantle boundary temperatures of approximately $30^\circ\text{C}$.

New Evidence
Analysis of the water plumes erupting from Enceladus's south polar region reveals the presence of silica nanoparticles ($SiO_2$). Laboratory experiments show that these nanoparticles can only form when alkaline water containing dissolved silica is heated to at least $90^\circ\text{C}$ at the seafloor.

Which of the following statements best describes how this new evidence affects Hypotheses 1 and 2?

### Water Absorption in Horned Lizards

Texas horned lizards (*Phrynosoma cornutum*) live in arid environments and are known for their ability to collect water from moist sand using their body surfaces. Two scientists discuss the mechanism by which this water enters the lizard's body.

Scientist 1
Horned lizards collect water through their skin via capillary action. The lizard's scales form a network of microscopic, hinge-like channels. When the lizard stands on damp sand or is rained on, capillary forces draw water along these channels toward the corners of the lizard's mouth. The lizard then actively gulps and swallows this water, importing it into the digestive tract for absorption. The skin itself is entirely impermeable to water, serving only as a physical transport network.

Scientist 2
Horned lizards absorb water directly through their skin cells (transdermal absorption) into their bloodstream, bypassing the mouth and digestive tract. The scale channels merely spread water evenly across the body to maximize the surface area available for absorption. The skin possesses specialized, moisture-sensitive micro-pores that open upon contact with liquid water, allowing direct diffusion into the subcutaneous capillaries. The lizard does not need to swallow to hydrate.

Which of the following experiments would best resolve the conflict between the two scientists' viewpoints?

A student measures the frequency ($f$, in hertz) and wavelength ($\lambda$, in meters) of sound waves propagating through a room at a constant temperature. The results are recorded in the table below:

Based on these results, which of the following statements best describes the relationship between the frequency and wavelength of the sound waves?

A passage on lunar swirls is shown below, followed by three hypotheses. Match each hypothesis to the statement that best describes its stance on the origin of the magnetic anomalies and the cause of the brightness contrast.

### Lunar Swirls
Lunar swirls are light-colored, winding patterns observed on the Moon's surface. They are always associated with localized crustal magnetic fields (magnetic anomalies), but their origin is debated.

Hypothesis 1 (Comet Impact)
Lunar swirls are created when a comet coma collides with the lunar surface. The gas and dust in the coma mechanically scour away the dark, weathered upper layer of lunar soil (regolith), exposing the brighter, unweathered soil underneath. The impact's high-energy plasma flow magnetizes the local iron-bearing minerals in the crust, forming the magnetic anomaly. This event occurs instantaneously, and the swirls are stable features that do not change over short timescales. Solar wind weathering does not play a significant role in creating the brightness contrast.

Hypothesis 2 (Solar Wind Shielding)
Lunar swirls are the result of ongoing shielding from solar wind weathering. Solar radiation and ions normally darken the lunar surface over millions of years. However, the localized crustal magnetic anomalies—which are remnants of an ancient, now-defunct global lunar dynamo—deflect the solar wind. The shielded areas remain bright, while unshielded areas surrounding them continue to darken. Swirl formation is a slow, gradual process, and the swirls are not currently undergoing active surface transport or dynamic changes.

Hypothesis 3 (Dust Transport)
Lunar swirls are formed by the electrostatic transport of fine dust. The Moon's surface is electrically charged by solar ultraviolet light. Near crustal magnetic anomalies (remnants of an ancient global magnetic field), localized electric fields are created. These electric fields selectively loft and transport very fine, highly reflective dust particles, concentrating them along the magnetic field lines. This electrostatic sorting is an active, dynamic process occurring daily, meaning the swirl patterns are constantly refreshed and can change over short periods. Solar wind shielding is not the main cause of the brightness contrast.

Deep-Sea Hydrothermal Vent Communities

Deep-sea hydrothermal vents support diverse communities of organisms in the absence of sunlight. Three researchers discuss the primary source of nutrients that forms the base of these food webs.

*Researcher 1*
The base of the food web relies entirely on local chemosynthesis by endosymbiotic bacteria. These bacteria live inside specialized tissues of host organisms, such as tube worms and clams, and oxidize hydrogen sulfide ($H_2S$) emitted from the vents. The host provides the bacteria with carbon dioxide and oxygen, while the bacteria synthesize organic compounds directly for the host.

*Researcher 2*
Local chemosynthesis occurs but is insufficient to sustain the high biomass of vent communities. Instead, these ecosystems rely on the downward drift of organic detritus (known as 'marine snow') from photosynthetic organisms living in the sunlit surface waters. The rich organic matter settles to the ocean floor, serving as the main source of nutrients.

*Researcher 3*
The primary nutrient source is chemosynthetic but does not rely on symbiosis. Free-living, chemolithoautotrophic bacteria in the water column and on seafloor rocks form dense bacterial mats. Mobile grazing invertebrates (such as crabs and snails) feed directly on these mats. These grazers are then consumed by larger predators, distributing nutrients throughout the ecosystem.

Based on the passage, match each statement describing a nutrient delivery mechanism to the researcher who proposes it.