Read Sections 6.1 and 6.2 before viewing the slide show.
Unit 21
Gases, Liquids, and Solids and Their
Transitions and Characteristics
•Identification of Molecular Level Differences (6.1)
•Terminology of Phase Transitions (6.1)
•Phase Diagrams (6.1)
•Supercritical Fluids (6.1)
•Comparison of Ionic and Molecular Compounds
(6.2)
Identification of Molecular Level Differences (6.1)
•There are fundamental differences at the molecular level between gases, liquids
and solids
•Gases have particles that are far apart and moving rapidly and randomly in their container
•Liquids have particles that are fairly close together but can still move past each other relatively
slowly
•Solids have particles that are fairly fixed in position but can vibrate about those positions.
•To get an impression of these differences, work with the PhET simulation called
States of Matter found at http://phet.colorado.edu/en/simulation/states-of-matter.
Either download it to your computer by hitting the blue download button or run it
directly by hitting the green Run Now button at the site.
•To see the molecular level differences between states, hit the Solid, Liquid, and
Gas buttons to the right side of the simulation. How does the appearance of the
atoms compare to the descriptions of the three states above?
Simulation Questions (6.1)
•For the following, continue using the States of Matter simulation at:
http://phet.colorado.edu/en/simulation/states-of-matter
•Place the state to Solid and increase the temperature by raising the slider at the
bottom of the simulation. What do you observe about the atoms as the
temperature increases?
•What do you observe about the atoms as the slider is moved down to cool the
container?
•This is a molecular level representation of what happens during processes with
which you are already familiar:
•Melting – conversion of solid to liquid (when you heat the solid)
•Vaporization – conversion of liquid to vapor (when you heat the liquid)
•Condensation – conversion of vapor to liquid (when you cool the vapor)
•Freezing – conversion of liquid to solid (when you cool the liquid)
•Sublimation – conversion of a solid directly to a vapor (cannot see this one with the demonstration –
dry ice is a typical example of a substance that sublimes under normal conditions)
Transition Temperatures (6.1)
•If you were asked the boiling point of water your response would likely be 212 ºF
(or 100 ºC ). Similarly for the freezing point the response would likely be 32 ºF (or
0 ºC). Those values, however, depend on the pressure at which they are
measured.
•Consider the boiling point, for example. You already know that liquid water can be
converted to vapor at room temperature – consider the fact that water evaporates
in a glass at room temperature. What makes 100 ºC special? Consider the
following:
•Some water molecules are always escaping into the vapor state and some are always condensing
into the liquid state.
•When the rate of the escape equals the rate of the condensation, the system is said to be at
equilibrium.
•The pressure of the molecules above the liquid at equilibirum is called the vapor pressure and its
value depends upon temperature.
•The boiling point occurs when the vapor pressure equals the pressure being exerted on the system.
•The typically quoted boiling point for water of 100 ºC is the temperature at which the vapor pressure
of water is 1 atmosphere. For any material, the normal boiling point is the temperature at which its
vapor pressure equals 1 atmosphere.
Phase Diagrams (6.1)
•The relationship between pressure and
temperature is often summed up in a figure
called a phase diagram, such as that to the
right for water. A few key observations:
•Three main regions are labeled – solid, liquid,
and vapor.
•The lines represent equilibrium lines between the phases
separated by the lines. For example, the BD line is the
equilibrium line between liquid and vapor. Notice that
water achieves a vapor pressure of 1 atmosphere
(101.3 kPa) at 100 ºC – the normal boiling point.
Image from http://www.kmacgill.com
At lower pressures, water will boil at a lower temperature
and at higher pressures a higher temperature.
•The triple point (point B) occurs at one temperature and pressure. At this point, all
three phases are in equilibrium.
•Point D represents the critical point. Above the temperature of 374 K no distinct liquid
phase can exist – only a dense vapor state exists. The temperature at point D is called
the critical temperature and the pressure is called the critical pressure.
Supercritical Fluids (6.1)
•A fluid that is confined at temperatures above its critical temperature and pressure is called a
supercritical fluid.
•Some supercritical fluids actually have practical uses. CO2 is one of the most important
supercritical fluids. Examples of its use:
•Supercritical CO2 is used to extract caffeine from coffee beans. It is environmentally safe, relatively cheap, and easily
recycled for reuse.
•Supercritical CO2 can be used in dry cleaning to replace some chlorinated solvents
•Supercritical CO2 is used as a degreaser.
•Supercritical CO2 can be used as an extracting solvent for a wide variety of chemicals.
Notice as the temperature goes up the line separating liquid and gas phases disappears until it is one
dense gas. Image from http://www.nasa.gov .
Comparison of Ionic and Molecular Compounds (6.2)
•Recall that ionic compounds form by the transference of one or more electrons
from a metal to a nonmetal, such as in NaCl. In molecular compounds electrons
are shared, such as in water.
NaCl
Image from http://www.chemistry.wustl.edu
H2O
Image from http://www.chem1.com
•Notice the sodium ions and chloride ions are not actually bonded to each other –
they are strongly attracted by the “+” and “-” charges, but are not connected by
bonds.
•Water, on the other hand, consists of separate molecules – H2O – with two
hydrogen atoms actually bonded to each oxygen atom.
•The different nature of ionic and molecular compounds leads to several important
property differences.
Comparison of Ionic and Molecular Compounds (6.2)
•A summary table of the differences in properties between ionic and molecular
compounds is given below.
Ionic Compounds
Molecular Compounds
Almost all are solids at room
temperature
Many are solids at room temperature,
but some are liquids or gases
High boiling and melting points
Lower boiling and melting points
High energy required to melt – about
10-100 times that of molecular
compounds
Much lower energy required to melt
Solutions of ionic compounds in water
conduct electricity
Solutions of ionic compounds do not
conduct electricity
Most are crystalline, hard, and brittle
Solids are usually much softer than
ionic compounds