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What properties distinguish ionic compounds from covalent compounds?

What kinds of tests could you do to confirm that a compound is ionic or molecular? Could you dissolve the supposed ionic compound in water to see if it readily dissociates or...?
Eva 5/31/99

electrical conductivity*
enthalpy of fusion*
enthalpy of vaporization*
ionic compound*
Eva, that's one way to tell them apart. Many differences in properties of ionic and molecular materials stem from the fundamentally different nature and strength of forces that hold these materials together. The attractive forces between positive and negative ions are much stronger than the attractive forces between neutral molecules. You can use that fact to explain differences in the following properties for ionic and molecular compounds:

  • Electrical conductivity of the compound in aqueous solution. Ionic compounds conduct electricity when dissolved in water, because the dissociated ions can carry charge through the solution. Molecular compounds don't dissociate into ions and so don't conduct electricity in solution.
  • Electrical conductivity of the compound in liquid form. Ionic compounds conduct electricity well when melted; metallic solids do as well. Covalent molecular compounds do not, because they usually don't transfer electrons unless they react.
  • Hardness. Molecular solids are usually much softer than ionic materials. Ionic crystals are harder but often quite brittle. Squeezing an ionic crystal can force ions of like charge in the lattice to slide into alignment; the resulting electrostatic repulsion splits the crystal.
  • Melting points and boiling points. In an ionic compound, the forces of attraction between positive and negative ions are strong and high temperatures are required to overcome them. The melting and boiling points of ionic compounds are usually very high. A smaller amount of energy is required to overcome the weak attractions between covalent molecules, so these compounds melt and boil at much lower temperatures than metallic and ionic compounds do. In fact, many compounds in this class are liquids or gases at room temperature.
  • Enthalpies of fusion and vaporization The enthalpy of fusion is the amount of heat required to melt one mole of the compound in solid form, under constant pressure. The enthalpy of vaporization is the amount of heat required to vaporize one mole of the compound in liquid form, under constant pressure. These properties are typically 10 to 100 times smaller for molecular compounds than they are for ionic compounds.

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! What properties distinguish ionic compounds from covalent compounds?

Copyright © 1997-2010 by Fred Senese
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Last Revised 02/15/10.URL: http://antoine.frostburg.edu/chem/senese/101/compounds/faq/properties-ionic-vs-covalent.shtml