Changes in state
Whenever a substance changes its state, like when ice melts to form
liquid water, it is said to undergo a "change in state". Changes
in state can occur when a substance is heated, cooled or even if an
external pressure change occurs. An example of this is when propane
gas is pumped into a sealed tank and it turns into a liquid, LPG. Liquid
carbon dioxide fire extingushers are also a good example of a change
in state brought on by pressure.
This topic will consider changes of state between solid, liquid and
gas brought about by changes in temperature only, as these are the most
commonly experienced changes. Remember changes in state are physical
changes, not chemical. The substance itself remains chemically the same,
i.e. water is always H2O whether it is ice, water or steam.
See The water
cycle for more on the changes of the state of water.
Energy and changes in state
Changes in state occur because energy is either
added or removed from a substance, affecting the way the particles interact
with each other. If a substance is heated, energy is added and the particles
will become more active; vibrating, rotating and even moving about faster.
If the substance has enough energy, it can overcome the bonding forces
holding the particles together and, in doing so, undergo a change in
state. Cooling, on the other hand, removes energy thus making the particles
less active and allowing the bonding forces to take hold within the
substance.
Melting - solid to liquid
Melting occurs when a solid is heated and turns to liquid. The
particles in a solid gain enough energy to overcome the bonding forces
holding them firmly in place. Typically, during melting, the particles
start to move about, staying close to their neighbouring particles,
then move more freely. For pure substances, the temperature at which
this change occurs is quite precise and is called the melting point
of the substance.
Freezing - liquid to solid
Freezing
occurs when a liquid is cooled and turns to a solid. Eventually the
particles in a liquid stop moving about and settle into a stable arrangement,
forming a solid. This is called freezing and occurs at the same temperature
as melting. Hence, the melting point and freezing point of a substance
are the same temperature. The melting and freezing point of a substance
are defined as the temperature above which, the substance is liquid
and below which, it is solid.
Boiling - liquid to gas
Boiling occurs when the particles in a liquid gain enough energy
to overcome the bonding forces holding them loosely in place in the
liquid and they become free, fast moving, individual particles in a
gas. For pure substances, the temperature at which this change occurs
is quite precise and is called the boiling point of the substance.
Condensation - gas to liquid
If a gas is cooled, its particles will eventually stop moving about
so fast and form a liquid. This is called condensation and occurs at
the same temperature as boiling. Hence, the boiling point and condensation
point of a substance are the same temperature. The boiling and condensation
point of a substance is defined as the temperature above which, the
substance is gas and below which, it is liquid.
Evaporation
Evaporation occurs when particles in a liquid pass directly into
the gas state, at a temperature below the boiling point of the liquid.
When a puddle "disappears" after rain, the water has evaporated.
Evaporation is dependent on individual particles gaining enough energy
to escape the surface of the liquid and become gas particles.
Sublimation
The change in state which occurs when a solid goes directly to a gas
is called
"sublimation'' Sublimation is like evaporation; it does not occur
at any particular temperature, but varies in rate with temperature.
Mothballs sublime and so does dry ice. Watch carefully and you will
see the solid slowly disappearing and forming a gas or odour without
first melting. For a solid to sublime, individual particles on the surface
of the solid obtain enough energy from their surroundings to jump off
the surface of the solid and become individual gas particles.
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