Passage IV

Three experiments were performed to study the surface tension and pH of colorless aqueous solutions of 4 different acids: nitric acid, formic acid, chloroacetic acid, and acetic acid. A liquid’s surface tension in millijoules per square meter (mJ/m²) is equal to the energy in millijoules required to increase the surface area of the liquid by 1 m².

Experiment 1

Twenty 250 g aqueous solutions—5 of nitric acid, 5 of formic acid, 5 of chloroacetic acid, and 5 of acetic acid— were prepared. Each of the 5 solutions of a given acid had a different percent by mass of acid (10%, 20%, 30%, 40%, or 50%). The surface tension of each solution and of distilled H₂O was measured at 25°C (see Figure 1).

Experiment 2

One gram of each of the 50% by mass acid solutions prepared in Experiment 1 was diluted with distilled H₂O to a total mass of 10 g, forming four 5% by mass acid solutions. Six drops of methyl violet, a pH indicator, were added to each of these solutions. (A pH indicator is a compound whose color is determined by the pH of the solution in which the compound is dissolved.) The color of each resulting solution at 25°C was recorded (see Table 1).

Experiment 3

One gram of each of the 20 solutions prepared in Experiment 1 was diluted with distilled H₂O to a total mass of 10 g. The pH of each resulting solution was measured at 25°C (see Figure 2).