ACT Science Practice Test 25

Phosphorus is an essential nutrient that can negatively affect water quality, primarily by promoting excessive plant and algae growth. When this occurs, plants and animals that live in the water are affected by the reduced sunlight and lower oxygen levels that develop as organic matter decomposes. For humans, algal blooms lead to a reduction in the quality of drinking water, a decrease in the use of the water source for recreational activities, and a decline in property value along waterfront areas.

Lakes are often classified according to their trophic state, which indicates their biological productivity. The least productive lakes are called oligotrophic. Bodies of water classified as oligotrophic are typically cool and clear, have relatively low nutrient concentrations, and provide excellent drinking water. The most productive lakes are called eutrophic and are characterized by high nutrient concentrations that result in algal growth, cloudy water, and low dissolved oxygen levels. Table 8.6 shows the phosphorus levels that are found in lakes with different trophic classifications.

TABLE 8.6

Lake managers collected data over approximately two decades in four different ecological areas of a large lake. Each area had a different target phosphorus level based on the natural ecological factors of the area, as indicated in Figures 8.7 through 8.10. For proper lake health, the level must be at or below the target amount. Levels in excess of the target amount lead to an imbalance in nutrient flow.

Figure 8.7

Figure 8.8

Figure 8.9

Figure 8.10

There is some evidence that ancient civilizations knew placing various metals together could create an electrical current. In the year 1800, Alexander Volta published experiments outlining his discovery of the voltaic pile, a device commonly referred to as the first electric battery. Volta stacked two different metals on either side of a wet felt disk and found that certain combinations produced an electrical voltage. By the early 1800s, many scientists were expanding on the idea of the voltaic pile by making apparatuses now known as voltaic cells. These cells generally contain two separate jars connected by a salt bridge, or porous membrane. Each jar contains a certain metal and a solution of the positive ions of the same metal. When different jars containing different metals are connected, an electrical voltage can be produced. A theoretical example of a copper/zinc voltaic cell is shown in Figure 9.1.

Figure 9.1

The last two centuries have seen a marked spike in demand for smaller batteries that can produce higher voltages for longer periods of time. This led a chemistry student to become interested in how using different metals in the voltaic cell could increase the voltage output of that cell. To conduct the experiment she chose four different types of metals that were available in strips from her local hardware store. These metals were zinc (Zn), lead (Pb), copper (Cu), and silver (Ag). She used 1-molar concentrated solutions of each of the various metal ions. She then made a salt bridge out of filter paper soaked in a potassium chloride brine solution. Many different combinations of metals were attempted, and the voltage output was measured with a standard voltmeter. The results of the experiment are recorded in Table 9.1.

TABLE 9.1

After the experiment was completed, the chemistry student looked at the literature to make sense of her results. She found two definitions particularly helpful. The anode was defined as the metal strip where oxidation occurs. The metal atoms were losing electrons and dissolving into the solution as metal ions. Electrons from the anode were free to move through the wire toward the cathode. The cathode was defined as the metal strip where reduction occurs. The cathode had an abundance of electrons from the anode. Metal ions in the solution around the cathode accepted those electrons and joined the strip as additional solid metal atoms.

The student also found tables of standard reduction potentials (Table 9.2). These tables compared how much voltage should be produced when the metal is placed in an electrochemical cell with a standard electrode. One table showed each metal as a cathode, and the other showed each as an anode. The student learned that these tables were used to calculate the theoretical voltage output of any electrochemical cell. (Any electrochemical cell has to have both a cathode and an anode.)

TABLE 9.2

The theoretical output voltage of an electrochemical cell is the sum of the standard potentials of the anode and the cathode.