This is an in-class activity that I use in my advanced general chemistry course to teach students how to rank the relative acidity of monoatomic cations and how to qualitatively predict the strength of the interaction of these cations with water (hydration and hydrolysis).

I teach the students that these properties trend with Z²/r (where Z is charge and r is radius). They are generally comfortable calculating Z²/r and comparing the result to a table in their textbook to assign relative acidity, but struggle to make this assignment qualitatively without looking at the textbook. Being able to qualitatively make these assignments helps students when we get to predicting solubility of compounds using Bronsted acidity and basicity. In the course as a whole, I emphasize the power and importance of being able to qualitatively predict trends in reactivity, in part to provide an alternative approach to the fall semester of the course--which focuses on quantitative aspects of physical chemistry--and in part to prepare students for organic chemistry, where they can't simply plug a product into an equation and generate a synthesis.