Ionic crystals - salts
Ionic
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.
The
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.
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.
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.
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.
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.
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