A Study of the Non-linear Variation in the Temperature of the Water Density Anomaly as a Function of Solute Concentration

In this study an experimental technique which permits detailed study of the temperature of maximum density of pure water and aqueous solutions is presented. The density of water as a function of temperature passes through a maximum at 3.98C. This temperature of maximum density (Tmd) changes when solutes are added to the water. This investigation is carried out by cooling a rectangular chamber containing a test fluid. Throughout the tests a 4°C temperature gradient is maintained. As the test fluid is cooled through its density maximum the normal single cell convection that occurs in the presence of a temperature gradient is replaced by a double cell. Monitoring this double cell is the basis of all tests carried out in this study. For solutes such as ionic salts and sugars, the temperature of maximum density decreases in a linear manner as the solute concentration increases (‘Despretz law’). It had been noted, however, in previous work that for monohydric alcohols such as methanol and ethanol the behaviour of the temperature of maximum density is non-linear, showing an initial rise above 4C as the solute concentration is increased, followed by a drop below 4C as the concentration continues to rise. Results presented here from more detailed studies indicate that the behaviour of the temperature of maximum density in such cases is highly non-linear, moving through several local maxima in the low concentration region for both ethanol and 2-propanol. Macroscopic and microscopic are investigated in an attempt to understand and explain this unexpected behaviou