The Foundations of Chemistry

resulting solutions, called hydrohalic acids, contain predominantly ionized HX (XCl,

Br, I). The ionization involves protonationof a water molecule by HX to form a hydrated

hydrogen ion and halide ion X(also hydrated). HCl is used as an example.

HF is only slightly ionized in aqueous solution because of the strong covalent bond.

In addition, the polar bond between H and the small F atoms in HF causes very strong

hydrogen bonding between H 2 O and the largely intact HF molecules.

The only gases that dissolve appreciably in water are those that are capable of

hydrogen bonding (such as HF), those that ionize (such as HCl, HBr, and HI), and

those that react with water (such as CO 2 ).

RATES OF DISSOLUTION AND SATURATION

At a given temperature, the rate of dissolution of a solid increases if large crystals are

ground to a powder. Grinding increases the surface area, which in turn increases the

number of solute ions or molecules in contact with the solvent. When a solid is placed in

water, some of its particles solvate and dissolve. The rate of this process slows as time

passes because the surface area of the crystals gets smaller and smaller. At the same time,

the number of solute particles in solution increases, so they collide with the solid more

14-5

O

H

H HF O

H

as well as HFH


 


  
  
  
 

O

H

H Cl H O

H

H

 Cl 
 H

H
transfer 

Oxygen gas is sufficiently soluble in
water to support a wide variety of
aquatic life.

550 CHAPTER 14: Solutions

A mortar and pestle are used for
grinding solids.

A saturated solution of copper(II)
sulfate, CuSO 4 , in water. As H 2 O
evaporates, blue CuSO 4 5H 2 O
crystals form. They are in dynamic
equilibrium with the saturated
solution.

A solution that contains less than the amount of solute

necessary for saturation is said to be unsaturated.

Concentration of solute

Supersaturated

Saturated

Unsaturated

(at constant T, P)

0