Properties of Ionic Compounds
Revised by Dr. Walt Volland July 12, 2013 all rights reserved copyright 2010-2013
Ionic
solids are poor conductors of electricity and heat. Ionic
solids are generally high melting (more than 150 degrees C). Ionic
solids are hard and brittle. Ionic
solids melt to form liquids that are electrical conductors because the ions are free to move. Ionic
solids that are water soluble, dissolve to form solutions that are
electrical conductors. ( Not all ionic substances are water soluble.)
Why ionic substances are the way they are Ionic solids are rigid because the multiple interactions between oppositely charged ions hold the charged particles in relatively fixed positions. Electrical conductivity depends on ions being able to move freely. The conduction of heat depends partly on the movement of electrons in a substance. Electric charge is "locked" in the lattice positions of the ions in the solids. This means there is poor mobility of charges and poor conductivity of electricity and heat.\
Ionic solids are brittle and hard because the electrostatic attractions in the solid again hold the ions in definite positions. The electrostatic attractions must be overcome to move the ions. When the ions in the solid are shifted by some very strong force the positions of ions shift so that like charged ions are close together. This results in strong repulsions and the like charged ions move apart. The solid shatters and does not simply deform like a metal. Try crushing a few grains of salt in the bowl of a spoon with another spoon. The particles do not deform, they shatter. Ionic solids melt when the ions are heated and have enough energy to slide past one another. They are mobile and can act to carry electrical charge through the liquid. This explains why a molten ionic substance conducts electricity, but a solid ionic material doesn't. The ions move through the liquid to carry charge from one place to another. The dissolving process is like a tug of war. Soluble ionic substances are ripped apart by the solvent when the solid is dissolved. The solvent pulls the ions out of the solid and breaks the forces holding the crystal together. The solvent isolates the ions in an envelope of solvent particles. The ions are free to move and carry electric charge through the solution. Nonsoluble ionic solids are held together so tightly that the ions cannot be pulled out of the lattice by the solvent. The attractive forces in the solid are stronger than the attractions between the solvent and the ions. The solvent can't pull the ions out of the crystal. |
Ionic compound name Melting
point in degrees celsius LiCl lithium chloride 613 45 g cold water,
128 g100 NaCl sodium chloride 801 209 g cold water,
284 g 100 KCl potassium chloride 776 35 g cold water,
57 g 100 RbCl rubidium chloride 715 77 g cold water,
139 g100 MgF2 magnesium fluoride 1396 0.0076 g cold water,
insoluble 100 MgCl2 magnesium chloride 708 54 g cold water,
72 g100 MgBr2 magnesium bromide 695-700 101 g cold water,
126 g 100 MgI2 magnesium iodide greater than 700 100 g 0 cold
water, 165 g 100
Revised by Dr. Walt Volland July 12, 2013 all rights reserved copyright 2010-2013
