ACT Science Practice Test 86

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.


When introduced into H2O, many solid substances are able to dissolve, or disperse evenly throughout the solvent. Salts have been found to dissolve easily when introduced into H2O, since they readily dissociate to yield ions that may interact directly with H2O. Molecular compounds, on the other hand, do not dissolve as easily, since their interactions with water typically do not permit ionization, the physical process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. Two experiments were conducted to better understand the solubility of salts and molecules in water at various temperatures. The solubility, S, was measured as follows:

S = (msub) / (mH20)

where msub was the mass of the substance dissolved in water, and mH20 was the mass of the water itself. △S, or the change in solubility (from 0℃), was calculated in the experiments for three salts and three molecules with increasing temperature. The mass of water was held constant at 100g for each of these experiments.

Figure 1 shows the results of comparing the solubilities of three salts with increasing temperature, while Figure 2 shows the results of comparing the solubilities of three molecules with increasing temperature. Molecular masses (MM) are shown for each substance.


Figure 1


Figure 2

1. Based on Figure 1, at 40℃ as the molecular masses of the salts increase, the △S:

A. decreases, because a greater mass of substance dissolves in the same mass of water.
B. decreases, because a smaller mass of substance dissolves in the same mass of water.
C. increases, because a greater mass of substance dissolves in the same mass of water.
D. increases, because a smaller mass of substance dissolves in the same mass of water.

2. Consider the trials represented in Figure 1 that occurred at 60℃. As the molecular mass of the substance decreased, the observed △S:

F. increased only.
G. increased, then decreased.
H. decreased only.
J. decreased, then increased.

3. If an additional trial had been done in which KCl dissolved in H2O at 102℃, while it was still in aqueous form, the △S most likely would have been:

A. less than 25.
B. between 25 and 35.
C. between 35 and 45.
D. greater than 45.

4. According to Figure 2, when NH3 was added to water at 20℃, the solubility of the resulting solution:

F. increased, because △S was positive.
G. increased, because △S was negative.
H. decreased, because △S was positive.
J. decreased, because △S was negative.

5. Based on Figures 1 and 2, which of the following combinations of solute and temperature at a known mH2O would produce the greatest increase in solubility?

A. CH4 (molecular compound, MM = 16) at 40℃
B. NaF (salt, MM = 42) at 40℃
C. CH4 (molecular compound, MM = 16) at 80℃
D. NaF (salt, MM = 42) at 80℃

Recombination of genes is usually associated with the sexual reproduction of cells, or meiosis. However, it can also occur when cells that undergo asexual reproduction, or mitosis, need to be repaired, such as after radiation exposure. This repair process, known as homologous recombination, aligns two copies of the same double strand of DNA, one with the error and one without. As seen in Figure 1, correct genes are transplanted from the correct strand to the one with errors (genes with errors are represented with a *).


Figure 1

The activities of some genes have been found to promote homologous recombination (HR). In an experiment to quantify the genetic control over HR, 4 scientists measured the frequency of HR per hour over a 24-hour period in isolated connective tissue cells from rats placed in growth media. They then lysed the cells, separated out the entire protein content, and used gel electrophoresis to count the amount of protein present in the cells (see Figure 2).


Figure 2

These scientists noticed that only a few specific proteins appeared to be responsible for promoting HR, and labeled the genes encoding them as W, X, Y, and Z. They engineered cells to express combinations of two active genes and recorded the HR. They then analyzed the DNA content of the lysed cells and calculated distances between four genes that encoded the relevant proteins (see Table 1).

Table 1
GenesHR
(events per hour)
Distance between genes (centimorgans)
W and X7520
X and Y12530
W and Z6015

Each of the 4 scientists then proposed individual models for the positions of the genes they studied, taking into account the findings in Table 1. Each model shows where genes may be located along a strand of DNA (see Figure 3). Each model correctly assumes that the lengths of the genes are insignificant compared to the length of the DNA.


Figure 3

A final experiment showed that rat connective tissue cells in which genes W and Y were active had an HR frequency of 45 times per hour.

6. All 4 models agree on the distance between which of the following pairs of genes?

F. Genes W and X
G. Genes W and Y
H. Genes X and Z
J. Genes Y and Z

7. According to Figure 2, if some of the connective tissue cells had a protein content of 3,500 molecules per cell, the HR of these cells is most likely closest to which of the following?

A. 50 events per hour
B. 100 events per hour
C. 150 events per hour
D. 200 events per hour

8. If Scientist 2's model is correct and an additional gene, Gene V, is 10 centimorgans from Gene X and 15 centimorgans from Gene Z, then Gene V is most likely between:

F. Genes W and X.
G. Genes W and Z.
H. Genes X and Y.
J. Genes X and Z.

9. The result of the final experiment studying the distance between Genes W and Y is consistent with models proposed by which of the following scientists?

A. Scientists 1 and 3
B. Scientists 1 and 4
C. Scientists 2 and 3
D. Scientists 3 and 4

10. Based on the information provided, HR would occur when connective tissue cells are exposed to:

F. growth media.
G. sexual reproduction.
H. asexual reproduction.
J. X-rays.

11. Which scientist's model proposes that Genes Y and Z are separated by 65 centimorgans?

A. Scientist 1's
B. Scientist 2's
C. Scientist 3's
D. Scientist 4's

12. Genes A and B are separated by 10 centimorgans on a chromosome. An organism has alleles A and B* on 1 chromosome and alleles A* and B on the homologous chromosome. If a single HR event occurred between these 2 genes as shown in Figure 1, the genotype of Genes A and B for the 2 chromatids involved in the crossover would be:

F. AB and AB.
G. AB and A*B*.
H. A*B and AB*.
J. A*B* and A*B*.