Why Substances Dissolve Understanding the Solution Process

Despite the central place of liquid solutions in chemistry, gaseous solutions and solid solutions also have vital importance and numerous applications. 

Gas-Gas Solutions All gases are infinitely soluble in one another Air is the classic example of a gaseous solution, consisting of about 18 gases in widely differing proportions. Anesthetic gas proportions are finely adjusted to the needs of the patient and the length of the surgical procedure. 

Gos-Solid Solutions When a gas dissolves in a solid, it occupies the spaces between the closely packed particles. Hydrogen gas can be purified by passing an impure sample through a solid metal such as palladium. Only the H2 molecules are small enough to enter the spaces between the Pd atoms, where they form Pd—H covalent bonds. Under high H2 pressure, the H atoms are passed along the Pd crystal structure and emerge from the solid as H2 molecules. 

Waxes are another familiar type of solid-solid solution. Most waxes are amorphous solids that may contain small regions of crystalline regularity. A natural wax is a solid of biological origin that is insoluble in water but dissolves in nonpolar solvents. 

As a qualitative predictive tool, like dissolves like is helpful in many cases. As you might expect, this handy macroscopic rule is based on the molecular interactions that occur between solute and solvent particles. To see why like dissolves like, let s break down the solution process conceptually into steps and examine 

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Although much of the explanation for why certain substances mix and form solutions and why others do not is beyond the scope of this text, we can get a glimpse at why solutions form by taking a look at the process by which ethanol, C2Fi50H, dissolves in water. Ethanol is actually miscible in water, which means that the two liquids can be mixed in any proportion without any limit to their solubility. Much of what we now know about the tendency of particles to become more dispersed can be used to understand this kind of change as well. 

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