Hazardous Materials

temperature, p. 49 tonne (t), p. 39 vapor, p. 36

vapor pressure, p. 53 volatile, p. 53 volume, p. 36

volumetric thermal•expansion coefficient p. 60 weight, p. 36

vapor density, p. 47

1+MSM!i Identify the common properties that characterize solids, liquids, and gases. Use the factor-unit method to convert measurements expressed in customary units into their equivalents in appropriate metric units and vice versa. Describe the concepts of density, specific gravity, vapor density, and vapor pressure and cite examples of their usefulness to emergency responders. Convert temperature readings on one temperature scale into their equivalents on the ocher temperature scales noted in this chapter. Describe the mechanisms that contribute to the spread of fire from one location to another. Convert heat measurements expressed in Beus to measurements expressed in calories and vice versa. Use the gas laws to calculate the volume of gases subjected to different temperature and pressure conditions. Describe the potential danger associated with the expansion of a heated gas or vapor that is confined in a storage vessel. Identify the general hazards that emergency responders encounter when exposed to cryogens.

At first glance, the mere number of hazardous materials is likely to overwhelm the average nonscientist. How is it possible to learn the individual properties of so many substances and recall them under the stressful conditions that often prevail when lives and property are in jeopardy?

Fortunately, we can associate the hazardous properties of many substances with their state of matter. We learn, for instance, that all gases possess certain common properties; on studying the chemistry of gases, we learn to identify these common properties and then turn our attention later to the features that cause individual gases to be regarded as unique substances.

In this chapter, we learn about some of the general properties of matter and energy and how they influence certain phenomena such as the spread of fire . Also, as we review the properties of matter and energy, we learn how they relate to the issues in fire science. Specifically, in this chapter we learn how the modes of heat transfer contribute to the propagation of fire, the reason water often effectively extinguishes a fire, and why gas cylinders are likely to rupture when their contents are excessively heated.

2.1 MATTER DEF INE D Each day, we encounter air, water, metals, stone, dirt, animals, and plants. These are the materials of which our world is made. Scientists refer to them as the different kinds of matter. When we search for common features among its forms, we note that ordinary matter possesses mass and occupies space. In other words, matter is distinguishable from empty space by its presence in it .

matter • Anything that possesses mass and occupies space

Chapter 2 Some Features of Matter and Energy 3 5

The qua ntity of matter possessed by an object regardless of it s location in the universe

The force with whic h a mass is pulled toward the center of Earth

The force of attraction between two bodies

physic al sta te of matter • Any form in which matter is gener- ally encountered : solid, liquid, and gas

The capacity of matter confined within a tank or conta iner

solid • Matter that pos- sesses a definite volume and a definite shape

li quid Matter that possesses a definite volume but lacks a def inite shape

gas • Matter that pos- sesses neither a definite volume nor a definite shape

The gaseous form of a substance that exists as a solid or liquid at normal ambient conditions

The concept of mass is closely related to the concept of weight. In our universe, every form of matter is attracted to all other forms by the force we call gravity. On Earth, the weight of matter is a measure of the force with w hich graviry pulls it toward Earth's cen- ter. As we leave Earth 's surface, the gravitational pull decreases until it becomes virtually insignificant . The weight of matter accordingly reduces to zero, yet the marter still pos- sesses the same mass as it did on the surface of Earth. For this reason, the expressions "has a mass of" and " weighs " are essentially equivalent on Earth's surface.

As usually experienced, matter exists in three different for_ms or states of aggregation: solid, liquid, and gas. 1 These are called the physical states of matter.

Because matter occupies space, a given form of matter is also associated with a definite volume o r capacity. Space should not be confused with air, because air is itself a form of matter. Volume refers to the actua l amount of space that a given form of matter occupies.

2.1-A SOLIDS A solid is the form of matter existing in a rigid state independent of the size and shape of its container. Consider this book. It retains its shape regardless of its position in space and does not need to be placed into a container to retain that shape . Left to itself, it will never spontaneous ly assume a shape different from what it has now.

Solids a lso occupy a definite volume at a given temperature and pressure. We can squeeze most solids with all our might o r heat or cool them, but their total volume change is relati vely small.

Certain solid plastics can behave differently from most solid s. Because of their inher- ent elasticity, they can assume different shapes and vo lumes when squeezed, stretched, or otherwise manipulated. Solid foams also can behave uncharacteristically, because they contain encapsulated air.

2 .1-B LIQUIDS A liquid is a form of matter that does nor possess a characteristic shape; racher, its shape ~ep~nds on the shape of the container it occupies. Consider water within a glass. The liquid takes the shape of the glass up to the level that it occupies. If we pour the water into a cup, the water takes the shape of the cup; or if we pour it into a bowl, the water takes th_e s_hape of the bowl. Of course, sufficient space must always be available for the warer wuhm the co~ta1~er; otherwise, the liquid overflows. Assuming that space is available, howe~er, an~ hqu~d ~ssumes whatever shape its container possesses.

Like sohds, liquids occupy a specific volume at a given temperature and pressure They rend to maintain a relatively fixed volume when they are exposed to a cha · · etther of_ the.se conditions. Liquids and solids are often considered incompressible, b;;.,eu:: the apphcanon of pressure barely changes their volumes When heated 1· 'd d d h h · · , 1qu1 s o ex pan muc r:iore t an soh~s d.o, ~ut nor_ nearly to the extent that gases expand. \Y/e revisit th; expansion of heated l1qu1ds m Section 2.11.

2 .1-C GASES A gas, or vapor, is anoch_er form _of matter that does not possess a characteristic sha e and assumes the shape of its co nramer. If a gas or mixture of ases · • • p a balloon, it assumes the shape of the balloon· or if it is t 7 cl such as air, is put mto the shape of the tire. ' rans erre mro a ure, it assumes

1 0ther physica l sta tes of maner are kn own beside s solid 1· 'd d

ve ry high temperatures, whereas a fift h state, ca ll ed the B~!~1Ej,~~e . gas. A fourth st~te, plasm a, exists only at peratures. _Alth_o ~gh we will not encounter materials in th e fo urth //;;1densate, ex1~ts only at very low tem - ous materials, it. is worthy to note that the sun, other stars, and oth n f I th sta~es during ?ur stud y o~ ha_za rd- pla sma state. It 1s the predomi nant state of ord 'n . h' er o~m s of mtcrga lac n c matter exist m rhe

1 ary matter wit in th e um ve rse. 36 Chapter 2 Some Features of Matter and Energy

Gases also lack a characteristic volume. When confined 10 a container with non- rigid, flexible walls, the volume that a confined gas occupies dep~nds on its temperatur~

d pressure. When confined to a balloon, for instance, the gas s volume expands an an ntracts depending on the prevailing temperature and pressure. When confined to a c~ntainer with rigid walls, however, the volume of the gas is forced to remain constant. ~his property of gases can cause rigid containers to explode, a topic we note later m Section 2.12. . . . .

The properties of sohds, hqmds, and gases noted in this section can now be used to formall y define the three states of matter:

1 Matter in the solid state possesses a definite volume and a definite shape. 1 Matter in the liquid state possesses a definite volume but lacks a definite shape. 1 Matter in the gaseous state possesses neither a definite volume nor a definite shape.

2,2 UNITS OF MEASUREMENT The necessity to measure, and to measure with accuracy, is essential to any kind of scien- tific or technological endeavor. To measure means to find the number of units in a sample of something. For instance, when we measure the distance from one point to another along a wall, we generally determine how many feet, yards, or meters are between the two points. The foot, yard, and meter are examples of the common units of length.

Two systems of units have survived the test of time: the United States customary system of weights and measures, and the SI or metric system. The United States is the only remaining industrialized country that does not exclusively use the SI system. The cus- tomary system is based on the use of an array of units that appear to have no obvious inter- relationship, such as inches (in.), feet (ft), yards (yd), miles (mi), ounces (oz), pounds (lb), pints (pt), quarts (qt), and gallons (gal).

The majority of the world's population and the worldwide scientific community use a system that is historically called the metric system of measurement. This system has been modified so that today, we actually use what is called the 51 system of units, where "SI" is an abbreviation of the system's official French name, Le Systeme International d'Unites. The SI system encourages the use of certain fundamental units from which all other mea- surements are constructed. These fundamental units are called the 51 base units. Examples of SI base units are the meter and kilogram, for length and mass, respectively.

Certain prefixes are used in the SI system to denote multiples and fractions of the units of measurement. Each prefix is a fraction or multiple of the number 10. For example, when we wish to refer to 1000 meters, we use the word kilometer. The prefix kilo means 1000 times the meter, the SI base unit.

The prefixes listed in Table 2.1 are commonly used in studying the chemistry of haz- ardous materials. These particular prefixes should be committed to memory. They are used to measure certain properties of matter, particularly its length, mass, and volume. It is appropriate to discuss each type of measurement separately.

2.2-A LENG'Jlt Today, the meter (m) is defined as the length of the path traveled by light in a vacuum in 1/299,792,458 of a second. In ordinary practice, we measure length in the metric system with_ a metric ruler. By so doing, we discover that one meter is slightly longer than a yard; spec1fically, one meter equals 39.37 inches (in .).

1 m = 39.37 in. One meter is equivalent to 100 centimeters (cm) and to 1000 millimeters (mm).

1 m = 100 cm = 1000 mm

customa ry system of weights and measures Any unit of weight and measures based on the yard and pound and used in normal com- merce within the United States

SI system of units • Known historically as the metric system, the scientific standard of measurement that employs a set of units describing length, mass, time, and other charac- teristics of matter

SI base units • The accepted units of mea- surement adopted internationally such as the meter for length, kilogram for mass, and kelvin for temperature

meter (m) • The SI unit of length in the metric system of measurement

Chapter 2 Some Features of Matter and Energy 3 7

gram (g) One one- thousandth of the mass of 1 kilogram

FIGURE 2.1 The rela- tionshi p between the inch and the centi meter. Note that 1 inch (in.) equals 2.54 centimeters (cm).

TABLE 2.1 Common Prefixes Used in the Metric System

PREFIX SYMBOL MEANING

hecta- h one hundred (102) times the SI base unit

kilo- k one thousand (103) times the SI base unit•

deci- d One-tenth (10-1) of the SI base unit

centi- C One-hundredth (10-2) of the SI base unit

milli- m One-thousandth (10-3) of the SI base unit

micro- µ one-millionth (10-6) of the SI base unit

nano- n One-billionth (1 o-9) of the SI unit one-trillionth (1 o-12) of the SI unit

- pico- p

'The SI base unit of mass is an exception.

For very large lengths, we use the kilometer (km); once again, 1000 meters is equivalent to 1 kilometer.

1 km= 1000 m

For very small lengths, we use the micron (µm) . One micron is one-millionth of a meter.

1 µm = 0.000001 m

The relationship between the inch and centimeter is shown in Figure 2.1 . One inch equals 2.54 centimeters.

1 in. = 2.54 cm

2.2-B MASS The SJ unit of mass is the kilogram. A bit of attention needs to be gi ven to constructing the multiples and fractions of mass measurements, because this unit of mass already con- tains a prefix (kilo-). The names of the various multiples and fractions of the unit of mass are constructed by attaching the appropriate prefix to the word gram (g), not kilogram. In other words, the gram is used as though it were the SI unit of mass, even though it actu· ally is not.

One kilogram is equivalent to 2.2 pounds. One gram is approximately the mass of a peanut.

Three common metric units of mass are the milligram, microgram, and kilogram. One one-thousandth of a gram is a milligram (mg) ; one one-millionth of a gram is a microgram (µg ); and 1000 grams is a kilogram (kg).

1 mg= 0.001 g 1 µg = 0.000001 g 1 kg = 1000 g

0 1 2 3 4 5 6 7 8 9 10 r, ...... :, ;,;;:~:;~:;, ...... , .. ,: .. , ·:' ... , .... , .. :~~:::., .. ,. .. I"' "'I. I"" ... , ... , ... "'I .. , ...... , .. ,.: ... :

0 1 2 3 4

38 Chapter 2 Some Features of Matter and Energy J

1 cubic meter FIGURE 2.2 Because this cube measures 1 meter on each edge, its volume is 1 cubic meter (m3), the approved SI unit of volume.

One milligram is approximately the mass of a grain of sand whereas one microgram is approximately the mass of a fleck of dust, '

For relatively large mass measurements, use is made of the metric ton, or tonne (t).

1 t = 1000 kg To verbally distinguish the ton and the tonne, the latter is pronounced "tunny."

2.2-C VOLUME The approved SI unit of volume is the cubic meter (m3). This unit is derived directly from the SI unit of length and is not a SI base unit itself. We can easily derive the cubic meter by considering the cube in Figure 2.2, which measures 1 meter on each edge. The volume of this cube is determined by taking the product of its length, width, and height.

Volume = 1 m X 1 m X 1 m = 1 m3

Using simple arithmetic, the volume of this cube is determined to be 1 cubic meter. Because it is possible to derive the cubic meter directly from a previously defined unit, it is unnec- essary to define some other unit as the SI unit of volume.

Although the cubic meter is the approved unit of volume in the SI system, another unit has been used for many years to measure volume. This unit is the liter (L) . One quart is slightly less than one liter.

946 ml= 1 qt

Chemists continue to use the liter for measuring volume because its size is so convenient for laboratory-scale measurements. The cubic meter is comparatively too large.

One liter is equivalent to one one-thousandth of a cubic meter.

1 l = 0.001 m3

If we construct a cube measuring 1 meter on each edge and then divide the resulting vol- ume into 1000 equal-size cubes, the volume of each small cube is one liter. Imagine further dividing each liter cube into another 1000 equal-size cubes. These subdivisions are illus- trated in Figure 2.3. Because the prefix mi/Ii- means one one-thousandth of a unit, the volume of each of the new cubes is one milliliter (ml).

1 l = 1000ml Formerly, the milliliter was known as a cubic centimeter (cm3). Although the use of the cubic centimeter has essentially been phased out in chemistry, it is still used in the medical field .

t onne (t) The metric unit of mass equivalent to 1000 kilograms

liter (L) The volume occupied by a cube measuring 10 centime- ters to an edge

Chapter 2 Some Features of Matter and Energy 39

r

Omnibus Trade and Competitiveness Act The federal statute that enhances the competi- tiveness of American industry throughout international markets based in part on the required use of the metric system

1m

1 1m

1 cubic meter 1 liter

10cm

1 '<.N. l'll JA"

10cm

1 liter

FIGURE 2.3 The cube on the left measures 1 meter on each edge and has been divided i~to lOOO equal- size cubes. The volume occupied by each of the smaller cubes is 0.001 c~bic meters, or 1 liter (L). The cube on the right measures 1 O centimeters on each edge and has been subd1v1ded into 1 _oo_o equal-size cubes. The volume of the larger cube is 1 liter, and the volume of each smaller cube is 1 milliliter (ml), or 1 cubic centimeter (cm 3).

2 .2-D GENERAL USE OF THE METRIC SYSTEM IN THE UNITED STATES In the international community, the United States is the sole industrialized nati_on that has not officially adopted the metric system. The United States' nonuse of the metnc system at one point was contributing to unsuccessful competition in some international ma~kets, To address this concern, Congress passed the Metric Conversion Act in 1975, Notwithstand- ing its passage, the act called only for voluntary compliance and accomplished little change in the manner by which Americans measure the properties of matter.

Then, in 1988, Congress passed the Omnibus Trade and Competitiveness Act, which mandated the following:

Identified the metric system as the preferred system for U,S. trade and commerce Required use of the metric system by all U,S. government agencies Required use of metric units for all federally funded construction projects costing over $10 million Required use of metric units for all federally assisted highway construction projects that began on or after October 1, 1996

This act obligated building and highway construction companies to begin using the metric system when seeking compensation for work on federally funded projects,

The use of metric units is also evident on warning labels for a variety of products whose manufacture or use is regulated by agencies of the federal government, especially when the relevant measurements have been obtained by scientists. For example, a high- salt (sodium chloride) diet has been linked with ailments like high blood pressure, heart attacks, and strokes, To warn about these potential ill effects, the FDA requires informa· tion concerning the dietary content of sodium to be printed on labels affixed to containers of prepared foods. Three labeling definitions are used to denote the sodium content per serving: sodium-free, less than 5 milligrams; very low sodium, less than 35 milligrams; and low sodium, less than 40 milligrams.

Even when the use of the metric system is not specifically mandated, consumer product manufacturers are printing metric units on labels together with their customary counterparts, Look at a ~everage can, for instance. Its volume may be listed as 32 fluid ounc~s (1_ qt) togethe_r with 946 milliliters. All in all, use of the metric system is slowly creepmg mto the Uruted States and taking us one step closer toward common world- wide usage.

40 Chapter 2 Some Features of Matter and Energy

2,3 CONVERTING BETWEEN UNITS OF THE SAME KIND Su ppose we wish to convert between gr~ms and kilograms , pounds and grams, or liters and ga llons. How do we accomplish this task? Problems like these are best solved by using a simple procedure called the factor-unit method. Briefly, it consists of the follow- ing key steps:

1 Identify the desired unit. 1 Choose the proper conversion factor(s). 1 Multipl y the given measurement by the conversion factor(s), being certain to multiply

and di vide numbers and cancel equal units.

Choosing the proper conversion factor is a crucial step to obtaining the correct answer 10 a problem. The conversion factor is a fraction numerically equal to 1 that equates the quaniiry in the numer~tor to the qua~tity in the denominator. For example, we know there are 1000 meters m 1 kilometer. Either of the following fractions serves as a correct conversion factor:

1000 m 1 km 1 km

or 1000m

The factor s are respectively read as follows: 1000 meters per kilometer; one kilometer per 1000 meters.

Suppose we know the height of Moscow's Mercury City Tower (at right ), the tallest building in Europe. It measures 338 .82 meters from ground level to the top. What is its height in kilometers?

Using the factor-unit method, we simply multiply the given mea- surement, 338 .82 meters, by a conversion factor that relates meters to kilometers.

1 km 338.82 m X lOOO m = 0.3388 km

Note that them in 338.82 m cancels with them in 1000 m . Then, divid- ing 338 .82 by 1000 gives 0.3388 kilometers (to four significant figures) . Individuals who are experienced in using the metric system simply move the decimal point from left to right, or vice versa, as the need arises.

Some of the more frequently used metric units and their equivalent customary counterparts are given in Table 2 .2. These relat10nsh1ps per- mit us to write conversion factors like the following:

lm 39.37 in.

TABLE 2.2

METRIC UNIT

1m

2.54 cm

1 kg

454 g

946 ml

2.54 cm 1 in.

1 kg 2.2 lb

946ml 1 qt

Common SI (Metric) Units and Their Customary Equivalents

CUSTOMARY UNIT

39.37 in.

1 in.

2.2 lb

1 lb

1 qt

factor-unit A procedure for changing a quantity expressed in one unit to a quantity of another unit by mul - tiplying, dividing, and arithmetically canceling numbers and units

conversion factor A fraction equal to 1 in which the magnitude of one unit is related in the numerator to the magnitude of another unit of the same type in the denominator

Chapter 2 Some Features of Matter and Energy 41

,l .,

I

SOLVED EXERCISE 2.1 . • for use by fire departments and others trained in

A class I standp ipe system contains a 2½-,n ch hose connection . eters handling heavy fire streams. Express t he equivalent length ,n cent,m . . . . .

. convert 2y, inches into ,ts equivalent length 1n centI- Solution: We first need one or more_ conversion f actors t~ t the following conversion factor: meters. Because one inch is 2.54 centim eters, w e can cons rue

2.54 cm 1 in .

. h ( 2 5 inches) into centimeters as follows: Through use of the factor-un it method, w e then convert 2½ inc es or ·

2 .54cm 64 cm 2. 5 lA.. X = ·

SOLVED EXERCISE 2.2

concentration The relative amount of one substance mixed or dissolved in a specified amount of a second substance

percentage (%) •The concentration of a mix- ture expressed as parts of a unit in 100 of the same units

A firefighte r must possess the physical agi lity to easily carry a 60-pound bundle Imm the base to th e top 01 a 148- foot ladder. Express the bundle mass in kilogram s an d t he length of the ladder 1n meters.

Solut ion: Accordin g to the information in Ta ble 2.2, 1 kg = 2.2 lb, and 1 m = 39.3 7 in. By th e factor-unit method, a 60-pound bu ndle is equ ivalent to a 27 -kilogram bundle.

60 Ill. x = 27 kg 2.2 Ill.

Furthermore, a 148-foot ladder is equivalent to a 45-mete r ladder.

148 ftx 12 lA.. x _ _ l _m_=45m 1 ft 39.37 1ft.

2 .4 CONCENTRATION Scientists often measure the amount of a substance as a stated unit in a given mass or volume of a mixture. This value is called the substance's concentration. A variety of units are used by professionals to measure concentration. Emergency responders often use con- centrations provided by the federal government in DOT, OSHA, and EPA regulations. For instance, when noting an airborne concentration of hydrogen cyanide at a fire scene, you may wish to compare it to OSHA's 8-hour permissible exposure limit in the workplace. This concentration is 11 milligrams per cubic meter (11 mg/m3).

Concentration is sometimes expressed as a percentage. The word percentage means parts per hundred. To calculate the percentage of an item in a total, we divide the number of items by the total number and multiply the product by 100. The percentage is expressed as a number followed by the percent sign(%) . In chemistry, the concentration of a sub- stance in a mixture is expressed as either a percentage by mass or a percentage by volume. In the former instance, we use mass units of the same type, and in the latter case, we use volume units of the same type. For example, suppose we have 100 grams of a solution in which 5 grams of table salt is dissolved. The concentration of table salt in the solution is expressed as 5% by mass. If we have 100 milliliters of a solution in which 5 milliliters of a substance is dissolved, the concentration of the substance in the solution is expressed as 5% by volume.

42 Chapter 2 Some Features of Matter and Energy

I

heat • The form of energy transferred from one body to another because of their temperature dif- ference; energy arising from atomic or molecu- lar motion

The nature of any process that absorbs heat from its surroundings

The

TABLE 2.6 f Some Common Liquids as a Function Vapor Pressures o

of Increasing Temperature

TEMPERATURE WATER (mmHg) ETHANOL (mm Hg)

BENZENE (mmHg) 'F 'C

5.6 15 14 -10 2.1

12 .2 27

32 0 4.6 23.6 45

50 10 9.2 43.9 75

68 20 17.5 78.8 118

86 30 31.8

LIQUID

222.2 271 122 50 92.5

167 75 289 .1 666.1 643

760 1693.3 1360

212 100

The evaporation rates of liquids are loosely classified as fast, medium, and slow when the numerical values are greater than 3.0, between 0.8 and 3.0, an~ less t_han 0.8, res_pec- tively. Using this subjective system, the evaporation rate of benzene 1s class1fu:d as medmm.

Generally speaking, the vapor of a substance 1s its most hazardous physical form. I'. is the vapor of a flammable liquid-not the liquid itself- that burns, and 1t 1s a toxic liq- uid 's vapor that causes adverse health effects when inhaled. Clearly, the vapor pressure of a flammable or toxic substance has a direct impact on its potential fire and health ha z- ards, respectively.

When a flammable or toxic liquid has a relatively low vapor pressure, little vapor evolves at the prevailing temperature. It can neither pose a significant fire and explosion hazard nor an inhalation toxicity hazard. A flammable liquid with a comparatively high vapor pressure, however, poses a fire and explosion hazard, and a poisonous liquid with a comparatively high vapor pressure poses an inhalation toxicity hazard.

A flammable or poisonous liquid with a relatively high vapor pressure presents unique transportation problems. During the course of its transportation, a liquid confined within a tank or other vessel can increase in temperature by absorbing heat from its surround- ings. As the temperature of the liquid increases, a considerable volume of vapor is pro- duced within the tank. This vapor enters the headspace above the liquid and exerts pressure on the walls of the confining vessel. To retain its integrity, the vessel must be constructed to withstand this internal pressure during the time that it is used to transport the liquid.

2.9 HEAT AND ITS TRANSMISSION Heat is the form of energy associated with the motion of atoms or molecules (Sections 4.4 and 4.6), small particles of which all matter is composed. Heat is manifested when changes in any of the following occur:

A material's temperature The physical state of a substance The chemical identity of a substance

nature of any process Heat is either absorbed or emitted as these processes occur. When heat is abso rbed, that emits heat into its the ph_enomenon is called an endothermic process; and when he at is released ro th e sur· surroundings roundmgs, the phenomenon is called an exothermic process . 54 Chapter 2 Some Features of Matter and Energy

In Section 2.6, it was noted that energy is often d · B · · h h I · d I

. s. One British thermal un·t B measure m nt1s t erma umts an ca one 1 , or tu, represents the heat that must be supplied to

· e the temperature of 1 pound of wat 1 d h · • • ra1s , . . er egree Fa renhe1t, specified at the tempera- e of waters maximum density 39 1°F (3 97°c) • · · rur . . , · • . One calorie, or cal, 1s defmed as the

amount of heat reqmred to raise the temperature f 1 f 1 d c I · 4 5

15 50c T h . o gram o water egree e sms,

British thermal unit (Btu) The amount of heat required to raise the temperature of

from 1 · to · · ~o. undred fifty-two (252) calories is equivalent to one British 1 pound of water

thermal umt, and 1 calorie 1s equivalent to 4.184 joules. 1 degree Fahrenheit

1 Btu = 252.0 cal = 1055 J 1 cal= 4 .187 J

When_ che_mical reactions occur, the reactants change their chemical identities by trans~ormmg mto. o~e or more other substances. Thermal energy accompanies these chemical changes; 1t 1s called their heat of reaction.

calorie The amount of heat required to raise the temperature of 1 gram of water 1 degree Celsius

Heat is evolved during a variety of combustion processes such as the burning of gaso- line, wood, natural gas, and other materials. When these fuels burn, they provide energy and serve as ~ources_ of heat, power, and light. On the negative side, however, they can occurs also pose a nsk of fire and explosion. When these materials burn in bulk, they usually

heat of reaction The energy that is absorbed or evolved when a chemical reaction

ignite secondary fires involving nearby materials. The proper control of the heat emitted during combustion is an extremely important

factor in fire control. Fires continue as self-sustained phenomena only when sufficient heat is released during combustion to substitute for the input energy initially provided from an ignition source. Conversely, many fires cannot be extinguished until some means is undertaken to reduce or eliminate this heat.

Emergency responders also encounter heat as a form of energy in situations having nothing to do with combustion phenomena. Paramedics, for example, often use instant heat packs and cold packs when administering first-aid to individuals with muscular inju- ries. One type of portable pack consists of a pouch of water and a solid chemical sub- stance: magnesium sulfate for heat packs, and ammonium nitrate for cold packs. When the separate packs are squeezed, the water commingles with the solids and produces their solutions. As a magnesium sulfate solution forms, the heat that is evolved may be used to warm an injury. As an ammonium nitrate solution forms, the heat that is absorbed may be used to cool an injury.

Heat is always transferred from warm materials to cooler ones. If several materials near one another have different temperatures, those that are warm become cooler, and those that are cool become warmer, until they achieve a common temperature. Heat is transmitted from one material to another or from one spot to another spot by three independent modes: conduction, convection, and radiation. The nature of each mode of heat transmission is an important factor associated with understanding how fire spreads.

2.9-A CONDUCTION The handle of a metal spoon that has been inserted into hot coffee becomes hot itself. This transfer of heat between two or more materials in contact-in this case, from the coffee to the spoon-is called conduction.

Every material conducts heat to some extent. Metals, such as silver, copper, iron, and aluminum, conduct heat most efficiently; nonmetals, such as glass and air, a re not good conductors of heat. Materials that are not good conductors are good insulators, because they delay the transfer of heat.

2·9·B CONVECTION Beat can also be transferred from spot to spot or even between two or mo_re substances ?Y the natural mixing of their component parts. This happens when cold milk or cream dis-

conduction The mech- anism by which heat is transferred to the parts of a stationary material or from one material to another with which it is in contact

Chapter 2 Some Features of Matter and Energy 55

I I I convection The mech- anism by which heat is transferred by the movement of a heated material from spot to spot

rad iation The mecha- nism by which heat is transferred between two materials not in contact

perses throughout hot coffee without the use of a spoo~, or when th~ a~r in a room gets warmer when it is hot outside the room. This transm1ss10n of heat ~1thin a substance or between substances by means of natural mixing is known as convection.

The circulation of warm air that exits a heat vent into a room is caused by convec- tion. The popular expression "heat rises" actually means "hot air rises." As warm air issues from a heat vent into a room, it rises toward the ceiling. The surrounding cool air descends to replace the warm air that rose, is heated at the heat vent, an~ then rises toward the ceiling. In this fashion, warm and cool air exchange to generate a1r currents.

The origin of the convection phenomenon is associa~e~ ':"ith ~~e _imp~ct that gravity has on matter. In zero gravity, convection can occur only if 1t 1s art1fic1ally mduced. When a substance burns convection moves its combustion products away from the flame to dis- sipate into the sU:rounding atmosphere; but where zero gravity exists, as in a spaceship orbiting Earth, the combustion products remain in the immediate area of the combustion zone, where they quickly extinguish the flame .

2 .9-C RADIATION Imagine a 200-watt light bulb hanging from a ceiling. When the light is turned on, heat can be felt when we hold our hands around the bulb. This transfer of heat from the bulb to our hands cannot be caused by conduction, since the air between the bulb and our hands is a poor conductor of heat. It also cannot be caused by convection, because hot air currents rise upward. The heat from the bulb is transmitted by a third means called radiation.

Unlike conduction and convection, radiation occurs even when there is no material contact between two objects . Heat from the sun radiates through space and warms Earth and other celestial bodies.

All matter radiates heat at elevated temperatures; that is, it exhibits the phenomenon called incandescence. When the temperature of a heated object exceeds approximately 932°F (500°C}, the radiation becomes visible. Burning flames , glowing coals, and molten metal are examples of matter hot enough to radi ate visible light. Hot objects ma y also radiate energy at lower temperatures, but the energy emitted is generally in the infrared region of the electromagnetic spectrum, which cannot be observed with the naked eye.

2.9-D SPREAD OF FIRE The conduction process does not significantly contribute to the sustenance and spread of a normal fire. Figure 2.9 illustrates that convection and radiation are the modes of hear transmission primarily responsible for sustaining and sprea ding freely burning fires in the following ways:

Convection affects the spread of fire by means of the natural movement of hot and cold air. As hot air rises, heat is transmitted to adjoining materials and initiates their ignition. Cool, fresh air simultaneously flows inward into the fi re to replace the hot air, thereby providing the fire with a supply of oxygen.

Radiation affects the spread of fire by transmitting heat, primarily in a lateral di- rection, from the immediate fire scene to nearby materials. The transmission of radi ant heat initiates the combustion of these materials.

heat capacity • The 2.10 CALCULATION OF HEAT amount of heat needed to raise the tempera- ture of 1 pound of a substance 1 degree Fahrenheit or 1 gram of a substance 1 degree Celsius

Two or more substances differ from one another by the quanti ty of hea t required to pro-_ duce a given temperature change in the same mass of each substance. This qu antity at ~eat is called the heat capacity. The heat capacities of some common liquids are p rovided m Table 2.7. For water, the magnitude of the heat capacity varies as a functi o n o f its cem· perature, as indicated. The units of heat capacity in the English a nd metric sys tems are rbe

56 Chapter 2 Some Features of Matter and Energy