|At the top the water molecules are tightly bound to each other by hydrogen bonds and the Ca2+ ions and Cl– ions are held together by strong ionic bonds. To form a solution these bonds must be broken, as shown in the middle. Finally, the ions form bonds with water molecules as shown at the bottom.|
Solutions are homogeneous mixtures of a solute in a solvent. In order for particles of a solute to dissolve they have to break the bonds that hold them together. This is endothermic and consumes energy. Likewise, when particles of the solvent separate to allow solute particles to come in between them they must break bonds holding them together. Of course this, too, is endothermic. If this were the whole story then the formation of solutions would be very unlikely to happen spontaneously. But it is not the whole story. The solute particles form new bonds to the solvent particles and new bond formation is exothermic. This pushes the formation of a solution toward spontaneity. There is also another important factor. Particles have more ways to be arranged when they are in a solution than when they are pure. Since situations with more ways to be arranged are more likely, solutions are likely to form.
Most solutions that we encounter in our daily lives have a nearly perfect balance of energy absorbed and energy given off so that solutions form without any obvious drama. Salt, sugar, maple syrup, honey, instant coffee, and powdered detergents all dissolve without any hint that heat is given off or absorbed. But some chemicals do clearly absorb or give off heat when they dissolve. For example, it is a common laboratory experiment to measure the temperature change in water when potassium nitrate (KNO3) is dissolved in it. When this salt dissolves it absorbs energy and the temperature of the water drops. Another example is anhydrous calcium chloride (CaCl2). This salt is sometimes used in pickling vegetables but in northern climates is also sold in large bags for the purpose of melting ice. This capacity of salt to encourage the melting of ice is the subject of another demonstration. It is an example of the application of a colligative property.
Calcium chloride releases heat when it dissolves, provided any water of hydration is missing from its crystal lattice. The release of heat can be quite impressive under the right circumstances. When the ions of the salt are taken apart they have to break very strong ionic bonds. And when the water molecules are pulled apart to accommodate the ions they must break the weaker but still powerful hydrogen bonds that hold them together. But as the water molecules surround the ions (usually six water molecules per ion) they form medium-strength ion-dipole bonds. The water molecules are said to hydrate the ions. So many of these hydration bonds form that the net result is the release of quite a lot of heat. The heat of solution of calcium chloride is –81.3 kJ/mol.