Lesson 8: Solubility

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Acids, Bases and Salts
   

An "acid" may be defined as a substance which yields hydrogen (H+) ions (or protons) when it dissociates in water.  In fact, there are two generally classifications of acids , strong and weak.  Their principal difference is based on ionization.  An example of a strong acid is hydrochloric acid (HCl).  Referring to an acid as "strong" is another way of saying it yields a high concentration of hydrogen ions.  By contrast, H2S (hyrdrogen sulfide) is a weak acid.

Hydrochloric (HCl), nitric (HNO3) and sulfuric (H2SO4) are among the strongest acids and carbonic (H2CO3), boric (H3BO4) and hydrosulfuric, another name for hydrogen sulfide (H2S) are among the weakest acids.  There are many others which are intermediate between these extremes.

A "base" is a substance which yields hydroxyl (OH-) ions when it dissociates in water.  A base which yields a high concentration of hydroxyl ions is a "strong" base and one which yields a low concentration of hydroxyl is a "weak" base.

"Salts" ordinarily yield neither hydrogen nor hydroxyl ions.  However, they do dissociate.  Sodium chloride dissociates as follows:


Since neither H+ nor OH- ions are produced, NaCl is a neutral salt - neither acid nor basic.  Of importance here also is the fact that both Na+ and Cl- are products of the reaction between a strong base (NaOH) and a strong acid (HCl).  Other salts which are products of a reaction between a strong acid and a weak base, such as aluminum sulfate (Al2(SO4)3) are acidic because when they ionize in water a secondary reaction takes place as follows:


Water is capable of ionizing as follows:

The aluminum cation essentially reacts with the hydroxyl of water to form an insoluble aluminum hydroxide:

It is important to note that the Al(OH)3 is not ionized as much as water itself and therefore aluminum ions remove hydroxyl ions from water to form an undissolved solid, thus leaving an excess of H+ ions in solution.  There is no tendency for H+ ions to combine with (SO4)-- becasue H2SO4 almost completely dissociates.  The phenomenon illustrated above where a salt reacts with water, is called "hydrolysis".  A similar situation in reverse prevails with salts formed from a weak acid and a strong base.  Sodium carbonate is an alkaline salt because of ionization, carbonate ions (CO3)-- tend to remove H+ ions from solution by forming very slightly ionized carbonic acid and thus leaving an excess of (OH)- ions present in the solution.

pH Value

Pure water is not considered simply H2O molecules.  Water molecules ionize to a slight degree, producing both hydrogen ions (or protons) and hydroxyl ions:


If we now use water as a reference point and consider it to be neutral it will be found to be both an acid and a base because it produces both hydrogen and hydroxyl ions.  Since these are present in identical concentrations, however, water is considered neutral.  The term "pH" was suggested to designate hydrogen ion concentration and the term can generally be defined as the logarithm of the reciprocal of the hydrogen ion concentration, or:


Since the reciprocal of the hydrogen ion concentration = 1  x  107, the logarithm of the reciprocal of the hydrogen ion concentration is 7.0;  the pH is 7.0.

 
 

The pH Scale

Some important facts about hydrogen ion concentration:

  1. When the hydrogen ion concentration is highest, pH is lowest, and the solution is most acidic.

  2. When the hydrogen ion concentration is lowest, pH is the highest, and the solution is most alkaline.

  3. Neutrality at pH 7 is midpoint in the scale;  pH values lower than 7 represent higher hydrogen ion concentration (more acid) than neutrality and pH values higher than 7 represent lower hydrogen ion concentrations (more alkaline) than neutrality.
     
     

Examples of pH (common substances)
 
 

Substance
Nominal pH Value
Battery Acid
0.5
Lemon juice
2.5
Orange juice
3.5
Beer
4.5
Milk (cows)
6.4
Water (pure)
7.0
Blood (human)
7.3
Lime (saturated solution)
12.4
Lye
13.5

Neutralization of Acids and Bases

Consider what happens when an acid solution and an alkaline solution each containing one equivalent of hydrogen ions and hydroxyl ions respectively, are mixed.  Essentially we have a strong acid and a strong base.  Both ionize extensively as follows:

 



The solution has mostly present, at least initially, a mixture of:  H+, Cl-, Na+ and OH-.  The reaction that takes place is one of "neutralization" between the H+ ions and OH- ions.p; In other words, the H+ ions and the OH- ions combine to form water which is only slightly ionized.  The effect is to remove H+ and OH- from the system until they are almost all gone.  When the reaction is completed the only H+ and OH- ions left are the insignificant concentration obtained from ionization of water, i.e., the pH.  There are, of course, free Na+ and Cl- ions present in abundance and in solution because NaCl is a highly ionizable salt.  Neutralization reactions involving acids and bases are very common in chemistry.

Facts about pH:

  • A weak measure of acids and bases (0-7 is acidic; 7-14 is basic)
  • Strong measurements  are expressed by normal, molal or percent
  • Salt causes a reading to be lowered
  • Organics (sugal R) cause the readings to increase
  • pH measures a logarithmic concentration of Hydrogen ions
  • Hydrogen ions cause corrosion
  • Salts ( organics) cause pH indicators to read radicals just like the pH meter, but it is not due to Hydrogen ion concentration