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
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 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.
The change in state which occurs when a solid goes directly to a gas is
"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.