ACT science practice test 50

Directions: Each passage is followed by several questions. After reading a passage, choose the best answer to each question and fill in the corresponding oval on your answer document. You may refer to the passages as often as necessary.

You are NOT permitted to use a calculator on this test.


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.

1. Which of the following best describes the independent variables of this investigation?

A. The types of metal used for the cathode and anode
B. The type of metal ion solution used in the anode jar
C. The amount of output voltage produced by various electrochemical cell configurations
D. The concentration of metal ion solution used in each electrochemical cell

2. Which variable should not be controlled for this experiment?

A. The surface area of the metal strip used for the anode
B. The type of metal strip used for the anode
C. The amount of time the electrochemical cell is allowed to operate
D. The concentration of potassium chloride used to make the salt bridge

3. A lead/silver electrochemical cell is expected to have an output voltage of 0.93 V. However, Experiment 6 from Table 9.1 shows a measured output voltage of 0.73 V. Which of the following might account for the difference?

A. Silver should have been the cathode, and lead should have been the anode, but the experimenter switched them.
B. Experiment 6 was the third time the silver ion solution had been used, and the concentration of silver ions had been diminished.
C. The experimenter mistakenly switched a lead strip with a zinc strip for the anode.
D. The experimenter used a 1.5-inch-wide strip of lead as the anode instead of the standard 1-inch strip.

4. Experiment 1 from Table 9.1 shows a zinc/lead cell with a measured output voltage of 0.63 V. Which of the following best describes the cell?

A. Zinc is the cathode and lead is the anode.
B. Zinc is the cathode and zinc ions are the anode.
C. Zinc is the anode and lead is the cathode.
D. Zinc is the anode and zinc ions are the cathode.

5. Figure 9.1 shows the electrochemical cell the student used for Experiment 2 where zinc was the anode and copper was the cathode. What can be inferred about the mass of the zinc and copper strips as the experiment progressed?

A. The mass of both strips increased.
B. The mass of both strips decreased.
C. The mass of the zinc strip increased, but the mass of the copper strip decreased.
D. The mass of the zinc strip decreased, but the mass of the copper strip increased.

6. Experiment 8 was conducted with an aluminum strip in Jar 1 and a copper strip in Jar 2. Which metal would be the anode?

A. Aluminum would be the anode.
B. Copper would be the anode.
C. Aluminum would be the anode until the metal dissolved to a certain point, and then copper would become the anode.
D. There is not enough information to determine which would be the anode.

7. Experiment 8 was conducted with an aluminum strip in Jar 1 and a copper strip in Jar 2. What would be the expected output voltage of the electrochemical cell?

A. 1.10 V
B. 1.66 V
C. 1.32 V
D. 2.00 V

8. Further experimentation finds that when electrochemical cells are hooked together in a series configuration, their output voltage is added together for total output voltage. How many copper/zinc electrochemical cells are required to light a diode that needs a minimum of 12.0 V to operate?

A. 2 cells
B. 10 cells
C. 11 cells
D. 12 cells