professor Dave here let's talk about phase changes
it's possible for matter to transform directly between any of the three phases we are probably aware of
melting and freezing which go between solids and liquids as well as boiling
and condensing which go between liquid and gas, in addition going directly from
solid to gas like dry ice is called sublimation, going directly from gas to
solid is called deposition. each process carries with it a particular change in
enthalpy. any liquid will have a vapor pressure as some particles will
spontaneously evaporate and as the temperature is raised the liquid will
eventually reach its boiling point which is where the vapor pressure is
equal to the atmospheric pressure. at this temperature bubbles of gas will
form within the liquid as opposed to only molecules at the liquid-gas
interface going into the gas phase. the normal boiling point of a liquid is
measured at one atmosphere or the pressure at sea level
atmospheric pressure decreases as the boiling point decreases which is why
water boils at a lower temperature at higher altitudes
the freezing point of a substance is the temperature at which a liquid turns to
crystalline solid, the melting point is the temperature at which a crystalline
solid melts. these are the same temperature they just represent
processes in opposite directions. as we heat a sample of ice it will melt and
then boil. on this diagram the flat sections represent the phase changes
themselves. in these sections the temperature doesn't rise because the
heat energy being added is going towards disrupting the lattice energy of ice or
overwhelming the hydrogen bonds occurring in liquid water so during
these periods the heat energy does not go towards heating the sample. once a
phase change is complete the heat energy returns to heating the sample. the energy
exchanged during melting and boiling is called the heat of fusion and the heat
of vaporization respectively
the phase change is denoted by these subscripts telling us when water is
considered solid, liquid, or gas and this is the energy needed per mole of water
to cause the phase change. this is specific to water because every compound
has a different lattice energy in the solid phase and makes intermolecular
contacts of differing strengths in the liquid phase. phase diagrams show what
phase a substance will be in at a particular temperature and pressure. for
water we can see the phases we would expect at atmospheric pressure. if the
pressure is dramatically lowered we can see how water would sublimate as the
temperature increases instead of melting first. on a phase diagram lines represent
equilibrium between two phases and the triple point represents an equilibrium
between all three. for water the line between solid and liquid slants up and
left because the solid form is less dense than the liquid form. for carbon
dioxide it is the liquid form that is less dense
so the line slants up and right. also notice that at atmospheric pressure
carbon dioxide is never a liquid, this is why dry ice which is solid carbon
dioxide typically sublimates. let's check comprehension.
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