Temperature and Heat

If a weather forecaster predicts temperatures between 30 C and 35 C, you know you can expect ao o

hot day. Many people - perhaps even you - think that temperature and heat are the same thing. Butthey are not. Temperature and heat are related, but they are not the same. In order to understand thedifference between temperature and heat, you will need to look more closely at how energy and themotion of molecules are related.

Kinetic EnergyCount Rumford observed that heat was produced when a hole was drilled in a cannon barrel. JamesPrescott joule observed that objects in motion produce heat. In both cases, work is being done. Whatdo you think of when you hear the word work? You may think of doing chores, such as washingdishes or raking leaves. Or perhaps going to work in an office comes to mind. But when scientistsspeak of work, they are referring to a force (a push or a pull) acting on an object and causing it tomove. A moving hammer can do work by hitting a nail and driving it into a piece of wood. Movingobjects can do work because they have energy. Energy of motion is called kinetic (kih-NEHT-ihk)energy. The faster an object moves, the more kinetic energy it has. So a fast - moving hammer cando more work than a slow moving one. You can test this by hammering a nail into a piece of wood.The faster you swing the hammer, the farther the nail is driven into the wood.Like all moving objects, molecules have kinetic energy because of their motion. Temperature is ameasure of the average kinetic energy of molecules. Adding heat to a substance increases theaverage kinetic energy of the molecules and causes a rise in temperature. Thus temperature is ameasure of how hot or how cold something is. The higher the temperature of a substance, the fasterthe molecules in that substance are moving, on the average. Likewise, alower temperature indicates that the molecules are moving more slowly. Inwhich pot of water would most of the water molecules be moving faster - apot at 90 C or one at 70 C? o o

Unlike temperature, heat depends on the mass of the substance present.For instance, 10 grams of water at 90 C have more heat energy than 5o

grams of water at the same temperature. This means that if you were tospill hot water on your hand by accident, 10 grams of water at 90 C wouldo

produce a more severe burn than 5 grams of water at 90 C!o

Measuring TemperatureYou would not want to put your hand into a pot of boiling water to find outhow hot the water is! And you might not always agree with someone else onhow hot or how cold something is. So you need a safe and accurate way ofmeasuring temperature. A thermometer is an instrument for measuringtemperature. Most common thermometers consist of a thin tube filled with aliquid, usually alcohol or mercury. Remember that as a liquid is heated, itsmolecules move faster and farther apart. So as the liquid in a thermometergets warmer, it expands and rises in the tube. The opposite happens as theliquid gets cooler. The molecules move slower and closer together. Theliquid contracts and drops in the tube.Along the tube of a thermometer is a set of numbers, called a scale, thatallows you to read the temperature. The Celsius scale is used to measuretemperature in the metric system. The unit of temperature on the Celsiusscale is the degree Celsius ( C). Water freezes at O C and boils at 100 C.o o o

Another metric temperature scale often used by scientists is the Kelvinscale. On this scale, temperature is measured in units called kelvins (K).You can convert Celsius degrees to kelvins simply by adding 273 to the

Figure 1 - A comparison of the

Celisus and Kelvin temperature

scales is shown here. Notice that

absolute zero is -273 C.o

Celsius temperature. For example, if a thermometer reads 10 C, the same temperature on the Kelvino

scale would be 273 + 10 = 283 IL A temperature of -5 C equals 268 K [273 + (-5)]. At whato o

temperature does water freeze on the Kelvin scale? At what Kelvin temperature does water boil? The main reason the Kelvin scale is useful to scientists is that the lowest reading on this scale, 0 K iso

the lowest temperature that can be reached. This temperature is often called absolute zero. Scientistshave now been able to reach a temperature only one millionth of a degree Celsius above absolutezero.You may not have guessed that there is a lowest possible temperature. Recall that temperature is ameasure of the energy of motion of molecules. What do you think happens at absolute zero?

Measuring HeatYou know that when you cook soup or boil water, heat energy must be added to the liquid in order toraise its temperature. Heat energy is needed to set molecules in motion. Temperature is a measure ofthis molecular motion.Heat cannot be measured directly. But changes in temperature - which can be measured directly -provide a way to measure heat indirectly. An increase in temperature indicates that heat is beingadded. A decrease in temperature indicates that heat is being removed.Heat is measured in units called calories. One calorie (cal) is defined as the amount of heat neededto raise the temperature of 1 gram of water 1 degree Celsius. For example, to raise the temperatureof 1 gram of water from 4 C to 5 C or from 20 C to 21 C 1 calorie of heat is needed. Another unit thato o o o

can be used to measure heat is the joule (J) named after James Prescott joule. One calorie is equalto 4.19 joules (1 cal = 4.19 J).Notice that the amount of heat needed for a given temperature change depends on the mass of thewater being heated. For example, 10 calories of heat will raise the temperature of 1 gram of water10 C If you had 10 grams of water instead of 1 gram, the same 10 calories would raise theo

temperature of the water only 1 C How many calories would beo

needed to raise the temperature of 10 grams of water 10 C?o

Specific Heat CapacityMass is not the only factor that determines temperature change. Thesame amount of heat will produce a different temperature change indifferent substances even if their masses are the same. That isbecause some substances absorb heat energy more readily than othersubstances.The ability of a substance to absorb heat energy is called its specificheat. The specific heat of a substance is the number of caloriesneeded to raise the temperature of I gram of that substance I degreeCelsius. The specific heat of water is 1 calorie per gram per degreeCelsius (1.00 cal/g• C. This is high compared with the specific heats ofo

most other substances.The high specific heat of water explains why the climate near an oceanor a large lake is usually mild. Water tends to heat up slowly, but it alsoloses heat slowly. This slow heating and cooling tends to keep theclimate near a large body of water relatively uniform.Figure 2 lists the specific heat values of some other commonsubstances. Specific heat is an important property because it can beused to help decide which substance should be used for a specificpurpose. For example, you can see by looking at Figure 2 that thespecific heat of aluminum is almost twice that of iron. That means thataluminum pots and pans hold about twice as much heat as pots andpans of the same mass made of iron.

Figure 2 - According to this table, which

would heat up faster; aluminum or

mercury?

Calculating Heat EnergySpecific heat can be used to calculate the amount of heat gained or lost by a substance. The heatgained or lost by a substance is equal to the product of its mass times the change in temperature

(ªT) times its specific heat. (The symbol ª is the Greek letter delta and when used in a math formula

means change; ªT means change in temperature.)

Heat gained or lost = Mass X ªT x Specific heat

Within a closed container, the heat lost by one substance must equalthe heat gained by another substance. A device that makes use of thisprinciple is called a calorimeter (kal-uh-RIHm-uht-er). A calorimeter isused to measure the heat given off in chemical reactions.

Figure 3 shows how a calorimeter is constructed. An insulated outercontainer surrounds an inner container filled with water. Inside theinner container is a chamber in which a chemical reaction takes place.Because the heat given off by the chemical reaction equals the heatgained by the water, the heat of the chemical reaction can becalculated. The temperature change, mass, and specific heat of thewater must be known in order to make the calculation. For example,suppose the surrounding water has a mass of 300 grams. If thetemperature of the water increases 5 C the heat given off by theo

chemical reaction is equal to 300g X 5 C X 1 cal/g • C = 1500 calories.o o

How much heat would be given off by a chemical reaction that raisedthe temperature of 150 grams of water 10 C?o

Potential EnergyWhen does heat energy not cause a change in the temperature of a substance? The answer to thisquestion is quite simple: when the heat energy is stored. Stored energy - in the form of heat or anyother kind of energy - is called potential (poh-TEHNshuhl) energy. Potential heat energy is present inchemical substances such as gasoline and other fuels. The stored heat energy is released when thefuels are burned, for example, in a car engine.

Figure 3 - The calorimeter is a device

used to measure the heat given off

durring a chemical reaction.

Foods also contain potential heat energy. The energy stored in foods can be measured in caloriesbecause when foods are "burned," they release heat energy. ("Burning" food in your body involvesthe process of respiration, in which food that is broken down into sugar is combined with oxygen torelease energy.) When sugars are burned in your body, heat energy needed to keep your bodyfunctioning is produced. The amount of heat a food gives off is indicated by the number of calories itcontains. There is one big difference, however. "Food calories" are really kilocalories (kcal). And 1kilocalorie is equal to 1000 calories. Food calories are usually written with a capital C to differentiatethem from calories with a small c. So the next time you are on a diet, you can tell your friends that youare watching your kilocalories!

Questions:

1. What is temperature? What is the difference between temperature and heat?

2. How does a thermometer measure temperature?

3. What is the most common metric scale used to measure temperature? What temperature

scale is most often used by scientists?

4. How would you convert a temperature in kelvins to degrees Celsius?

5. Do you ever think a temperature of absolute zero will ever be reached? Why or why not?

6. How can heat be measured? What units are used to measure heat?

7. What is specific heat? Why is it important?

8. What is a calorimeter? How does it work?

9. Which would require more heat energy – raising the temperature of 100 grams of water

from 40 C to 100 C or raising the temperature of 1000 grams of water from 80 to 90 C?o o o o

Show your calculations.