16.1 Thermal Energy and Matter

Reading StrategyPreviewing Copy the table below. Beforeyou read, preview the figures in this sectionand add two more questions to the table. Asyou read, write answers to your questions.

Key ConceptsIn what direction doesheat flow spontaneously?

What is the temperatureof an object related to?

What two variables isthermal energy related to?

What causes thermalexpansion?

How is a change intemperature related tospecific heat?

On what principle does acalorimeter operate?

Vocabulary◆ heat◆ temperature◆ absolute zero◆ thermal expansion◆ specific heat ◆ calorimeter

In the 1700s, most scientists thought heat was a fluid called caloricthat flowed between objects. In 1798, the American-born scientistBenjamin Thompson (1753–1814), also known as Count Rumford,challenged this concept of heat. Rumford managed a factory thatmade cannons. Figure 1 shows how a brass cylinder was drilled tomake the cannon barrel. Water was used to cool the brass so that it didnot melt. Rumford observed that the brass became hot as long as thedrilling continued, producing enough heat to boil the water. Soon afterthe drilling stopped, however, the water stopped boiling. When thedrilling resumed, the water again came to a boil. Based on his observa-tions, Rumford concluded that heat could not be a kind of matter, butinstead was related to the motion of the drill.

Work and HeatA drill is a machine that does work on the cannon. Remember that nomachine is 100 percent efficient. Some of the work done by the drilldoes useful work, but some energy is lost due to friction. Frictioncauses the moving parts to heat up. The more work done by the drill,the more that friction causes the cannon to heat up.

Heat is the transfer of thermal energy from one object to anotherbecause of a temperature difference. Heat flows spontaneouslyfrom hot objects to cold objects. Heat flows from the cannon to thewater because the cannon is at a higher temperature than the water.

Which has more thermal energy,a cup of tea or a pitcher of juice?

Questions About Thermal Energy and Matter

b. ?

d. ?

c. ?

e. ?

a. ?

Answers

474

Figure 1 Count Rumfordsupervised the drilling of brasscannons in a factory in Bavaria.From his observations, Rumfordconcluded that heat is not a formof matter.

474 Chapter 16

FOCUS

Objectives16.1.1 Explain how heat and work

transfer energy.16.1.2 Relate thermal energy to the

motion of particles that makeup a material.

16.1.3 Relate temperature to thermalenergy and to thermalexpansion.

16.1.4 Calculate thermal energy,temperature change, or massusing the specific heat equation.

16.1.5 Describe how a calorimeteroperates and calculate thermalenergy changes or specific heatusing calorimetrymeasurements.

Build VocabularyConcept Map Have students constructa concept map of the vocabulary termsin this section. Instruct students to placethe terms in ovals and connect the ovalswith lines on which linking words areplaced. Students should place the mainconcept (Thermal Energy and Matter) at the top. As they move away from themain concept, the content shouldbecome more specific.

Reading StrategySample answers: a. A pitcher of juiceb. Why did Rumford conclude that heat is not a form of matter? c. The brass washot enough to make water boil onlyduring drilling, so the heat must berelated to the motion of the drill. d. Howis specific heat related to temperature?e. The lower a material’s specific heat, the more its temperature rises when agiven amount of energy is absorbed by a given mass.

INSTRUCT

Work and HeatFYIIt is common usage to talk about heatflowing. More precisely, it is thermalenergy that flows. It is always correct touse “heat” as a verb; using “heat” as anoun should be avoided.

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Reading Focus

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Section 16.1

Print• Reading and Study Workbook With

Math Support, Section 16.1 and Math Skill: Calculating with Specific Heat

• Math Skills and Problem SolvingWorkbook, Section 16.1

• Transparencies, Chapter Pretest andSection 16.1

Technology• Probeware Lab Manual, Lab 7• Interactive Textbook, Section 16.1• Presentation Pro CD-ROM, Chapter Pretest

and Section 16.1• Go Online, NSTA SciLinks, Specific heat

Section Resources

TemperatureHow do you know something is hot? You might use a thermometer tomeasure its temperature. Temperature is a measure of how hot or coldan object is compared to a reference point. Recall that on the Celsiusscale, the reference points are the freezing and boiling points of water.On the Kelvin scale, another reference point is absolute zero,which is defined as a temperature of 0 kelvins.

Temperature is related to the average kineticenergy of the particles in an object due to their randommotions through space. As an object heats up, its particlesmove faster, on average. As a result, the average kinetic energyof the particles, and the temperature, must increase.

Why does heat flow from a high to a low temperature?One way that heat flows is by the transfer of energy in col-lisions. On average, high-energy particles lose energy, andlow-energy particles gain energy in collisions. Overall, col-lisions transfer thermal energy from hot to cold objects.

Thermal EnergyRecall that thermal energy is the total potential and kineticenergy of all the particles in an object. Thermal energydepends on the mass, temperature, and phase (solid, liquid,or gas) of an object.

Thermal energy, unlike temperature, depends on mass.Suppose you compare a cup of tea and a teapot full of tea.Both are at the same temperature, so the average kineticenergy of the particles is the same in both containers.However, there is more thermal energy in the teapot becauseit contains more particles.

Now consider how thermal energy varies with tempera-ture. You can do this by comparing a cup of hot tea with acup of cold tea. In both cases, the tea has the same mass,and the same number of particles. But the average kineticenergy of particles is higher in the hot tea, so it also hasgreater thermal energy than the cold tea.

Figure 2 shows the particles in a cup of hot tea and in apitcher of lemonade. The tea is at a higher temperaturebecause its particles move a little faster, on average. But theyare only moving slightly faster, and the pitcher of lemonadehas many more particles than the tea. As it turns out, thepitcher of lemonade has more thermal energy than the cupof hot tea.

What is thermal energy?

5375& Associates

A

B

Figure 2 Thermal energy depends on mass andtemperature. A The tea is at a highertemperature than the lemonade because itsparticles have a higher average kinetic energy.B The lemonade is at a lower temperature, butit has more thermal energy because it has manymore particles. Inferring In which liquid arewater particles moving faster, on average?

Thermal Energy and Heat 475

Build Reading LiteracyMake Inferences Refer to page 472Din this chapter, which provides theguidelines for making inferences.

Students’ understanding often dependson how they apply prior knowledgetoward making inferences about newsituations. Have students read the twoparagraphs at the bottom of p. 474.Invite students to describe situationsthat are similar to the drill heated byfriction. Then, ask students to make aninference: Based on what you haveread, why do your hands feel warmerafter you rub them together? (Some ofthe work done is lost to friction, and so isconverted to thermal energy.)Logical

Temperature

Students may assume from common-usephrases such as heat transfer and heatflow that heat is a moving substance.Emphasize that heat is a flow, or transfer,of thermal energy from one object ormaterial to another, just as work is atransfer of mechanical energy. Whileconvection involves the movement ofparticles of a fluid from one place toanother, there is obviously no flow ofmatter when thermal energy is trans-ferred from one solid to another. Anamount of heat, like an amount of work,refers to how much energy is transferred.Verbal

Thermal EnergyUse VisualsFigure 2 Stress that the particles in both liquids are mostly water molecules,and that these are not simple spheres.Ask, In what ways can a water moleculemove? (Each molecule can move to thesides in three dimensions, rotate, andstretch along its molecular bonds.) Pointout that some kinetic energy is present in each of these motions, and this affectsthe overall temperature of the liquid.Visual

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Thermal Energy and Heat 475

Customize for English Language Learners

Reading/Learning LogConcepts such as heat, temperature, andthermal energy are easy to misunderstand andconfuse with each other. Be sure that Englishlanguage learners have a clear understanding

of these concepts by having them construct a Reading/Learning Log. Have students writewhat they understand in the left column, andwhat they still have questions about in theright column.

Answer to . . .

Figure 2 The average speed of waterparticles is greater in the tea becausethe average kinetic energy is greater.

Thermal energy is thetotal potential and kinetic

energy of all the particles in an object.

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Thermal Contraction and ExpansionIf you take a balloon outside on a cold winter day, it shrinks. Can youexplain why? As temperature decreases, the particles that make up theair inside the balloon move more slowly, on average. Slower particlescollide less often and exert less force, so gas pressure decreases and theballoon contracts. This is called thermal contraction.

If you bring the balloon inside, it expands. Thermal expansion isan increase in the volume of a material due to a temperature increase.

Thermal expansion occurs when particles of matter move fartherapart as temperature increases. Gases expand more than liquids andliquids usually expand more than solids. A gas expands more easilythan a liquid or a solid because the forces of attraction among parti-cles in a gas are weaker.

Thermal expansion is used in glass thermometers. As temperatureincreases, the alcohol in the tube expands and its height increases. Theincrease in height is proportional to the increase in temperature. In anoven thermometer, a strip of brass and a strip of steel are bondedtogether and wound up in a coil. As the coil heats up, the two metalsexpand at different rates, and the coil unwinds. This causes the needleto rotate on the temperature scale.

Specific Heat When a car is heated by the sun, the temperature of the metal doorincreases more than the temperature of the plastic bumper. Do youknow why? One reason is that the iron in the door has a lower specificheat than the plastic in the bumper. Specific heat is the amount of heatneeded to raise the temperature of one gram of a material by one degreeCelsius. If equal masses of iron and plastic absorb the same heat, theiron’s temperature rises more. The lower a material’s specific heat,the more its temperature rises when a given amount of energy isabsorbed by a given mass.

Specific heat is often measured in joules per gram perdegree Celsius, or J/g•�C. Figure 3 gives specific heats fora few common materials. It takes 4.18 joules of energy toraise the temperature of 1.00 gram of water by 1.00 degreeCelsius. How much energy is needed to heat 2.00 grams ofwater to the same temperature? You would have to addtwice as much energy, or 8.36 joules.

What is thermal expansion?

Cooling Air

Procedure1. Inflate a round balloon and

then stretch its openingover the mouth of a 2-Lbottle. Use a tape measureto measure and record theballoon’s circumference.

2. Put a dozen ice cubes intoa plastic bucket. Add coldwater to the bucket to adepth of 15 cm. Submergethe bottom of the bottle inthe ice water and tape thebottle in place.

3. After 10 minutes, measureand record the circum-ference of the balloon.

Analyze and Conclude1. Observing How did

the volume of air in theballoon change?

2. Inferring Explain why theair behaved as it did.

Material (at 100 kPa) Specific Heat (J/g•�C)

Water

Plastic (polypropylene)

Air

Iron

Silver

4.18

1.84–2.09

1.01

0.449

0.235

Specific Heats of Selected Materials

Figure 3 Specific heat is the heat needed to raise the tempera-ture of 1 gram of material by 1ºC. Analyzing Data Which material in the table has the highest specific heat? The lowest?

Thermal Contraction and Expansion

Cooling Air

ObjectiveAfter completing this activity, studentswill be able to• describe the effect of temperature on

the volume of a gas.

Skills Focus Measuring, Comparingand Contrasting

Prep Time 20 minutes

Materials round balloon, 2-L plasticbottle, metric tape measure, plasticbucket, ice

Advance Prep The circumference ofthe balloon can be found by wrapping a string around the balloon and thenmeasuring the length of the string.

Class Time 20 minutes

Safety Students should wear safetygoggles and lab aprons and must wipeup any spills immediately to avoid falls.

Teaching Tips• Students can calculate the volume of

the balloon from its circumference by approximating the shape of the balloon as a sphere and using theequation C � 2�r to find the radius.The volume is then given by V � �r3.Make sure students include the bottleto determine the total volume of theenclosed air.

Expected Outcome The ballooncontracts as the air in the bottle cools.

Analyze and Conclude1. The volume decreased when cool.2. Cooling the air inside the bottle andballoon reduced the kinetic energy of itsparticles, and therefore the pressure onthe balloon, causing it to contract.Visual, Logical

Specific HeatBuild Science SkillsAnalyzing Data Have students examinethe specific heat values in Figure 3. Ask,Which substance requires nearly 1 J ofenergy to raise the temperature of 1 gby 1°C? (Air) What amount of energywould be required to raise the temper-ature of 2.00 g of water by 1.00°C?(4.18 � 2.00 � 8.36 J) Logical

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Section 16.1 (continued)

Thermal Contraction One of the fewexceptions to thermal expansion is water nearits freezing point. Over most temperatureranges, water increases in volume as itstemperature increases, but between 0°C and4°C water actually contracts as it gets warmer.This unusual behavior occurs because hydro-gen bonding between water molecules in ice

arranges them in a way that occupies a greatervolume than in liquid water. This results in ice being less dense than liquid water. In thewinter, when a pond begins to freeze, thedenser, warmer water sinks to the bottom, and the cooler, less dense ice floats. This formsa layer of warmer water at the bottom of thepond, in which fish are able to live.

Facts and Figures

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Thermal Energy and Heat 477

The heat (Q) absorbed by a material equals the product of the mass(m), the specific heat (c), and the change in temperature (ΔT).

Specific Heat Q � m � c � ΔT

In this formula, heat is in joules, mass is in grams, specific heat is inJ/g•°C, and the temperature change is in degrees Celsius.

Calculating Specific HeatAn iron skillet has a mass of 500.0 grams. The specific heat of ironis 0.449 J/g•°C. How much heat must be absorbed to raise theskillet’s temperature by 95.0°C?

Read and UnderstandWhat information are you given?

Mass of iron, m � 500.0 g

Specific heat of iron, c � 0.449 J/g•°C

Temperature change, ΔT � 95.0°C

Plan and SolveWhat unknown are you trying to calculate?

Amount of heat needed, Q � ?

What formula contains the given quantities andthe unknown?

Q � m � c � ΔT

Replace each variable with its known value.

Q � 500.0 g � 0.449 J/g•°C � 95.0°C

� 21,375 J � 21.4 kJ

Look Back and CheckIs your answer reasonable?

Round off the data to give a quick estimate.

Q � 500 g � 0.5 J/g•°C � 100°C � 25 kJ

This is close to 21.4 kJ, so the answer is reasonable.

1. How much heat is needed toraise the temperature of 100.0 gof water by 85.0°C?

2. How much heat is absorbed by a 750-g iron skillet when itstemperature rises from 25°C to 125°C?

3. In setting up an aquarium, theheater transfers 1200 kJ of heatto 75,000 g of water. What isthe increase in the water’stemperature? (Hint: Rearrangethe specific heat formula to solvefor ΔT.)

4. To release a diamond fromits setting, a jeweler heats a10.0-g silver ring by adding 23.5 J of heat. How much does the temperature of thesilver increase?

5. What mass of water will changeits temperature by 3.0°C when525 J of heat is added to it?

For: Links on specific heat

Visit: www.SciLinks.org

Web Code: ccn-2161

Build Math SkillsFormulas and Equations Studentsshould become familiar with rearrange-ments of the formula Q � m � c � �T, so that any one of these quantities can be calculated in terms of the other three.Have students rearrange the equation in order to calculate specific heat c � Q/(m � �T), mass m � Q/(c � ΔT),and temperature ΔT � Q/(m � c).Logical, Portfolio

Direct students to the Math Skills in the Skills and Reference Handbookat the end of the student text foradditional help.

Solutions1. Q � m � c � ΔT� (100.0 g)(4.18 J/g•°C)(85.0°C) � 35.5 kJ 2. Q � m � c � ΔT� (750 g)(0.449 J/g•°C)(125°C � 25°C)� (750 g)(0.449 J/g•°C)(100°C) � 34 kJ3. ΔT � Q/(m � c)� 1,200,000 J/(75,000 g � 4.18 J/g•°C)� 3.8°C 4. ΔT � Q/(m � c)� 23.5 J/(10.0 g � 0.235 J/g•°C) � 10.0°C 5. m � Q/(ΔT � c)� 525 J/(3.0°C � 4.18 J/g•°C) � 42 g Logical

For Extra HelpMake sure students start by writing out the equation required to solve eachproblem. Then, check that they are ableto solve the equation for the unknownvariable using basic algebra skills. Logical

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Thermal Energy and Heat 477

Answer to . . .

Figure 3 Silver has the lowest specific heat. Water has the highestspecific heat.

Thermal expansion is theincrease in volume of a

material due to a temperature increase.

Download a worksheet on specificheat for students to complete, andfind additional teacher supportfrom NSTA SciLinks.

Additional Problems1. Gold has a specific heat of 0.13 J/g•°C. If asample of gold with a mass of 250 g undergoes a temperature increase of 4.0°C, how much heatdoes it absorb? (130 J)2. A piece of iron at a temperature of 145.0°C coolsoff to a temperature of 45.0°C. If the iron has amass of 10.0 g and a specific heat of 0.449 J/g•°C,how much heat is given up? (449 J)Logical, Portfolio

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Section 16.1 Assessment

Reviewing Concepts1. In what direction does heat flow on its

own spontaneously?

2. How is the temperature of an objectrelated to the average kinetic energy ofits particles?

3. Name two variables that affect thethermal energy of an object.

4. What causes thermal expansion of anobject when it is heated?

5. How do the temperature increasesof different materials depend on theirspecific heats?

6. What principle explains how a calorimeteris used to measure the specific heat of asample material?

Critical Thinking 7. Applying Concepts Why is it necessary to

have regularly spaced gaps between sectionsof a concrete sidewalk?

8. Predicting An iron spoon and silver spoonhave the same mass. Which becomes hotterwhen both are left in hot tea for one minute?(Hint: Use the specific heats given in Figure 3.)

9. Calculating If it takes 80.0 joules to raise thetemperature of a material by 10.0�C, howmuch heat must be added to cause anadditional increase of 20.0�C?

10. The specific heat of copper is0.39 J/g•°C. How much heat is neededto raise the temperature of 1000.0 g ofcopper from 25.0°C to 45.0°C?

11. A peanut burned in a calorimetertransfers 18,200 joules to 100.0 g ofwater. What is the rise in the water’stemperature? (Hint: Rearrange thespecific heat formula to solve for �T.)

Measuring Heat Changes A calorimeter is an instrument used to measure changes inthermal energy. A calorimeter uses the principle that heatflows from a hotter object to a colder object until both reachthe same temperature. According to the law of conservationof energy, the thermal energy released by a test sample is equalto the thermal energy absorbed by its surroundings. Thecalorimeter is sealed to prevent thermal energy from escaping.

Figure 4 shows how a calorimeter can be used to measurethe specific heat of aluminum. A known mass of water isadded to the calorimeter. The mass of the sample of aluminumis measured. The aluminum is heated and then placed in thewater. The calorimeter is sealed. As the aluminum cools off,the water is stirred to distribute thermal energy evenly. Thewater heats up until both the aluminum and the water are at thesame temperature. The change in temperature of the water ismeasured. The thermal energy absorbed by the water is calcu-lated using the specific heat equation. Since this same amountof thermal energy was given off by the sample of aluminum,the specific heat of aluminum can be calculated.

Figure 4 A calorimeter is used to measurespecific heat. A sample to be tested is heatedand placed in the calorimeter. The lid is put onand the temperature change is observed.Hypothesizing Why does the calorimeterneed a stirrer?

ThermometerStirrer

Lid

Water

Aluminumsample

Calorimeter

Measuring HeatChanges

Calorimetry

Purpose Students observe a calorimetermeasuring changes in thermal energy.

Materials plastic foam cup with lid,thermometer, water, iron bolt (~75 g)

Procedure Place the iron bolt in afreezer for one hour prior to thedemonstration. Fill the cup two-thirds fullwith room-temperature water and recordits temperature. Place the cold iron in thewater and cover the cup. After threeminutes, record the temperature of thewater. Ask what students can infer aboutthe bolt’s initial temperature.

Safety Wipe up spills immediately.

Expected Outcome Students shouldconclude that the bolt was colder thanthe water. Visual, Group

ASSESSEvaluate UnderstandingAsk students to write a summaryparagraph relating thermal energy,temperature, and heat.

ReteachUse Figure 2 to summarize key conceptsabout thermal energy.

Solutions10. Q � m � c � ΔT � (1000.0 g) �(0.39 J/g•°C) � (45.0 � 25.0°C) � 7800 J11. ΔT � Q/(m � c)� 18,200 J/(100.0 g) (4.18 J/g•°C) � 43.5°C

If your class subscribesto the Interactive Textbook, use it toreview key concepts in in Section 16.1.

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Section 16.1 (continued)

6. A calorimeter uses the principle that heatflows from a hotter object to a colder objectuntil both reach the same temperature.7. The gaps provide space for concrete slabsto expand into so they do not buckle.8. Both spoons absorb the same energy and have the same mass. Because silver has alower specific heat (0.235 J/g•°C) than iron (0.449 J/g•°C), the silver becomes hotter.9. Doubling the temperature change doublesthe energy required, so 160 joules must be added.

Section 16.1 Assessment

1. Heat flows spontaneously from hot objectsto cold objects.2. Temperature is related to the averagekinetic energy of the particles in an object dueto their random motions through space.3. Mass of the object, temperature4. Particles of matter tend to move fartherapart as temperature increases.5. The lower a material’s specific heat, the moreits temperature increases when equal amountsof thermal energy are added to equal masses.

478 Chapter 16

Answer to . . .

Figure 4 The stirrer keeps tempera-ture uniform.