Ionic crystals - salts
Arranging ions Ionic crystal properties Water solubility High melting point solids
Cleave and shatter Electrical conductivity

Click for larger imageIonic crystals, or salt crystals as they are more commonly known, have properties and characteristics that are unique and interesting to investigate. This topic looks at the properties of salts and their behaviour and should be read in conjunction with Ions and salts, Making salts - the rules of electrovalency and NaCl, sodium chloride, or common salt.

Arranging ions
Ionic crystals are created when huge numbers of positive and negatively charged ions come together to form a crystal. The ions combine in such a way that the total of all the positive charges is equalled by the total of the negative charges.

Click for larger imageThe ions arrange themselves in such a way that each positive ion is surrounded by negative ions and each negative ion by positive ions. The overall arrangement of ions in a crystal is called a lattice.


The simplest ionic arrangements occur when two ions of equal but opposite charge come together in a ratio of one to one. Sodium chloride, NaCl, is an example of the simplest type of ionic arrangement.

Forming salt crystal.

More complex arrangements form when ions of different charge and size form crystals, but many of the properties discussed here remain the same, no matter how complex the crystal structure.

Ionic crystal properties
Ionic or salt crystals:

  • are often water soluble
  • are high melting point solids
  • cleave and shatter when struck with a sharp edge
  • are composed of charged particles, ions, but do not conduct electricity

Water solubility
Many salts are soluble. The basic units of salts are electrically charged ions, therefore attracting polar water molecules. If the water molecules can pull the ions from the crystal, the salt will dissolve. If they cannot then the salt will be insoluble.

Ionic dissolution.

High melting point solids
The bonding between the ions in salts tends to be very strong and it takes a lot of heat energy to disrupt the ionic crystal lattice. When the salt does melt, it forms individual ions in the liquid state. Some salts have such high melting points that they chemically decompose rather than melt. eg. calcium carbonate.

Melting salt.

Cleave and shatter
If a salt crystal is struck with a sharp edge or hard object it will begin to vibrate. If hit hard enough, one layer of ions can slip over another layer, turning the overall strong attractive forces between neighboring oppositely charged ions into the repulsion of neighboring like charged ions. The crystal literally explodes, shatters or cleaves along a plane to form multiple crystals that are smaller than their parent crystal, but otherwise look exactly the same.

Cleaving ionic salt.

Electrical conductivity
To conduct an electric current a substance must have charged particles that are free and mobile. These are electrons in a metal, or ions in a solution or salt melt.

Although salts have both ions and electrons, neither are free to move in the solid crystal lattice. The electrons are well locked up in their individual ions and the ions are immobilised in the crystal lattice. When a salt melts or is dissolved in water, its ions are free to move and then conduct electricity.

Copyright owned by the State of Victoria (Department of Education and Early Childhood Development). Used with Permission.

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Ions and salts
Making salts - the rules of electrovalency
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