The solid state
Of
the four states of matter, solids are probably the most familiar to us
and the easiest to understand and describe. We build solid structures,
surround ourselves with solid objects and value the solid state because
it is unchanging and predictable.
Solids in the Universe
Solids are the third most common state in the Universe, in the form of
cosmic dust, Earth-like planets, comets and other stray pieces of rock.
To form solids, the matter involved must have fairly large, heavy particles.
The known Universe is 99% hydrogen and most of the rest is helium. These
are the two lightest elements. Relatively very few heavy atoms and molecules
have been formed since the beginning of time, some 12 -15 billion years
ago.
Solids on Earth
Most of the outer layer of our Earth is solid. This solid mantle sits
on a liquid outer core which surrounds an inner solid core. Two thirds
of the Earth's surface is covered in liquid water. By mass, solids are
the second most common state, behind liquids.
What makes a solid?
Solids are typified by an unchanging shape and by the fact that they occupy
a set volume. Substances which are solids under normal conditions (25°C
and 1 atmosphere pressure) are usually made of large heavy atoms or molecules
and are tightly bonded together. Although the particles in a solid may
vibrate and spin with kinetic energy, they cannot break free of the strong
bonds holding the particles together in a lattice structure.
Physical types of solids
Pure substances that are solids generally fall into one of two categories:
crystals or glasses. Crystals, typically, have well defined lattice structures,
while glasses have a disrupted structure. They are sometimes likened to
a "suspended liquid", that is, having the chaos of a liquid,
but the rigidity of a solid.
Complex solids such as wood, paper, rocks, plastics and foams are made
up of mixtures of different substances forming a overall solid. Within
this solid structure there can be liquids and gases as well as solids.
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