ACT Science Practice Test 37

Students crystallized an impure, solid compound. First, they added just enough hot solvent to dissolve the compound. They then allowed the hot solution to cool, whereupon crystals began to form. Finally, the solution was placed in an ice bath to complete the crystallization process (see Figures 1 and 2). Figure 3 illustrates the progression of crystallization.

1 Solution added, undersaturated

2 Solution cools to saturation

3 Concentration decreases with crystal growth

4 Crystal growth during main cooling cycle

5 Supersaturated

Figure 3

Solubility (the amount of solute that will dissolve in a specific solvent) and crystallization are directly related. Since crystallization cannot begin until the solution becomes saturated, the faster a compound dissolves, the more quickly it can begin to form into crystals.

Study 1

Students tested the solubility of four different substances. The temperature was measured in °C, and water was used as the solvent. The results of the study are displayed in Figure 4.

Figure 4

Blood type is a hereditary trait. The type is established by the genes inherited from the mother and father. The ABO system is widely accepted as the best blood classification system. In the ABO system, there are four types of blood: A, B, AB, and O. The combination of inherited genes is known as the genotype and the actual blood type is known as the phenotype. The genes ensure that only the blood cells of the proper blood type remain in the body.

Table 1

The Rh (+/–) factor is inherited separately from the blood type. It is possible to have the Rh+ phenotype yet still carry the recessive gene for Rh–. The Rh+ (R) is the dominant gene and Rh– (r) is recessive. Table 2 shows the Rh phenotypes resulting from the various genotypes inherited from parents.

Table 2

The surface of every red blood cell is covered with proteins. Rh factor and blood type determine the proteins and the compatibility of donated blood as shown in Table 3.

Table 3

In a study of the effects of Ritalin and Adderall on children with ADHD, subjects were given one of four possible doses of medication. Their behavior in social and academic settings was then monitored and rated. The four possible doses were placebo (P), Ritalin given once in the morning (R1), Ritalin given twice daily (R2), or Adderall given once in the morning (A1).

The results for each group were averaged. Figure 1 shows the average behavioral rating (on a scale of 0–15, with 0 meaning no undesirable behavior) at various time periods throughout the day. Figure 2 shows the percentage of children who demonstrated side effects at a moderate or severe level on at least one day.

Figure 1

Figure 2

A solution consists of a solute dissolved in a solvent. For example, in a saltwater solution, the salt is the solute and the water is the solvent. Osmosis is the movement of solvent molecules across a barrier in order to equalize solution concentrations. When a saltwater solution is placed in a u-shaped tube with a selectively permeable barrier or semipermeable membrane, the phenomenon of osmotic pressure can be observed. Osmotic pressure is the pressure required to prevent the flow of a solvent across a barrier. In Figure 1, saltwater was poured in the right half of the tube and pure water was poured in the left half of the tube. As water passes from left to right in the figure below due to osmosis, the level of solution on the right rises. Eventually, the weight of the solution due to gravity becomes sufficient to prevent the further flow of water from right to left.

Figure 1

The following experiments were carried out to study how varying the molecular weight percentage of solvents and the temperature of solutions affects osmotic pressure. Table 1 shows the molecular weight percentages of the different solvents studied and Table 2 displays the temperatures of the various solutions used.

Table 1

Table 2

Experiment 1

A 1000 ml u-shaped tube was fitted with a semipermeable membrane and 300 ml of solution was added on the right half and 300 ml of water was added on the left half. Table 1 shows the four solutions used in four separate trials of Experiment 1. The percent weight was measured by computing the ratio of the weight of the solute (salt) to the weight of the solution after dissolution. Every 5 seconds, the height of the solution on the right side of the tube was measured and recorded. Figure 2 shows the results of Experiment 1 recorded at 20°C.

Figure 2

Experiment 2

Four more trials were performed but only the 10% solution was used. For each trial, the temperature was varied as shown in Table 2 and the results were graphed in Figure 3. In each trial, the height was measured 5 minutes after the water and solution were poured.

Figure 3