ACT Science Practice Test 99

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.


PASSAGE VII

Several scientists considered some different environmental factors and their influence on the growth of certain bacteria. The following experiments used Salmonella bacteria to measure the effect of pH levels, nutrients, and temperature on the number of bacteria produced within a given time period.

Experiment 1

A known quantity of Salmonella bacteria was placed in each of 3 Petri dishes with the same nutrient concentration at the same temperature. The pH level of each nutrient concentration in each dish was varied according to Table 1. On the pH scale, 7 represents neutral, values less than 7 indicate an acid, and values greater than 7 indicate a base. The lids of the Petri dishes were replaced after the bacteria were added and the dishes were left alone. After 6 hours, the percent growth of Salmonella bacteria was recorded (Table 1).

Experiment 2

A known quantity of Salmonella bacteria was placed in each of 3 Petri dishes with different nutrient concentrations in the form of organic compounds. The temperature and pH level (neutral 7) were held constant in each sample. The lids of the Petri dishes were replaced after the bacteria were added and the dishes were left alone. After 6 hours, the percent growth of Salmonella bacteria was recorded (Table 2).

Experiment 3

A known quantity of Salmonella bacteria was placed in each of 3 Petri dishes at different temperatures. The pH level and nutrient concentrations were held constant. The lids of the Petri dishes were replaced after the bacteria were added and the dishes were left alone. After 6 hours, the percent growth of Salmonella bacteria was recorded (Table 3).

1. According to Table 1, what might best contribute to the growth of Salmonella bacteria?

A. A pH level above 9
B. A pH level below 5
C. A pH level near 7
D. A pH level near 5

2. According to the results of the three experiments, which combination of the three factors studied would be expected to produce the highest percent growth?

F. pH level of 5, organic compound in Dish 2, temperature of 40°C
G. pH level of 7, organic compound in Dish 2, temperature of 10°C
H. pH level of 5, organic compound in Dish 1, temperature of 40°C
J. pH level of 9, organic compound in Dish 1, temperature of 90°C

3. Which of the following conclusions is strengthened by the results of Experiment 1?

A. Salmonella bacteria reproduce most efficiently in an acidic environment.
B. Salmonella bacteria reproduce most efficiently in a neutral environment.
C. Salmonella bacteria cannot reproduce in a basic environment.
D. Salmonella bacteria cannot reproduce in an acidic environment.

4. Bacteria will generally reproduce until all of the nutrients available have been depleted. How could Experiment 2 be altered to maximize the length of time that bacteria will reproduce?

F. Change the observation time from 6 hours to 12 hours.
G. Regularly re-supply each group of bacteria with unlimited nutrients.
H. Increase the rate of growth by decreasing the pH levels.
J. Do not test the effect of different nutrient combinations on growth.

5. Which of the following was the independent variable in Experiment 3?

A. pH level
B. temperature
C. organic compound
D. growth of bacteria

6. The experiments recorded the percent growth that occurred over a 6-hour period. Bacteria often reproduce at a rate that drastically varies from one stage to the next. The best way to study the different stages of growth would be to record the percent growth:

F. after 2 hours only.
G. after 4 hours, then again after 6 hours.
H. after 8 hours only.
J. every 15 minutes for 3 hours.