4.7
Solubility of Ionic Compounds
Solubility is the measure of the maximum amount of solute that can be dissolved in a given quantity of solvent at a given temperature and pressure. So…
The solubility of a solute is its maximum possible concentration at solubility equilibrium in a given amount of solvent. Solubility is affected by temperature and other physical conditions.
Substances that dissolve in water are called water-soluble. A simple water-soluble ionic compound like sodium chloride dissolves in water by breaking up into monatomic ions. Here, it is more favorable for the water molecules and ions to interact in solution than it is for the ions to remain in the ordered solid.
A more complex water-soluble ionic compound like sodium nitrate contains ions that are composed of multiple atoms covalently bound together, or polyatomic ions. When sodium nitrate dissolves, the polyatomic nitrate ions do not split into nitrogen and oxygen. Instead, the ions disperse in solution as intact units.
Substances that do not dissolve in water are water-insoluble. For example, silver chloride is a water-insoluble ionic compound. In this case, it is more favorable for the ions to remain in the ordered solid than to interact with water and disperse in the solution.
The solubility of an ionic compound in water depends on the ion pair that makes up the compound. Chemists have formulated a set of empirical guidelines to predict the solubility of ionic compounds in water. Exceptions to these guidelines are rare.
All nitrates and acetates are soluble. Similarly, all ammonium and non-lithium alkali metal compounds are soluble, as are nearly all lithium salts.
Sulfate compounds are soluble, with the exception of its salts with lead, mercury, and silver – remember the acronym LMS or the phrase Let Me See – and calcium, barium, and strontium – remember the acronym CBS or the phrase Come By Soon.
All chloride, bromide, and iodide salts are soluble, with the exception of their salts with LMS – lead, mercury, and silver – as well as copper(I) and thallium.
Moving to insoluble compounds, sulfides and hydroxides are insoluble, with the exception of their salts with alkali metals and barium. In addition, ammonium sulfide is soluble, and strontium hydroxide is soluble when heated.
Similarly, carbonates and phosphates are insoluble, with the exception of their ammonium and non-lithium alkali metal salts.
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Q1: What is solubility and how is it measured?
Solubility is the maximum amount of solute that can dissolve in a given quantity of solvent at a specific temperature and pressure. It is typically measured in molarity (M) or moles per liter (mol/L). Solubility varies depending on the solute-solvent pair and environmental conditions, making it a key property for predicting whether ionic compounds will dissolve in water.
Q2: Why do some ionic compounds dissolve in water while others do not?
Whether an ionic compound dissolves depends on the balance between interionic forces in the solid and ion-dipole forces between ions and water molecules. For soluble compounds like sodium chloride, water interactions are more favorable than the solid state. For insoluble compounds like silver chloride, strong interionic forces exceed water's ability to separate ions, so the solid remains largely undissolved.
Q3: What is the difference between monatomic and polyatomic ions in solution?
Monatomic ions contain a single atom, such as K+ or Cl−, while polyatomic ions are groups of atoms bonded together that carry a charge, like NO3− or SO42−. When soluble ionic compounds dissolve, monatomic ions separate individually. Polyatomic ions remain intact as single units and do not split into individual atoms in solution.
Q4: How do water molecules interact with ions during dissolution?
Water is a polar molecule with positive and negative ends. When ionic compounds dissolve, the positive hydrogen end of water attracts negative ions, while the negative oxygen end attracts positive ions. This ion-dipole interaction surrounds individual ions, reducing the strong forces binding them in the solid and allowing them to move into solution as solvated ions.
Q5: What are the key solubility rules for predicting ionic compound behavior?
Chemists use empirical solubility rules based on ion pairs. All nitrates and acetates are soluble, as are ammonium and alkali metal compounds. Chlorides, bromides, and iodides are soluble except with silver, lead, and mercury. Sulfates are soluble except with lead, mercury, silver, calcium, barium, and strontium. Carbonates and phosphates are insoluble except with ammonium and alkali metals.
Q6: What happens when an insoluble ionic compound is placed in water?
When an insoluble compound like silver chloride contacts water, the strong interionic forces binding the ions in the solid are stronger than the ion-dipole forces between water and individual ions. The water molecules cannot overcome these forces, so the ions remain bonded in the solid state. Most of the compound stays undissolved, though technically some dissolves to less than 0.01 M.
Q7: How do solubility rules help predict precipitation reactions?
Solubility rules allow chemists to predict which ionic compounds will precipitate when solutions are mixed. When two soluble salts combine and form an insoluble product, precipitation occurs. By applying empirical guidelines for nitrates, sulfates, chlorides, carbonates, and hydroxides, students can determine whether precipitation reactions formation insoluble product will occur in aqueous solutions.