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By the end of this section, you will be able to:
  • Define phase transitions and phase transition temperatures
  • Explain the relation between phase transition temperatures and intermolecular attractive forces
  • Describe the processes represented by typical heating and cooling curves, and compute heat flows and enthalpy changes accompanying these processes

We witness and utilize changes of physical state, or phase transitions, in a great number of ways. As one example of global significance, consider the evaporation, condensation, freezing, and melting of water. These changes of state are essential aspects of our earth’s water cycle as well as many other natural phenomena and technological processes of central importance to our lives. In this module, the essential aspects of phase transitions are explored.

Vaporization and condensation

When a liquid vaporizes in a closed container, gas molecules cannot escape. As these gas phase molecules move randomly about, they will occasionally collide with the surface of the condensed phase, and in some cases, these collisions will result in the molecules re-entering the condensed phase. The change from the gas phase to the liquid is called condensation    . When the rate of condensation becomes equal to the rate of vaporization    , neither the amount of the liquid nor the amount of the vapor in the container changes. The vapor in the container is then said to be in equilibrium with the liquid. Keep in mind that this is not a static situation, as molecules are continually exchanged between the condensed and gaseous phases. Such is an example of a dynamic equilibrium    , the status of a system in which reciprocal processes (for example, vaporization and condensation) occur at equal rates. The pressure exerted by the vapor in equilibrium with a liquid in a closed container at a given temperature is called the liquid’s vapor pressure    (or equilibrium vapor pressure). The area of the surface of the liquid in contact with a vapor and the size of the vessel have no effect on the vapor pressure, although they do affect the time required for the equilibrium to be reached. We can measure the vapor pressure of a liquid by placing a sample in a closed container, like that illustrated in [link] , and using a manometer to measure the increase in pressure that is due to the vapor in equilibrium with the condensed phase.

Three images are shown and labeled “a,” “b,” and “c.” Each image shows a round bulb connected on the right to a tube that is horizontal, then is bent vertically, curves, and then is vertical again to make a u-shape. A valve is located in the horizontal portion of the tube. Image a depicts a liquid in the bulb, labeled, “Liquid,” and upward-facing arrows leading away from the surface of the liquid. The phrase, “Molecules escape surface and form vapor” is written below the bulb, and a gray liquid in the u-shaped portion of the tube is shown at equal heights on the right and left sides. Image b depicts a liquid in the bulb, labeled, “Liquid,” and upward-facing arrows leading away from the surface of the liquid to molecules drawn in the upper portion of the bulb. A gray liquid in the u-shaped portion of the tube is shown slightly higher on the right side than on the left side. Image c depicts a liquid in the bulb, labeled, “Liquid,” and upward-facing arrows leading away from the surface of the liquid to molecules drawn in the upper portion of the bulb. There are more molecules present in c than in b. The phrase “Equilibrium reached, vapor pressure determined,” is written below the bulb and a gray liquid in the u-shaped portion of the tube is shown higher on the right side. A horizontal line is drawn level with each of these liquid levels and the distance between the lines is labeled with a double-headed arrow. This section is labeled with the phrase, “Vapor pressure.”
In a closed container, dynamic equilibrium is reached when (a) the rate of molecules escaping from the liquid to become the gas (b) increases and eventually (c) equals the rate of gas molecules entering the liquid. When this equilibrium is reached, the vapor pressure of the gas is constant, although the vaporization and condensation processes continue.

The chemical identities of the molecules in a liquid determine the types (and strengths) of intermolecular attractions possible; consequently, different substances will exhibit different equilibrium vapor pressures. Relatively strong intermolecular attractive forces will serve to impede vaporization as well as favoring “recapture” of gas-phase molecules when they collide with the liquid surface, resulting in a relatively low vapor pressure. Weak intermolecular attractions present less of a barrier to vaporization, and a reduced likelihood of gas recapture, yielding relatively high vapor pressures. The following example illustrates this dependence of vapor pressure on intermolecular attractive forces.

Questions & Answers

An atom or group of atoms bearing anelectrical charge such as the sodium and chlorine atoms in a salt solution.
Adazion Reply
Hello guys! Answer me questions nah
Adazion
pls wat is periodic table
Prince
it's a list that shows the chemical element arranged according to their properties.
Adazion Reply
what is the chemical equation for ideal gas?
Adazion
what's Boyle and gas law?
Adazion
what's the meaning of this℃ in atomic table
Adazion
wat are ions
Sinyene
What is periodic table
SIRAJO Reply
Table that shows the elements in order of their atomic number
Uzair
organization of everything known in the universe in groups and periods. The structure is based on increasing mass and reaction properties.
Kate
How to mix chemical
Ukeh Reply
why the elements of group 7 are called Noble gases
isaac Reply
they aren't. group 8 is the noble gasses. they are snobs that don't mix with others like nobles, they have full valence shells so they don't form bonds with other elements easily. nobles don't mingle with the common folk...
Jessica
the group 7elements are not the noble gases . according to modern periodic group 18 are called noble gases elements because their valence shell are completely field so that they can't gain or loss electron so they are not able to involve in any chemical reaction.
Leena
Group 7 element they are not noble gases they halogen and halogen mean salt formers
SIRAJO
what is chemistry
Daniel Reply
chemistry is the branch of science which deal with the composition of matter
SHEDRACK
chemistry is an organized way to think about matter
Kate
Chemistry is the branch of science that deals with the composition, properties and uses of matter
Bamgbose
The branch of science concerned with the substances of which matter is composed, the investigation of their properties and reactions, and the use of such reactions to form new substances.
Aminu
discuss the orbital stracture of the following methane,ethane,ethylene,acetylene
khadija Reply
Why phosphurs in solid state have one atom but in gas state have four atoms
Shehab Reply
Are nuclear reactions both exothermic reactions and endothermic reactions or what?
Blessed Reply
to what volume must 8.32 NaOH be diluted to its analytical concentration 0.20 M
Sheriza Reply
weight in mg 1.76 mole of I
Sheriza
the types of hydrocarbons
Ohanaka Reply
u are mad go and open textbook
Emmanuel
hahahahahahahahahahahahaha
Jessica
aliphatic and aromatic hydrocarbons
Osakue
stupid boy Emmanuel
Ohanaka
saturated and unsaturated
Leena
aromatic hydrocarbon aliphatic hydrocarbon
SIRAJO
I don't use to see the messages
Adazion Reply
Hhhhhh
SIRAJO
how can you determine the electronegativity of a compound or in molecules
Shalom Reply
when u move from left to right in a periodic table the negativity increases
reeza
Are you trying to say that the elctronegativity increases down the group and decreases across the period?
Ohanaka
yes and also increases across the period
reeza
for instance when you look at one group of elements in a periodic table electronegativity decreases when you go across the table electronegativity increases. hydrogen is more electronegative than sodium, potassium of that group. oxygen is more electronegative than carbon.
reeza
i hope we all know that organic compounds have carbon as their back bone
Madueke
OK,Thank you so much for the answer. I am happy now
Adazion Reply
can I ask you a question now
Osakue
yes
hanna
what is the oxidation number of nitrogen, oxygen and sulphur
Osakue
5, -2 & -2
hanna
What is periodic table
SIRAJO
What is an atom?
Adazion Reply
is a smallest particle of a chemical element that can exist
Osakue
can I ask a question
Osakue
it is a substance that cannot be broken down into simpler units by any chemical reaction
Madueke
An atom is the smallest part of an element dat can take part in chemical reaction.
Idris
an atom is the smallest part of an element that can take part in a chemical reaction nd still retain it chemical properties
Precious
Is the smallest particles of an element that take part in chemical reaction without been change
John

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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