MCAT General Chemistry Review

Chapter 9: Solutions

Introduction

What do first aid instant cold packs and sweet tea have in common? Not much—you might think—but both, in fact, demonstrate the same principles of solution chemistry. Instant cold packs contain two compartments, one holding water and the other ammonium nitrate. When the barrier between the two compartments is broken, it allows the ammonium nitrate to dissolve into the water. Sweet tea is made by dissolving a large amount of sugar into strongly brewed tea.

The creation of both the ammonium nitrate and sugar solutions is an endothermic process. However, the formation of the ammonium nitrate solution is much more endothermic than the formation of the sugar solution. This is why ammonium nitrate is useful in instant cold packs. When it dissolves in water, the system must absorb an amount of energy equal to  of ammonium nitrate. The heat is absorbed from the surrounding environment, so the pack feels cool to the touch.

Although the dissolution of sugar into water is not as strongly endothermic, we nevertheless have an intuitive understanding of the process’s endothermicity because we all know that the easiest way to dissolve lots of sugar into water (such as in tea or coffee) is to heat up the water and then add the sugar. Because heating the water increases the solubility of sugar, it must be that the dissolution of sugar into water is an endothermic process—think of Le Châtelier’s principle and changes in temperature from Chapter 6 of MCAT General Chemistry Review.

In this chapter, our focus will be on the characteristics and behaviors of solutions, examining the nature of solutions, the formation of aqueous solutions, measurements of solution concentration, and finally, the qualitative and quantitative evaluation of solution equilibria.