ACT Science Practice Test 44

Groundwater is water stored beneath the surface of the Earth. Groundwater chemistry in 2 bodies of water—drawn from an aquifer and from beneath a wetland—was studied during a 2000 summer drought and again during the next summer, which had normal rainfall. Figure 1 shows the methane (CH4) gas concentration in the groundwater at various depths in the aquifer and wetland. Figures 2 and 3 show the groundwater conductivity (directly proportional to the concentration of the dissolved ions) and pH at various depths in the aquifer and wetland, respectively. Also shown are the locations of the water table and the depths of the aquifer and wetland below this level.

Figure 1

Figure 2

Figure 3

Polyatomic ions can be represented by the combination of symbols

(X_aZ_b)ⁿ

where a is the number of atoms of Element X, b is the number of atoms of Element Z, and n is the total positive or negative charge for the entire polyatomic ion. If a or b is equal to 1, then the number 1 is omitted. If n is equal to 0, indicating a net neutral charge, then the number 0 is also omitted. For example, (NH₄)⁺¹ represents an ammonium ion, which contains 1 nitrogen atom (the number 1 is omitted), 4 hydrogen atoms, and a net polyatomic ion charge of +1. Since polyatomic ions carry a charge, they are very soluble in water, as opposed to neutral molecules.

Some atoms that comprise polyatomic ions are able to donate or accept different numbers of electrons, depending on the atoms with which they interact. In these different situations, they are said to have different oxidation states. For instance, while oxygen (O) typically has a constant oxidation state of -2, meaning it typically only accepts 2 extra electrons, chlorine (Cl) can have oxidation states of -1, +1, +3, +5, and +7, meaning that it has the ability to either accept 1 extra electron (-1) or donate either 1, 3, 5, or 7 electrons. The specific ions that feature the different oxidation states of chlorine are listed below.

The energy required to remove an electron from an atom, thereby giving that atom a more positive oxidation state, is known as an ionization energy. The ionization energies for removing each of the first four electrons from elements with atomic numbers 11-15, as measured in electron-volts (eV), are shown in Figure 1.

Figure 1

Simple diffusion (SD) is the process by which an uncharged solute in water migrates directly across an uncharged membrane, while facilitated diffusion (FD) is the process by which a charged or polar solute travels through a channel or transporter that crosses the membrane. Figure 1 illustrates how two solutes can diffuse, one by SD and one by FD.

Figure 1

Solutes that cross a membrane by SD or by FD show different rates of flow across a membrane, also known as flux. As a solute crosses a membrane by SD, the flux follows a linear pattern over time, with smaller solutes having the greatest increase in flux over time. As a solute crosses a membrane by FD, the flux follows a logarithmic pattern, leveling off at a maximum flux since there are only a limited number of channels or transporters through which the solute can travel.

Experiment 1

One scientist introduced five different solutes of the same concentration to similar membranes at a constant temperature. The molecular masses of these solutes are shown in Table 1.

This scientist then measured the time it took for the solute to reach equilibrium, which is a state of equal concentration of the solute on both sides of the membrane. The results are shown in Figure 2.

Figure 2

Experiment 2

Mixtures of solutes are subsequently introduced near three different membranes with different properties. The results of these three trials are presented in Figure 3.

Figure 3

The term "evolution" is often used in the context of biological changes in organism populations over time, but it can also be applied to the change in the chemical composition of the Earth's atmosphere. The hypotheses of two studies claim that this chemical evolution has altered the types of chemicals found in the atmosphere between the early stages of Earth's existence and the present day.

Study 1

Based on the hypothesis that volcanic eruptions were the source of gases in the early Earth's atmosphere, scientists recreated four model volcanic eruptions in closed chambers, each containing different percentages of the same volcanic particulate matter. They then observed the gases in the air above this model over time. The percent composition of this air after 1 day, when the air achieved a steady state of constant gas concentrations, is represented in Table 1.

Since the experiment provided only a suggestion of the gas levels in the early Earth's atmosphere, the scientists then analyzed the amount of trapped gases in sediment layers, which indicate the changing atmospheric levels of gases over billions of years. The data collected on O₂ and H₂O vapor are presented in Figure 1.

Study 2

A separate study used the same volcanic models as in Study 1, but it hypothesized that the scientists in Study 1 underestimated the amount of H2 in the early Earth atmosphere. They proposed a different composition of gases, highlighting an increased H₂ level in the atmosphere, also represented in Table 1. Based on these new data, the scientists proposed an alternative graph for the changing atmospheric levels of O₂ and H₂O vapor, also shown in Figure 1.

Figure 1