THE SOLUTION PROCESS AND SOLUBILITY

7.4 THE SOLUTION PROCESS AND SOLUBILITY

Very little happens to simple molecules, such as N 2 and O 2 , when they dissolve in water. They mingle with the water molecules and occupy spaces that open up between water molecules to accomodate the N 2 and O 2 molecules. If the water is heated, some of the gases are driven out of solution. This may be observed as the small bubbles that appear in heated water just before it boils. A fish can extract some of the oxygen in water by “breathing” through its gills; just 6 or 7 parts of oxygen in

a million parts of water is all that fish require. Water saturated with air at 25 ° C contains about 8 parts per million oxygen. Chapter 12 discusses how only a small amount of an oxygen-consuming substance can use up this tiny portion of oxygen in water and cause the fish to suffocate and die.

Although N 2 and O 2 dissolve in water in the simple form of their molecules, the situation is much different when hydrogen chloride gas, HCl, dissolves in water. The hydrogen chloride molecule consists of a hydrogen atom bonded to a chlorine atom Although N 2 and O 2 dissolve in water in the simple form of their molecules, the situation is much different when hydrogen chloride gas, HCl, dissolves in water. The hydrogen chloride molecule consists of a hydrogen atom bonded to a chlorine atom

C will dissolve 82 g of this gas. When HCl dissolves in water ( Figure

7.5 ), the solution is not simply hydrogen chloride molecules mixed with water molecules. The water has a strong effect upon the HCl molecule, breaking it into two parts, with the 2 electrons in the chemical bond staying with the chlorine. This forms

a positively charged hydrogen ion, H + , and a negatively charged chloride ion, Cl-. O

H–Cl( g )

H + ( aq ) + Cl - ( aq )

Solution

Figure 7.5 HCl dissolving in water. Water breaks apart a hydrogen chloride molecule to form a hydrogen ion, H + , and a chloride ion, Cl-.

In water solution, the chloride ion is surrounded by the positive ends of the water molecules, which are attracted to the negatively charged Cl- ion. This kind of attraction of water molecules for a negative ion has already been shown in Figure

7.3 . The H + ion from the HCl molecule does not remain in water as an isolated ion; it attaches to an unshared electron pair on a water molecule. This water molecule, with its extra hydrogen ion and extra positive charge, becomes a different ion with a

formula of H O + 3 ; it is called a hydronium ion. Although a hydrogen ion in solution is indicated as H + for simplicity, it is really present as part of a hydronium ion or larger ion aggregates (H

2 ,H 7 O 3 ).

(Hydronium ion)

Figure 7.6 A hydrogen ion, H + + , bonds to a water molecule, H 2 O, to produce a hydronium ion, H O . Bonding to additional water molecules may form larger aggregates, H O + ,H O 3 + 5 2 7 3 .

The solution of hydrogen chloride in water illustrates a case in which a neutral molecule dissolves and forms electrically charged ions in water. While this happens with other substances dissolved in water, the hydrogen ion resulting when substances like HCl dissolve in water is particularly important because it results in the formation of a solution of acid. So rather than calling this a solution of hydrogen chloride, it is called a hydrochloric acid solution.