ACT Science Practice Test 56

PASSAGE V

Near the end of the 19th century, British engineer Osborne Reynolds ran a set of experiments to observe and predict the transition between laminar (steady) and turbulent flow of a liquid through a pipe. In Reynolds' experiments, dye was forced through a liquid to show visually when the flow changed from laminar to turbulent. Laminar flow is common only in cases in which the flow channel is relatively small, the fluid is moving slowly, and its viscosity (the degree to which a fluid resists flow under an applied force) is relatively high. In turbulent flow, the speed of the fluid at any given point is continuously undergoing changes in both magnitude and direction. Reynolds demonstrated that the transition from laminar to turbulent flow in a pipe depends upon the value of a mathematical quantity equal to the velocity of flow (V) times the diameter of the tube (D) times the mass density (ρ) of the fluid divided by its absolute viscosity (μ). The "Reynolds number," as it is called, is determined by the following equation:

Several students designed similar experiments to observe flow rates of different liquids. To conduct the experiments, the students were given the following apparatus:

o Liquid supply tank with clear test section tube and 'bell mouth' entrance

o 1 Rotameter to measure the velocity of flow (flow rate)

o Tap water

o Motor oil

o 4, 10-ft long smooth pipes of various diameters: 0.25-inch, 0.50-inch, 0.75-inch, 1.0-inch

Figure 1 illustrates an approximation of the set-up of each experiment.

Figure 1

Figure 2 shows approximate viscosities of the water and motor oils used in the experiments.

Figure 2

Experiment 1

In Experiment 1, students began with a pipe of diameter 0.25 inches. The pipe was set first at a 15° angle and tap water was released steadily from the tank into the pipe. The velocity of flow (V) was measured. The pipe was then set at a 30° angle, a 45° angle, and a 60° angle, water was released steadily from the tank into the pipe, and the velocity of flow was measured. The process was then repeated for each diameter of pipe using the same amount of water each time. All data were recorded in Table 1. Temperature of the water was held constant at 20°C.

Experiment 2

In the second experiment, the tap water was replaced by Motor Oil A and the processes were repeated. The results are given in Table 2.

Experiment 3

In a third experiment, the tap water was replaced by Motor Oil B and the processes were repeated.

PASSAGE VI

Certain preservatives known as sulfites are often added to fruit products to keep the fruit fresher longer. Use of sulfites is controversial because studies have linked sulfites to severe reactions in some asthmatics. Students performed 2 experiments to measure sulfite levels.

Experiment 1

Four solutions, each containing a different amount of sulfite dissolved in H₂O were prepared. A coloring agent was added that binds with sulfite to form a red compound that strongly absorbs light of a specific wavelength, and each solution was diluted to 100 mL. A blank solution was prepared in the same manner, but no sulfite was added. A colorimeter (a device that measures how much light of a selected wavelength is absorbed by a sample) was used to measure the absorbance of each solution. The absorbances were corrected by subtracting the absorbance of the blank solution from each reading (see Table 1 and Figure 1).

Figure 1

Experiment 2

A 100 g fruit sample was ground in a food processor with 50 mL of H₂O and the mixture was filtered. The food processor and remaining fruit were then washed with H₂O, these washings were filtered, and the liquid was added to the sample solution. The coloring agent was added and the solution was diluted to 100 mL. The procedure was repeated for several fruits, and the absorbances were measured (see Table 2).