ACT Science Practice Test 2

Comets originate from regions of our solar system that are very far from the sun. The comets are formed from debris thrown from objects in the solar system: they have a nucleus of ice surrounded by dust and frozen gases. When comets are pulled into the earth's atmosphere by gravitational forces and become visible, they are called meteors. Meteors become visible about 50 to 85 km above the surface of Earth as air friction causes them to glow. Most meteors vaporize completely before they come within 50 km of the surface of Earth.

The Small Comet debate centers on whether dark spots and streaks seen in images of the Earth's atmosphere are due to random technological noise or a constant rain of comets composed of ice. Recently, images were taken by two instruments, UVA and VIS, which are located in a satellite orbiting in Earth's magnetosphere. UVA and VIS take pictures of the aurora borealis phenomenon, which occurs in the magnetosphere. The UVA and VIS technologies provide images of energy, which cannot be seen by the human eye.

The pictures taken by VIS and UVA both show dark spots and streaks. Scientists debate whether these spots and streaks are due to a natural incident, such as small comets entering the atmosphere, or random technological noise. The layers of Earth's atmosphere are shown in Figure 1.

Figure 1

Two scientists debate whether there is a constant rain of comets burning up in Earth's magnetosphere.

Scientist 1

Small comets are pulled into Earth's atmosphere by gravitational effects and burn up in the magnetosphere. They are about 20 to 30 feet in diameter and burn up in the magnetosphere because they are much smaller than the comets that become meteors. Comets with larger radii will burn up in portions of the atmosphere much closer to Earth. About 30,000 small comets enter the Earth's magnetosphere every day. The dark spots and streaks on UVA and VIS images occur when the small comets begin to boil in the magnetosphere, releasing krypton and argon and creating gaseous H₂O, which interacts with hydroxyl, OH^-, radicals. Images taken by these instruments at different points in time show the same frequency of dark spots and streaks and give conclusive evidence in favor of the Small Comet theory. If the spots and streaks were due to random technological noise, then the frequency of their appearance would fluctuate.

Scientist 2

The dark spots and streaks in the UVA and VIS images are due to technological noise, not small comets. If the Small Comet Theory were true, and 20 small comets bombarded Earth's atmosphere per minute, there would be a visible bright object at least twice every five minutes. This is because, as objects enter the Earth's mesosphere, they burn up, creating large clouds of ice particles. As the ice particles vaporize, they have a brightness in the sky approximately equal to that of Venus. Because comets rarely enter Earth's atmosphere, such bright flashes are rare occurrences, far less than two times every five minutes, so the Small Comet theory cannot be correct. Further, since comets originate from regions of space beyond the orbit of the farthest planet, they contain argon and krypton. If the Small Comet theory were true and Earth were bombarded by 30,000 comets per day, there would be 500 times as much krypton in the atmosphere as there actually is.

A cotton fiber is composed of one very long cell with two cell walls. During a 2-week period of cell life called elongation, cotton fibers grow 3 to 6 cm. The level of hydrogen peroxide in cotton fiber cells during elongation is very high. Scientists wanted to study whether the level of hydrogen peroxide affected the length of the cotton fiber.

The amount of hydrogen peroxide is controlled by an enzyme called superoxide dismutase (SOD). This enzyme turns superoxide into hydrogen peroxide. Four identical lines of cotton fiber plants were created. Each line was able to express only one of three types of superoxide dismutase. The gene for SOD1 was incorporated into L1, the gene for SOD2 was incorporated into L2, and the gene for SOD3 was incorporated into L3.

Experiment

Five cotton plants of each line were grown in nutrient solution until cotton fibers completed the elongation period. The average length of cotton fibers and the average concentration of hydrogen peroxide were determined. This information is shown in Table 1.

Next, because the scientists had determined the average elongation period, they measured the amount of hydrogen peroxide and the length of the cotton fibers halfway through their elongation period. This information is shown in Table 2.

Finally, the scientists measured the amount of hydrogen peroxide and the length of cotton fibers on the first day of the elongation period. This information is shown in Table 3.

Convection is a heat transfer process caused by moving liquid or gas currents from a hot region to a cold region. As a liquid or gas cools, it gets more dense. An example of a convection process is a cup of hot coffee: the liquid toward the top is cooled by the air, so it becomes more dense and sinks to the bottom of the cup; the hotter liquid toward the bottom of the cup is less dense, so it rises toward the top. See Figure 1, below.

Figure 1

The temperature of the liquid at the hot end of the insulated system is higher than the temperature at the cool end of the system. The difference (ΔT) between the hot liquid at the bottom and the cold liquid at the top changes depending on the starting temperature of the system. Table 1 gives ΔT for 500 mL of water in an insulated container with a height of 6.0 cm and a cross-sectional area of 4.0 cm² when the container is heated to different temperatures.

Figure 2 shows how ΔT changes with cross-sectional area for 500 mL of 100°C water in a container with a height of 6.0 cm. Figure 3 shows how ΔT changes with height for 500 mL 100°C water in a container with a cross-sectional area of 4.0 cm².

Figure 2

Figure 3

Metallic alloys, solid mixtures of metal, are useful for coin production when they contain a high percentage of zinc. When electric current is applied to zinc in the presence of precious metal solutions of silver nitrate, copper sulfate or potassium gold cyanide, the precious metals plate (form a coating) on the zinc surface.

- Silver nitrate, formed when silver dissolves in nitric acid, reacts with zinc to form solid silver and zinc nitrate.

- Copper sulfate, formed when copper dissolves in sulfuric acid, reacts with zinc to form solid copper and zinc sulfate.

- Potassium gold cyanide contains reactive gold ions.

A chemist performed experiments on precious metal plating.

Experiment 1

The chemist obtained 4 coin-like samples of a high percentage zinc alloy. All samples were circular, had a radius of 1 cm, and had the same thickness. The mass of each coin was recorded. Each coin was wired via a battery to a strip of either pure silver or copper metal. Coins wired to silver were placed in dilute nitric acid and coins wired to copper were placed in dilute sulfuric acid. Electric current of either 1,000 milliamperes (mA) or 2,000 mA was applied for 30 minutes to each sample. The coins were removed and the increase in mass from precious metal plating was recorded in milligrams. Results of the experiment are shown in Table 1.

Experiment 2

The chemist completely dissolved equal amounts of pure silver in 4 beakers of nitric acid. He then placed equivalent coin-like samples of zinc into the beakers for different lengths of time measured in minutes (min). The coin surfaces developed a silver metal coating without any electric current applied. The concentrations of silver coating on the coin and zinc nitrate in the surrounding solution were determined in parts per billion (ppb) and recorded in Table 2.