Dissolution Processes:

Iodine
Sodium Hydroxide (a Base)
Hydrogen Chloride (an Acid)
Dissolution with Reaction

When a substance dissolves, it may separate into ions (an electrolyte) or it may remain in molecular form (a non-electrolyte).  If it completely separates into ions so that practically none of the molecular form remains, it is called a strong electrolyte.  If some ions are formed, but a substantial amount of the molecular form remains, it is called a weak electrolyte.

top

Non-Electrolyte:  Iodine
    Iodine (I2) is an example of a non-electrolyte.  This is a purple-black solid at room temperature.  When heated, it turns to a gas (sublimation) producing a beautiful violet cloud.  It dissolves readily in many organic solvents, producing violet solutions in most cases and yellow-brown solutions in alcohols.  Iodine is not very soluble in water, but produces a slightly yellow solution.  No ions are produced when iodine dissolves in any of these solvents.
    The animation shows what happens when iodine is placed in water.  Only a small amount dissolves (about 0.3 grams of iodine per kilogram of water), and any excess remains as the solid.  This is not a static situation, however.  Molecules of iodine are continually dissolving from the solid phase, and dissolved molecules are continually re-attaching themselves to the solid - a dynamic equilibrium.  If more solid is added, no more iodine will dissolve.  If more water is added (lowering the concentration of dissolved iodine), iodine will dissolve until the concentration is again about 0.3 grams per kilogram.
top




Strong Electrolyte: Sodium Hydroxide (a Base)
    When sodium hydroxide (NaOH) dissolves in water, it separates into positively - charged sodium ions (cations) and negatively - charged hydroxide ions (anions).  These ions move around in the water, free and independent of each other, though cations tend to be surrounded more closely by anions and vice versa.  Sodium hydroxide is quite soluble in water (more than 50% by weight), but if more than this amount is added to water at room temperature, there will be solid remaining on the bottom of the container.
    The ions are pictured as specific shapes, but they are actually surrounded by molecules of water.  The number of water molecules influenced by a specific ion probably varies considerably with time.
    Each mole of sodium hydroxide that dissolves will produce one mole of sodium ions and one mole of hydroxide ions.  When 5 moles of sodium hydroxide is dissolved in 10 kilograms of water, this is called a 0.5 molal* solution of sodium hydroxide (sometimes more properly called a 0.5 formal solution).  The solution is actually 0.5 molal in sodium ions and 0.5 molal in hydroxide ions.
    molality (m) = moles of solute per kilogram of solvent (moles/kG)
top




Strong Electrolyte: Hydrogen Chloride (an Acid)
    Pure hydrogen chloride (HCl) is a gas at room temperature and atmospheric pressure.  It is very soluble in water and, like all gases, it's solubility depends on the partial pressure of the gas.  HCl is a strong electrolyte and when it dissolves in water it separates almost completely into positively - charged  hydrogen ions and negatively - charged chloride ions.  This aqueous solution is usually called hydrochloric acid.
    The ions are surrounded by water molecules, like the sodium and hydroxide ions above, but hydrogen ion has a special relationship with water.  When water (H2O) and hydrogen ion interact, they form the hydronium ion (H3O+) and probably other species with more water molecules involved, like (H9O4+).  All of the hydrogen atoms in these clusters are interconnected through oxygen atoms, and electrons are free to move around through these bonds and hydrogen bonds.  The positive charge may then be transferred very rapidly from one hydrogen atom to another.  This property contributes a great deal to the reactivity of acidic solutions.  It is not terribly important whether the ion is written as hydrogen ion (H+) or hydronium ion (H3O+), but some textbooks (and some teachers) prefer one form over the other.
    Each mole of hydrogen chloride that dissolves produces one mole of hydrogen or hydronium ions and one mole of chloride ions.  If 5 moles of hydrogen chloride gas are dissolved in enough water to make 10 kilograms of solution, it is called a 0.5 molal (or 0.5 formal) solution of hydrochloric acid.  The solution is actually 0.5 molal in hydrogen or hydronium ions and 0.5 molal in chloride ions.

top




Dissolution with Reaction:
    In this case, we start with sodium hydroxide already dissolved in water (note that there are 5 sodium ions and 5 hydroxide ions.  In the vapor above, we have introduced 4 hydrogen chloride "molecules".  As in the earlier example of hydrogen chloride gas dissolving, when the gas molecules strike the water they dissolve and separate into hydrogen (or hydronium) ions and chloride ions.  However, when both hydrogen ions and hydroxide ions are present in appreciable concentrations, they react to form a molecule of water (or hydronium ion reacts with hydroxide ion to form two molecules of water).  These new water molecules that are formed are no different from the water molecules that were already present so they seem to disappear, but we know that the total number of water molecules increases.  This reaction has no effect on the sodium ions and chloride ions - they continue to move about freely in the solution as they did in the previous examples.
    We started with 5 hydroxide ions and 4 HCl molecules, and the 4 hydrogen ions reacted with 4 of the hydroxide ions, so one hydroxide ion remains.  The solution was basic when we started, and is still basic at the end - but considerably less basic than it was initially.  If we had added 5 HCl molecules, all of the hydroxide ions would react (actually, there would be an extremely low concentration of both hydrogen and hydroxide ions) and the solution would be neutral.  If we had added 6 HCl molecules, one hydrogen (or hydronium) ion would remain and the solution would be acidic.
    If we had dissolved 5 moles of sodium hydroxide in 10 kilograms of water, the solution would be 0.5 molal in sodium ions and 0.5 molal in hydroxide ions.  When 4 moles of hydrogen chloride is added to this solution, it separates into 4 moles of hydrogen ions and 4 moles of chloride ions.  The 4 moles of hydrogen ions combine with 4 moles of hydroxide ions to form 4 moles (about 0.072 kG) of water (H2O).  The final solution contains the original 5 moles of sodium ions, the remaining 1 mole of hydroxide ions, 4 moles of chloride ions from the HCl, and there is now 10.072 kilograms of water.
top