http://www.ck12.org Chapter 13. States of Matter
MEDIA
Click image to the left for more content.Carbon dioxide is another substance that sublimes at atmospheric pressures. Carbon dioxide in the solid state is
known as dry ice. Dry ice is very cold (−78°C), so it is used as a coolant for goods such as ice cream that must
remain frozen during shipment. Because the dry ice sublimes rather than melting, there is no liquid mess associated
with its change of state as the dry ice warms.
Crystalline Solids
The majority of solids are crystalline in nature. Acrystalis a substance in which the particles are arranged in
an orderly, repeating, three-dimensional pattern. Particles of a solid crystal may be ions, atoms, or molecules,
depending on the type of substance. The three-dimensional arrangement of a solid crystal is referred to as the crystal
lattice. The general nature of crystal lattices of ionic compounds was introduced in the chapter,Ionic and Metallic
Bonding. Different arrangements of the particles within a crystal cause them to adopt several different shapes.
Crystal Systems
Crystals are classified into general categories based on their shapes. A crystal is defined by its faces, which intersect
with one another at specific angles, which are characteristic of the given substance. The seven crystal systems are
shown below (Table13.2), along with an example of each. The edge lengths of a crystal are represented by the
letters a, b, and c. The angles at which the faces intersect are represented by the Greek lettersα,β, andγ. Each of
the seven crystal systems differs in terms of the angles between the faces and in the number of edges of equal length
on each face.
TABLE13.2: Seven Basic Crystal Systems and an Example of Each
Crystal System Diagram Example
Cubic
a = b = c;α=β=γ= 90°Pyrite