Sodium chloride can exist as a solid below its melting point (801 °C)
CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
Sodium chloride can exist as a solid below its melting point (801 °C), as a molten liquid above that temperature, and as an aqueous solution when dissolved in water. A research team records the qualitative electrical conductivity of a sample of NaCl as it is gradually heated from 25 °C to 900 °C in a dry, closed system and then cooled back to 25 °C while water vapor is introduced to dissolve the solid residual.
Assume that the mass of NaCl remains constant and that no chemical decomposition occurs throughout the experiment.
(a) Sketch a qualitative graph of electrical conductivity (vertical axis) versus temperature/time (horizontal axis) for the entire experiment, clearly indicating the solid phase, the phase transition at 801 °C, the molten phase, cooling, and the subsequent formation of the aqueous solution.
(b) Explain the key features of your graph by discussing the availability and type of mobile charge carriers in each phase of NaCl.
(c) During the molten phase, the researchers apply a steady voltage and measure a current of 2.4 A through the liquid. Upon complete dissolution in water at 25 °C, the current increases to 3.6 A under the same voltage. Provide a molecular-level explanation for the higher current in the aqueous solution despite the lower temperature.
(d) Predict how the addition of a non-volatile, polar molecular solute (such as sucrose) to the aqueous NaCl solution would affect its conductivity. Justify your prediction based on intermolecular interactions and charge carrier concentration.