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How a Thermos Keeps Soup Hot for Hours

5th grade nonfiction passage about how a vacuum flask works: 228 words, six questions, an answer key and vocabulary.

5th grade nonfiction how a vacuum flask works 228 words

How a Thermos Keeps Soup Hot for Hours

A bowl of soup can stay hot for hours inside a lunch bag, and it almost seems like magic. The secret belongs to the vacuum flask, which many people call a thermos. It blocks heat in three different ways.

Heat can travel in three ways. It can pass through solid things, which is called conduction. It can ride along in moving air or liquid, which is called convection. It can also cross empty space as invisible rays, which is called radiation. A flask has a defense against each one.

A flask is built like a bottle inside another bottle. Between the two walls, almost all of the air has been pumped out. This empty gap is called a vacuum. With so little air in the gap, almost nothing is there to carry heat out by conduction or convection. The soup has a hard time passing its warmth to the outside.

That leaves radiation. To stop it, the inner bottle often has a shiny, mirror-like coating. Shiny surfaces bounce heat rays back instead of letting them escape, so much of the warmth is sent back toward the soup.

The stopper matters too. It closes the one opening where warm air could drift out. The same design works the other way around. A flask of cold lemonade stays cool because heat from the outside air has trouble getting in.

Questions

  1. What do many people call a vacuum flask? in the text

  2. What are the three ways that heat can travel? in the text

  3. What has been pumped out from between the two walls of a flask? in the text

  4. Why does the empty gap slow down heat? think about it

  5. What does the shiny coating do with heat rays? in the text

  6. Why would a flask of cold lemonade stay cool? think about it

Words to know

vacuum
an empty space with almost no air in it
conduction
heat moving through a solid thing
convection
heat carried along by moving air or liquid
radiation
heat that crosses space as invisible rays
coating
a thin layer spread over a surface
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One passage, three levels

The same topic and facts written three ways, so a mixed class can read and talk about one text. Each group’s sheet shows only a shape; the key says which is which.

● Easier · 193 words · Flesch-Kincaid grade 2.9

A bowl of soup can stay hot for hours in a lunch bag. The secret is a vacuum flask. Many people call it a thermos. It stops heat from getting out in three ways.

Heat can move in three ways. It can pass through solid things. This is called conduction. It can ride along in moving air or liquid. This is called convection. It can also cross empty space as rays you cannot see. This is called radiation. A flask stops all three.

A flask is like a bottle inside another bottle. Between the two walls, almost all of the air is pumped out. This empty gap is called a vacuum. With so little air in it, almost nothing can carry heat out. The soup cannot easily pass its warmth to the outside.

The inner bottle often has a shiny coating, like a mirror. Shiny things bounce heat rays back. So the warmth goes back toward the soup.

The stopper closes the one opening where warm air could drift out.

The flask works the other way too. Cold lemonade stays cool, because heat from the outside air has a hard time getting in.

▲ This passage · 228 words · Flesch-Kincaid grade 4.8

A bowl of soup can stay hot for hours inside a lunch bag, and it almost seems like magic. The secret belongs to the vacuum flask, which many people call a thermos. It blocks heat in three different ways.

Heat can travel in three ways. It can pass through solid things, which is called conduction. It can ride along in moving air or liquid, which is called convection. It can also cross empty space as invisible rays, which is called radiation. A flask has a defense against each one.

A flask is built like a bottle inside another bottle. Between the two walls, almost all of the air has been pumped out. This empty gap is called a vacuum. With so little air in the gap, almost nothing is there to carry heat out by conduction or convection. The soup has a hard time passing its warmth to the outside.

That leaves radiation. To stop it, the inner bottle often has a shiny, mirror-like coating. Shiny surfaces bounce heat rays back instead of letting them escape, so much of the warmth is sent back toward the soup.

The stopper matters too. It closes the one opening where warm air could drift out. The same design works the other way around. A flask of cold lemonade stays cool because heat from the outside air has trouble getting in.

■ Harder · 262 words · Flesch-Kincaid grade 7.2

A bowl of soup can remain steaming hot for hours inside a lunch bag, which almost seems like magic. The secret belongs to the vacuum flask, which many people call a thermos, and it works by blocking heat in three different ways.

Heat is able to travel in three distinct ways. It can pass directly through solid materials, a process known as conduction. It can be carried along by moving air or liquid, which is called convection. It can also cross empty space in the form of invisible rays, which is called radiation. A well-made flask has a defense against each of these routes.

The flask is constructed like a bottle tucked inside another bottle. Between the two walls, nearly all of the air has been pumped away, leaving an empty gap known as a vacuum. Since so little air remains in that gap, almost nothing exists there to carry heat outward by conduction or convection, and the soup struggles to pass its warmth to the outside world.

That leaves radiation. To stop it, the inner bottle frequently has a shiny, mirror-like coating. Shiny surfaces bounce heat rays back instead of letting them slip through, so a good share of the warmth is sent straight back toward the soup.

Even the stopper plays a part, because it seals the one opening where warm air could otherwise drift out. The same clever design also works in reverse. A flask filled with cold lemonade stays cool for a long while, since heat from the surrounding air has trouble finding a way in.

Download as one PDF

How to use it

  1. Press Read it big (or B) to put the text on the projector, or Full screen (F) to fill the whole screen.
  2. Ask the questions one at a time and use Show answer when the class has decided. A shows or hides every answer.
  3. Print student copy for the class, or Print with answers for yourself.

Ideas for class

  • Before reading, put vacuum, conduction, convection, radiation and coating on the board with their meanings from the Words to know section and have students predict what the text will be about.
  • Read it aloud once while students follow along, then have them reread it alone or with a partner. At a calm 100 words a minute it takes about 2 minutes to read aloud.
  • Use the Read it big button (or B) to put the text on the projector, and Show answer on one question at a time so the class commits to an answer first.
  • Question 4 asks the reader to infer. Ask students to point to the words in the passage that led them to their answer.
  • Ask students to write one question the passage did not answer, then look for the answer in a book or on a classroom screen.
  • Print the student copy for independent work and keep the answer-key copy for yourself.

Related

Questions

What reading level is "How a Thermos Keeps Soup Hot for Hours"?

It was written for 5th grade. It has 228 words and works out to a Flesch-Kincaid grade level of about 4.8. That is a formula's estimate from sentence length and syllables, and it is not a Lexile or a guided reading level, so check it against your students.

What do the questions ask?

There are six. 4 ask for something the text says directly and 2 ask the reader to work something out from clues. Each has an answer in the key, and the answers are written as a teacher would accept them, not as the only possible wording.

How long does it take to read?

About 2 minutes read aloud at 100 words a minute. For a one-minute fluency check, paste it into the fluency timer and use the word count.

Can I print it?

Yes. Print student copy gives the passage, the questions with lines to write on, and the word meanings. Print with answers gives the same page with the answers filled in. Both are made in your browser.

Is this passage from a published book?

No. It was written for Peabody Teach, so there is no book to buy and no login to read it.

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