What Is Solubility?
Solubility is the property of a solid, liquid or gaseous substance referred to as the solute, to dissolve in a solvent (usually a liquid) and form a solution. Solubility indicates the upper concentration a compound reaches in a solution. Therefore, the relation between the solute and solvent is very important in determining solubility. Strong solute-solvent attractions equate to greater solubility while weak solute-solvent attractions equate to lesser solubility. Different compounds vary greatly in their solubilities, owing to differences in their structures and properties.
The Solubility Rules
The solubility rules are only for ionic solids’ ability to dissolve in water. While we can calculate the solubility by measuring each substance and following an equation, the solubility rules allow us to determine the solubility of a substance before you attempt to create it. Below are the solubility rules for common ionic solids.
- Fluorides such as Barium Fluoride (BaF2), Magnesium Fluoride (MgF2) and lead fluoride (PbF2) are usually insoluble.
- Phosphates such as calcium phosphate (Ca3(PO4)2 and silver phosphate (Ag3PO4) are often insoluble.
- Most sulfides of transition metals are highly insoluble, including Cadmium sulfide (Cds), Iron II sulfide (FeS), Zinc II sulfide (ZnS) and Silver sulfide (Ag2S). Arsenic, antimony, bismuth and lead sulfides are insoluble.
- All silver salts are insoluble with exception of silver nitrate (AgNO3) and Silver acetate (Ag(C2H3O2).
- Generally, Salts containing nitrate ion (NO3– ) are soluble.
- Salts containing chloride ion (Cl-), Barium ion (Br- ) or Iodide ion (I-) are generally soluble.
- Generally chromates are insoluble. Examples include plead (II) chromate (PbCrO4) and Barium Chromate (BaCrO4).
- Most sulfate salts are soluble. Major exceptions to this rule include Calcium Sulfate (CaSO4), Barium sulfate (BaSO4), lead sulfate (PbSO4), Silver Sulfate (Ag2SO4) and Strontium sulfate (SrSO4).
- Salts containing Group I elements Ions (Li+, Na+, K+, Cs+, Rb+) are soluble. There are few exceptions to this rule. Salts containing the ammonium ion (NH4+) are soluble.
- Carbonates are generally insoluble. Examples include calcium carbonate (CaCO3), strontium carbonate (SrCO3), barium carbonate BaCO3, iron carbonate (FeCO3) and lead carbonate (PbCO3).
- Most hydroxide salts are slightly soluble. Hydroxide salts of group I elements (sodium, potassium, lithium, rubidium, cesium and francium) are soluble. Hydroxide salts of group II elements (Calcium, strontium, Barium, magnesium and beryllium) are slightly soluble. Hydroxide salts of transition metals and aluminum (Al3+) are insoluble. Examples include Iron III hydroxide- Fe (OH)3, Aluminum Hydroxide-Al (OH)3, Cobalt Hydroxide-Co(OH)2.
How To Use Solubility Rules
- Identify the compound whose solubility you want to check. This is very important when writing the empirical formula for easy identification of the ions that make up the compound.
- Look up each ion in the solubility rules. Check the left-hand and column for the general rule and look in the right-hand column to make sure you noted any exceptions.
- Another method is looking up ions in the solubility chart. This is perfectly easy. Cations are listed across the top and anions are listed vertically. Find the cell where your cation column and anion raw meet to determine solubility of the resulting compound.
Importance Of Solubility Rules
- The solubility rules help determine which substances are soluble and to what extent.
- Solubility rules are helpful in predicting the end states of the substances involved.
- Solubility rules are important when building chemical equations and reaction hypothesis.