Dissolution of Calcium Hydroxide in Water: A Guided Inquiry in University and High School Chemistry Laboratories

In this laboratory experiment, a guided inquiry exploring the physicochemical principles of the dissolution of Ca(OH)2(s) in water is proposed for laboratory classes in university and high school. As part of students’ inquiry, two experimental approaches are used. One is the change in solubility with temperature revealed by measuring the pH values of the suspended solution of Ca(OH)2(s) at various temperatures, which is then extended to its thermodynamic relationship via examining the temperature dependence of the solubility constant. The other method is to determine the enthalpy of solution using the calorimetric measurements. Due to the poor solubility of Ca(OH)2(s), for determining the enthalpy of the Ca(OH)2(s) solution, development of an energy diagram composed of several paths of a reaction that involve the dissolution of Ca(OH)2(s) as a component process and using Hess’s law are essential. A combination of these experimental approaches yields a stepwise students’ inquiry for revealing the source of the changes in Ca(OH)2(s) solubility with temperature, which may be flexibly adapted as an appropriate program depending on the targeted students. The two experimental procedures are presented by critically examining the experimental results. Based on the results of educational practices, typical guided inquiry constructions suited for the university and high school chemistry courses are proposed.