Ch 10 Section 4 Flashcards
some liquid chemical substances have odors that are easily detected because some molecules at the upper surface of the liquid have enough energy to
overcome the attraction of neighboring molecules
a phase is any part of a system that has
uniform composition and properties
condensation is the process by which a
gas changes to a liquid
a gas in contact with its liquid or solid phase is often called a
vapor
if the temperature of the liquid remains constant and the cap remains closed, the rate at which molecules move from the liquid phase to the vapor phase remains
constant
near the beginning of the evaporation process, very few molecules are in the
gas phase, so the rate of condensation is very low
but as more liquid evaporates, the increasing number of gas molecules causes the
rate of condensation to increase
eventually, the rate of condensation equals the rate of evaporation, and a state of
equilibrium is established
equilibrium is a dynamic condition in which two opposing changes occur at
equal rates in a closed system
even though molecules are constantly moving between liquid and gas phases, there is no
net change in the amount of substance in either phase
vapor molecules in equilibrium with a liquid in a closed system exert a pressure
proportional to the concentration of molecules in the vapor phase
the equilibrium vapor pressure of the liquid is the pressure exerted by a vapor in
equilibrium with its corresponding liquid
the increase in equilibrium vapor pressure with increasing temperature can be explained in terms of the
kinetic-molecular theory for the liquid and gaseous states
increasing the temperature of a liquid increases the
average kinetic energy of the liquid’s molecules
this energy change increases the number of molecules that have enough energy to
escape from the liquid phase into the vapor phase
the resulting increased evaporation rate increases the number of
molecules in the vapor phase, which in turn increases the equilibrium vapor pressure
because all liquids have characteristic forces of attraction between their particles, every liquid has a specific
equilibrium vapor pressure at a given temperature
the stronger these attractive forces are, the smaller the percentage of liquid particles that can
evaporate at any given temperature is
a low percentage of evaporation results in a low
equilibrium vapor pressure
volatile liquids are liquids that
evaporate readily
volatile liquids have relatively weak forces of
attraction between their particles
ether is a typical
volatile liquid
nonvolatile liquids do not
evaporate readily
nonvolatile liquids have relatively strong attractive
forces between their particles (e.g. molten ionic compounds)
equilibrium vapor pressures can be used to explain and defined the concept of
boiling
boiling is the conversion of a liquid to a
vapor within the liquid as well as at its surface
if the temperature of the liquid is increased, the equilibrium vapor pressure
also increases
the boiling point of a liquid is the temperature at which the equilibrium vapor pressure of the liquid
equals the atmospheric pressure
the lower the atmospheric pressure is, the lower the
boiling point is
at the boiling point, all of the energy absorbed is used to evaporate the liquid, and the temperature remains
constant as long as the pressure does not change
if the pressure above the liquid being heated is increased, the temperature of the liquid will rise until the vapor pressure equals the
new pressure and the liquid boils once again→principle behind pressure cooker
the pressure cooker is sealed so that the steam pressure builds up over the surface of the boiling water, the pressure increases the boiling temp of h20, resulting in
shorter cooking time
vacuum evaporator causes boiling at lower-than-normal
temperatures
vacuum evaporators remove water from milk & sugar solutions. under reduced pressure, the water boils away at a temperature low enough to
avoid scorching milk or sugar