The freezing point osmometer is different from the vapor pressure osmometer in that it is sensitive to which substance?

The freezing point osmometer is specifically sensitive to the presence of solutes that affect the freezing point of a solution. One significant example of this is ethanol, which can lower the freezing point significantly compared to other solutes. This characteristic makes the freezing point osmometer particularly effective in detecting changes in the composition of solutions where such solutes are present.

In contrast, while the vapor pressure osmometer also measures osmotic concentration, it operates on a different principle, measuring the vapor pressure of a solvent above a solution to determine osmolality. This means that the vapor pressure technique may not be as sensitive or applicable to all solutes as the freezing point method is for substances like ethanol, which has a pronounced effect on freezing point depression.

The other options relate to operational characteristics of the osmometer but do not address the specific sensitivity aspect that distinguishes the freezing point osmometer. For instance, cooling a sample or requiring a thermoelectric module does not directly correlate with the assay's sensitivity to certain substances, and calibration with aqueous standards is important for both methods, but again, does not pinpoint the unique sensitivity of the freezing point osmometer to ethanol or similar solutes.