Suggesting Experiments to Resolve Viewpoints

Two scientists discuss the conditions necessary for the rusting of iron ($Fe$).

Scientist 1
Rusting is a chemical reaction that requires only iron and oxygen ($O_2$) gas. When iron is exposed to $O_2$, it reacts to form iron oxide (rust). Water ($H_2O$) is not necessary for this reaction to occur.

Scientist 2
Rusting is a chemical reaction that requires only iron and liquid water ($H_2O$). When iron is exposed to $H_2O$, it reacts to form rust. Oxygen ($O_2$) gas is not necessary for this reaction to occur.

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

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

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

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

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

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

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

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

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

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?

Three scientists discuss the Mpemba effect, a phenomenon where initially warm water freezes faster than initially cold water under identical cooling conditions.

Scientist 1
The effect is primarily driven by mass loss due to evaporation. As warm water cools, it loses a significant portion of its mass to evaporation. Because less mass requires less heat removal to reach its freezing point, the initially warm water freezes first.

Scientist 2
The effect is caused by dissolved gases. Cold water naturally contains a higher concentration of dissolved gases (such as oxygen and carbon dioxide) than warm water. These gases act as solute impurities, lowering the freezing point of the cold water and inhibiting rapid ice crystallization.

Scientist 3
The effect is due to convection currents. When warm water is placed in a freezer, a steep temperature gradient between the hot core and the cold surface creates rapid, sustained convection currents. This enhances the rate of heat transfer to the environment compared to the weaker convection in initially cold water.

Match each scientist's hypothesis with the corresponding experimental outcome that would invalidate that hypothesis.

A team of marine biologists is investigating the source of organic carbon that supports the food web in the Mariana Trench, located at a depth of over 10,000 meters. The scientists propose three different hypotheses to explain where the organic carbon originates.

* Hypothesis 1: The organic carbon in the trench is derived from dead photosynthetic plankton sinking from the sunlit surface waters.
* Hypothesis 2: The organic carbon is produced locally in the trench by chemosynthetic bacteria that utilize geothermal chemical energy from deep-sea hydrothermal vents.
* Hypothesis 3: The organic carbon consists of terrestrial plant debris transported from land down the slopes of submarine canyons during storm events.

Match each hypothesis with the experimental observation that would most directly invalidate (disprove) it.

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?

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.

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?

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.

### 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?