4.3.1 Basic Material from Physics
and Chemistry
4.3.1.1 Atoms and Molecules
Basic Material from Physics and
Chemistry

In this section we will look at basic
background material from physics and
chemistry needed to understand the
nature of the chemical hazards we will be
modeling.
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Atoms
Matter is made up of atoms and molecules.
 Atoms are the smallest components of the
basic chemical elements, which include
hydrogen, carbon, iron, uranium, etc.
 As of November 2011, 118 elements have
been identified – the first 98 occur naturally
on earth, 80 are stable, and the others are
radioactive and decay into other elements.
 Periodic Table of the Elements:
http://www.atomic-elements.info/
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Atoms
Atoms are made up of a nucleus, surrounded
by orbiting electrons.
 The nucleus is composed of protons and
neutrons – usually an atom has the same
number of protons as neutrons.
 The number of protons in an atom
determines its atomic number, so for
example, carbon has 6 protons, hence an
atomic number of 6.
 An atom’s atomic weight is determined by its
total number of protons and neutrons, which
is approximately twice its atomic number.
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Atomic Weight
Looking at Table 4.2 on page 111 of our textbook,
we see that an element’s atomic weight is not an
integer – for example, carbon has an atomic
weight of 12.011.
 The reason for this is that atoms of the same
element may have different numbers of neutrons
– for example carbon atoms usually have 6
neutrons, but may have 7 or 8 neutrons, leading
to an atomic number of 13 or 14.
 Carbon atoms with more than 6 neutrons are
radioactive – the different types of carbon are
called isotopes of carbon.

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Atomic Weight
Radioactive carbon 14 is used for carbon
14 dating to determine the age of fossils
or old artifacts.
 To get an element’s atomic weight,
scientists have determined how much of
each isotope of an element occurs in the
universe and have computed a weighted
average.
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Molecules
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Most substances are made up of a combination of
atoms.
A molecule is a collection of atoms bound together in
particular combinations and structures.
For example, water molecules are made up of two
oxygen and one hydrogen atom – we denote water
by H2O.
Another example of a molecule is CH4, natural gas
(methane), which is used for cooking – it is made up
of 4 hydrogen atoms and one carbon atom.
Ozone molecules (O3) are comprised of three
oxygen atoms!
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Molecular Weight
The molecular weight of a molecule is the
sum of the atomic weights of the atoms
making up the molecule.
 For example, the molecular weights of the
molecules on the last slide are:
 Water: 2H + 1O = 2(1.008) + 1(15.9994)
= 18.0154
 Methane: 1C + 4H = 1(12.011) +
4(1.008) = 16.043
 Ozone: 3O = 3(15.9994) = 47.99982
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Molecular Weight

What would be the molecular weights of
these hydrocarbons:
◦
◦
◦
◦
◦
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Acetylene (C2H2)
Trichloroethylene (C2HCI3)
Propane (C3H8)
Butane (C4Hl0)
Ethanol (C2H5OH).
Note that many hazardous materials such as
these which are used as fuels, solvents, etc.,
are made up of hydrocarbons!
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4.3.1 Basic Material from Physics
and Chemistry
4.3.1.2 Physical Properties of Matter
States of matter
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There are three common states of matter –
solid, liquid, and gas.
We will mostly be concerned with liquids or
gases, especially the transition from liquids
to gases as spilled hazardous liquids
evaporate or react to form gases.
These gases can be toxic or flammable and
may move from the scene of an accident to a
location with an unprepared or unsuspecting
population.
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Density
Definition: The density of a substance is its
mass per unit volume.
 Typical units are: lb/ft^3, gm/cm^3, etc.
 For example,

◦ Water has a density of 62.4 lb/ft^3 or one
g/cm^3.
◦ Solid rock has a density of about 200 lb/ft^3.
◦ Air has a density of 0.004 lb/ft^2 at sea level.
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Specific Gravity
A quantity related to density is specific gravity.
 Definition: The specific gravity of a substance is the ratio of its
density to the density of water.
 For example, the specific gravity of solid rock with a density
of 200 lb/ft^3 would be:
 Specific gravity of rock

= (density of rock)/(density of water)
= (200 lb/ft^3)/(62.4 lb/ft^3)
= 3.21
In metric units, the same rock would have a specific gravity of
3.21 g/cm^3, since the density of water is 1 g/cm^3.
 For this reason, one may encounter instances of the terms
“density” and “specific gravity” being used interchangeably.
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Density and Temperature
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Most substances will expand when heated.
Thus, the same amount of material which is
heated will require more volume, which
means it will have a lower density!
Can you think of a substance that doesn’t
behave this way?
◦ http://www.ehow.com/info_8272150_typesmaterials-shrink-heated.html
◦ http://phys.org/news/2011-11-incredible-materialreveal-scandium-trifluoride.html
◦ http://www.ncnr.nist.gov/AnnualReport/FY1998/r
h1.pdf
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Evaporation
Consider an open container of some
chemical liquid, such as water, antifreeze,
alcohol, etc.
 What would you expect to happen over
time?
 Which would you expect to evaporate
more quickly?
 Intuitively, evaporation is the process of a
liquid turning into a gas!
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Evaporation
We will need to understand the process
of evaporation, since it will play a major
role in spills of hazardous materials!
 Here are two principles to keep in mind
when considering evaporation:
 1. The rate of evaporation is proportional to
the surface area (all other factors being
equal).
 2. The rate of evaporation increases as the
temperature of the liquid increases.
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Evaporation and Surface Area
Physically, evaporation is a process in which
molecules close to the surface of a liquid that
have sufficient kinetic energy to break through
the surface do so and escape individually into the
space above the liquid.
 The molecules that escape become part of the
gaseous or vapor component of the chemical.
 It follows that if the liquid has a larger surface
area, then more molecules will be able to escape
– for example, doubling the surface area will
double the rate of evaporation!
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Evaporation and Surface Area
Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment
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Evaporation and Temperature
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The temperature of a substance such as a
liquid is a measure of the average kinetic
energy of the substance’s molecules.
Thus, when heat is applied to a liquid, the
liquid will gain more energy, causing the
liquid’s molecules to increase their
movement, in turn increasing the liquid’s
average energy.
It follows that a proportion of the molecules
near the liquid’s surface will have higher
energy, leading to an increase in the
evaporation rate!
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Evaporation and Temperature
Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment
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Vapor Pressure
Let’s consider a “simple experiment”!
Place a beaker of chemical under a larger closed
glass cover.
 Initially, all of the chemical is inside of the beaker.
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Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment
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Vapor Pressure
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The rest of the space inside the glass cover is
filled with some other material such as air that
doesn’t react with the chemical (i.e. it is “inert”
with respect to the chemical.
Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment
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Vapor Pressure
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Now, suppose the material in the beaker begins to
evaporate.
Some of the molecules from the beaker join those in the
vapor space inside the cover, outside of the beaker.
Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment
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Vapor Pressure
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Since none of the original gas molecules have any place to
go, there is an increase in the total number of molecules in
the same space.
This results in an increase in the pressure under the cover.
Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment
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Vapor Pressure
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This process will eventually slow down and reach
an equilibrium point once there are so many
chemical molecules in the vapor space that the
chemical is no longer able to evaporate.
Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment
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Vapor Pressure
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A similar effect happens on hot days when
there is high relative humidity – there is so
much water vapor in the air that sweat
produced on a body is unable to evaporate!
Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment
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Vapor Pressure
Technically, what is really happening is that at all times there are molecules
with sufficient energy leaving the liquid and entering the vapor space as
well as molecules in the gas space colliding with and rejoining the liquid.
 Initially, more molecules leave the liquid than enter, but as time increases,
the rates even out until evaporation no longer is able to take place – at
this point the system is at equilibrium.
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Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment
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Vapor Pressure
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Once equilibrium is reached, since there are more total gas
molecules in the vapor space than there were initially, the total
pressure will be higher.
The amount of pressure increase, denoted P, is called the vapor
pressure of the chemical at the system’s current temperature.
Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment
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Vapor Pressure
Since increasing the temperature would increase evaporation,
it follows that vapor pressure would also increase.
 Thus, vapor pressure is an increasing function of
temperature!
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Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment
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Vapor Pressure

Note that if the gas in the vapor space is truly inert with
respect to the chemical in the beaker and undergoes no
interactions with the chemical (in either liquid or gaseous
form), then the vapor pressure P is independent of initial
pressure!
Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment
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Vapor Pressure
While in practice, idealized conditions like this never occur, it
turns out that the only factor that significantly impacts vapor
pressure is the system’s temperature.
 For this reason, when working with vapor pressure, one must
know the temperature of the system in question.
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Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment
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Boiling Process
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Suppose a beaker that contains a liquid chemical is gradually heated
to higher and higher temperatures, in a fashion similar to heating a
pot of water on a stove.
Keep in mind that ordinary atmospheric pressure of the air is
always pushing down on the liquid – in the figure this is
represented by an imaginary piston.
Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment
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Boiling Process
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If the vapor pressure of the liquid is increased enough so that it is
greater than the atmospheric pressure, then vapor bubbles of the
chemical can expand rapidly, causing the effect known as boiling.
At this point, the chemical can enter the vapor form throughout the
liquid, not just at the surface, since vapor bubbles create their own
vapor space where they develop!
Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment
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Boiling Process
Under boiling conditions, much larger quantities of the chemical can
move into the vapor state rapidly – the primary limiting factor is the
heat provided.
 The reason for this is that it takes a certain amount of heat energy to
change a fixed amount of a given chemical from liquid to gaseous form.
 This is true for both evaporation and boiling.
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Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment
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Boiling Process
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As an example, 540 calories of heat energy are
needed to convert one gram of water from liquid
to gas under normal conditions.
This amount of energy is called water’s heat of
vaporization.
Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment
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Boiling Process
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Once a liquid reaches its boiling point, all of the heat energy being
applied to it is used up to convert more of the liquid to gaseous
form.
At this point, the liquid’s temperature essentially stays constant
right at the boiling point, rather than continuing to rise higher.
Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment
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Boiling Process
To illustrate these ideas, consider the process of boiling an
egg.
 In New York, NY (at sea level), if it takes 4 minutes to boil
the egg at 100 degrees Celsius, will it take the same amount
of time at the same temperature in Denver, CO?
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Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment
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Boiling Process
Since Denver’s elevation is one mile, the atmospheric
pressure is lower, so to heat the water to boiling
requires less heat.
 Thus, the egg will take longer to cook than 4 minutes.
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Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment
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Boiling Process
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Pressure cookers are specially designed pots with a screw-on lid
designed to build up the pressure inside the pot to a pressure
higher than atmospheric pressure.
Then, to reach the boiling point, more heat needs to be applied to
get the pot’s ingredients to boil, which means the food cooks more
quickly!
Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment
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Boiling Point
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We tend to equate “boiling point” with “high
temperature,” most likely because when
cooking, this is usually the case.
For many dangerous chemicals, it turns out
that the boiling point is already achieved at
room temperature because the chemicals
have very high vapor pressures.
For example, propane gas is stored in small
metal cylinders or bottles under high
pressure.
At room temperature, under normal
atmospheric conditions, propane will boil.
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Boiling Point
Similar to a pressure cooker, by holding the
propane liquid at a pressure at or slightly
above vapor pressure, the liquid doesn’t boil.
 But what if a tanker truck full of propane
crashes and breaks open?
 The propane liquid would pour on the
ground, boil just like water on a hot griddle,
and form a highly flammable expanding gas
cloud that can float off into a surrounding
neighborhood.
 The goal of this chapter is to analyze
situations like this!
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Mixtures vs. Pure Substances
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All of the scientific concepts discussed so far have
been for “pure substances,” i.e. materials
consisting of a single chemical.
The chemical may be made up of molecules that
consist of more than one element, but each
molecule is identical.
Water, methane, or ozone would be a pure
substance.
A “mixture” of water and other chemicals such as
acetone or gasoline, or a mixture of gasoline and
oil, etc. is not a pure substance.
Many hazardous materials are in fact a mixture of
chemicals.
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Mixtures vs. Pure Substances

A natural question to ask is: How does a
mixture’s chemical properties relate to the
chemical properties of the mixture’s constituents?
◦ For example, how are boiling points, vapor pressures,
etc. affected?
It turns out that mixtures are much more
complicated than pure substances – instead of a
boiling point, a mixture may have a range of
temperatures through which they boil.
 Also, chemicals within a mixture may interact in
ways that alter the chemical properties of the
individual chemicals in the mixture.
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Resources
http://www.atomic-elements.info/
 http://www.ehow.com/info_8272150_type
s-materials-shrink-heated.html
 http://phys.org/news/2011-11-incrediblematerial-reveal-scandium-trifluoride.html
 http://www.ncnr.nist.gov/AnnualReport/FY
1998/rh1.pdf
 Charles Hadlock – Mathematical
Modeling in the Environment, Section 4.3
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