a) Informally, we may differentiate different
states of matter by its appearance, mobility, etc.
The descriptions are qualitative and sometimes
b) Using the classical scientific language, we can
classify the states of matter into solid,
liquid or gas by its symmetry. Those
contrasting states of matter are the result of
different symmetries. For example, the
translation symmetry is preserved while the
rotation symmetry is broken in solid. In
contrast, the translation symmetry is broken while
the rotation symmetry is untouched in gas. The
liquid state lies in between these two.
c) There are new states of matter that are beyond
the scope of classification by symmetry alone. We
have to introduce some other terms such as the
topology, etc. A good example is the
topological insulator, which is surface conducting
while the bulk is insulating. The topological
insulator is clearly different from the regular
insulator, but can't be differentiated by the
language of symmetry only.
The four states of matter that we commonly deal
with are solid, liquid, gas, and plasma.
You personally have experience with three of the
four of these: the ground that you stand on is
solid, the water you drink is liquid, and the air
that you breathe is gas. Plasma is different
from the others in that it is affected by magnetic
fields, which gasses are not, but otherwise
behaves much like a gas. The sun is made out of
plasma. What I don't understand is what you
are referring to as "evidence" - you know how the
solid ground, liquid water, and gaseous air behave
around you, and how they are different. We can do
experiments on them to know what their properties
are in more detail, but the experiences you
have with them are evidence themselves.
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