8.1  Formation of Solutions (continued)

Dissolving

Recall that a solution is a homogeneous mixture of two or more substances. Every solution has two types of components. A solute is a substance whose particles are dissolved in a solution. The substance in which the solute dissolves is called the solvent. For example, seawater is a solution in which salt is the solute and water is the solvent.

Solutes and solvents can take the form of a solid, liquid, or gas. The solution takes the state of the solvent. Figure 2 lists some common solutions and the states of their respective solutes and solvents. Air, for instance, is a solution of several gases dissolved in another gas. Nitrogen, making up about 78 percent of air, is the solvent. Oxygen, carbon dioxide, argon, and other gases are solutes.

Figure 2  A stainless steel pot or pan is a solution of chromium and nickel in iron. In a solution, the solvent is the substance in the greatest quantity.

You are probably most familiar with solutions in which water is the solvent. Carbonated drinks, hot tea, and seawater are just a few examples of the many water-based solutions you might have encountered.   Substances can dissolve in water in three ways—by dissociation, dispersion, and ionization.

Dissociation of Ionic Compounds For a solute to dissolve in water, the solute and solvent particles must attract one another. However, the particles within the solute are attracted to one another, and the particles within the solvent are attracted to one another. So before a solution can form, the attractions that hold the solute together and the solvent together must be overcome.

Figure 3 illustrates how a sodium chloride crystal dissolves in water. Sodium chloride is an ionic compound. Water is a polar molecule, and is attracted to the ions in the solute. The crystal dissolves as the sodium and chlorine ions are pulled into solution, one by one, by the surrounding water molecules. The process in which an ionic compound separates into ions as it dissolves is called dissociation.

Figure 3  When an ionic compound dissolves in water, the charged ends of water molecules surround the oppositely charged ions.

Dispersion of Molecular Compounds When you place a piece of hard candy on your tongue, the sweet taste spreads, or disperses, throughout your mouth. The water in your saliva dissolves the sugar and flavoring in the candy. Sugar dissolves in water by dispersion, or breaking into small pieces that spread throughout the water.

Both sugar and water are polar molecules, so they attract one another. Because the water molecules are constantly moving, they collide frequently with the surface of the sugar crystals, as shown in Figure 4. Attractions form between the water molecules and the exposed sugar molecules. When enough water molecules have surrounded a sugar molecule, the attractions between them are great enough to overcome the attractions holding the sugar molecule to the surface of the crystal. The sugar molecule breaks free, and is pulled into solution by the water molecules.

Figure 4  Saliva dissolves the sugar in hard candy by dispersion. As water molecules collide with sugar crystals, attractions develop between the water molecules and sugar molecules at the surface of the solid.

As more sugar molecules break free of the crystal, another layer of sugar molecules is exposed to the water, and the process repeats. The solute particles become evenly spread throughout the solvent.

Ionization of Molecular Compounds Hydrogen chloride, HCl, is a molecular compound in which a hydrogen atom and a chlorine atom share a pair of electrons. Recall that a hydrogen atom has only one proton and one electron. When HCl gas dissolves in water, the hydrogen proton from each HCl molecule is transferred to a water molecule. For each HCl molecule that reacts, a hydronium ion, H₃O⁺, and a chloride ion, Cl⁻, are produced.

Notice that when hydrogen chloride and water form a solution, two molecular compounds react to form two ions. The process in which neutral molecules gain or lose electrons is known as ionization. Unlike dissociation and dispersion, which are physical changes, dissolving by ionization is a chemical change. The solution that results contains new substances. When a solute dissolves by dissociation, the ions pulled into solution are the same ions present in the solute. When a solute dissolves by ionization, the ions in solution are formed by the reaction of solute and solvent particles.