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In the last lesson we talked about objects being able to absorb heat and not increase in temperature. When objects reach their “speed limits” (their phase change points) the temperature no longer increases or decreases until the object completely changes phase. In this lesson, we are going to talk about another way objects can be heated without changing temperature.

Heat Capacity

Think of a dry sponge. Now imagine putting that sponge under a slowly running faucet. The sponge would continue to fill with water until it reached a certain point and then water started to drip from it. You could say that the sponge had a water capacity. It could hold so much water before it couldn’t hold any more and the water started dripping out. Heat capacity is similar. Heat capacity is how much heat an object can absorb before it increases in temperature. This is also referred to as specific heat. Specific heat is how much heat energy a mass of a material must absorb before it increases 1C. Each material has its own specific heat. The higher a material’s specific heat, the more heat it must absorb before it increases in temperature. Water is unique in that it has a very large specific heat. Liquid water’s specific heat is over 4 which is very high. In comparison, granite is .8, aluminum is .9, rubbing alcohol is 2.4 and gold is .1. To get the same amount of rubbing alcohol and liquid water to increase the same amount of temperature, you would need to pump about twice the amount of heat into the water. To get the same amount of gold and liquid water to increase the same amount of temperature, you would need to pump 40 times the amount of heat into the water! In other words, it takes more energy to heat water then it does to heat alcohol, gold, or for that matter most other things.

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Experiment 1

Matchless Balloons

 

Caution: This experiment uses flame! Be sure to have adult supervision and please be careful.

You Need:Nonpoppingballoon

Balloon

Water

Matches

Sink

1. Put the balloon under the faucet and fill the balloon with some water.

2. Now blow up the balloon and tie it, leaving the water in the balloon. You should have an inflated balloon with a tablespoon or two of water at the bottom of it.

3. Carefully light the match or the lighter and hold it under the part of the balloon where there is water.

4. Feel free to hold it there for a couple of seconds. You might want to do this over a sink or outside just in case!

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So why didn’t the balloon pop? The water absorbed the heat! The water actually absorbed the heat coming from the match so that the rubber of the balloon couldn’t heat up enough to melt and pop the balloon. Water is very good at absorbing heat without increasing in temperature which is why it is used in car radiators and nuclear power plants. Whenever someone wants to keep something from getting too hot, they will often use water to absorb the heat.

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Experiment 2

The Race to Chill

 

Caution: You will be using a microwave and heating items to high temperatures. Be sure to have adult supervision and be careful.

You need:

A variety of materials such as a slice of bread, water, marshmallow, corn syrup, butter.

Glasses, the smaller the better

A thermometer that can reach temperatures over 212F or 100C

Microwave

Timer

Paper and pencil

 

1. Take your slice of bread and smash it into a ball.

2. Put it on a plate and place it into the microwave for 30 seconds.

3. Carefully take the bread out and stick your thermometer into the ball of bread.

4. Continue to take the bread’s temperature every 30 seconds as it cools to room temperature. Record the temperatures.

5. Repeat this for the marshmallow, water or whatever else you feel comfortable heating up in a microwave. Please be careful!

6. Feel free to create and compare line graphs for each material.

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Did you notice that some things cooled much faster then other things? That’s heat capacity. The bread probably cooled quite quickly compared to the water. Bread has a lower heat capacity then water. Have you ever grabbed a piece of pizza, taken a bite out of it and burned your mouth? Blame heat capacity. The crust of the pizza has a low heat capacity, so it loses heat and cools quickly. The cheese, however, has a higher heat capacity. It loses heat much slower then the crust and as such, it stays hotter much longer. So even though the crust is cool to the touch, that cheese can be quite hot! Metal has low heat capacity. It gains temperature quickly and loses it quickly. This is why you can touch your bread pan long before you can touch your freshly baked bread.

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Experiment 3

The Slow Chill

 

Caution: You will be using a stove and heating water to high temperatures. Be sure to have adult supervision and be careful.

You need:

2 coffee mugs

One or two thermometers that can reach temperatures over 212F or 100C

Water

Pot

Stove

Measuring cups

Timer

1. Fill the pot with about 2 cups of water.

2. Turn on the stove and heat the water until it reaches 150F (66C).

3. Carefully pour about a cup of water into one mug and about a half a cup of water into the second mug.

4. Take the temperature of the water in both mugs every 30 seconds.

5. If you wish, you can create a nice line graph of your data.

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Do you remember inertia? The more mass something has the more force is needed to get it to move, stop moving or change direction. Well, you can think about heat capacity kind of like inertia. In fact, sometimes the term thermal inertia is used to describe how quickly or slowly something will change temperature. In your experiment, which mug of water changed temperature faster? The mug with less water in it right? The smaller amount of water lost heat much faster then the larger amount and so its temperature dropped much quicker. The more of a substance you have, the harder it is to get it to change temperatures. A gallon of water will increase in temperature much slower then a teaspoon of water. A large pizza will take longer to warm up then a slice of pizza.

Remember that heat is a type of energy. Thermal energy to be precise. As an object gains thermal energy the molecules inside the object may vibrate faster depending on the heat capacity of the object. When the molecules move faster, you’ll see the temperature of that object increase. Heat capacity is how much thermal energy an object can absorb before its molecules vibrate faster and cause an increase in temperature. Water has a high heat capacity and so it absorbs a lot of heat before it increases in temperature. Aluminum has a low heat capacity and so its temperature increases with only small amounts of heat.

In A Nutshell

- Heat capacity is how much heat an object can absorb before its temperature increases.

-Specific heat is how much heat energy a mass of a material must absorb before it increases 1C.

-Heat capacity is influenced by the specific heat of the material and/or the amount of the material.

-Each material has its own specific heat. The higher a material’s specific heat is, the more heat it must absorb before its temperature increases.

-A larger amount of something will have a higher heat capacity then a smaller amount of something.

-Water has a very high heat capacity.

Did You Get It?

1. What is heat capacity?

2. Which would cool down faster, a bottle of maple syrup or a teaspoon of maple syrup?

3. Owww!! I just burned my mouth on a piece of pizza! The strange thing is the crust is just warm. What happened?

4. When I eat at a fast food restaurant I always eat my fries before the burger since the fries get cold so much faster. Which has a higher heat capacity, the fries or the burger?

5. Why do I fill a hot water bottle with hot water and not just hot air?

 

Answers

 

1. Heat capacity is how much heat an object can absorb before its temperature increases.

2. The teaspoon. The smaller the amount the less heat capacity it has.

3. The cheese has a much higher heat capacity then the crust. So the cheese stays hot much longer.

4. The burger holds onto it’s heat longer then the fries. The burger has a higher heat capacity.

5. Water has a higher heat capacity so it cools much more slowly then air. A hot air bottle will be cool in a

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