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Why Salt Melts Ice

7th grade nonfiction passage about freezing point: 260 words, six questions, an answer key and vocabulary.

7th grade nonfiction freezing point 260 words

Why Salt Melts Ice

After a winter storm, road crews spread salt on icy streets, and before long the ice begins to melt. This seems strange, because salt is not warm. The explanation lies in how water freezes.

Pure water freezes at zero degrees Celsius, or thirty-two degrees Fahrenheit. At that temperature, the water molecules slow down enough to lock into the regular pattern of a crystal. Even in solid ice, however, a very thin film of liquid water usually covers the surface, and molecules are constantly leaving the crystal and rejoining it.

When salt touches the ice, it dissolves in that thin film and makes salty water, called brine. The dissolved salt gets in the way. Water molecules that try to rejoin the crystal keep bumping into salt particles, so fewer of them can lock into place. Meanwhile, molecules on the ice continue to break away. The balance tips toward melting, and the freezing point of the mixture drops below zero. Ice that was perfectly happy at thirty degrees Fahrenheit finds itself too warm to stay solid.

The melting itself takes heat from the surroundings, which is why a mix of ice and salt feels very cold. People use this trick in the kitchen. When making homemade ice cream, they pack ice and salt around the container, and the chilly brine freezes the cream.

Salt has limits. Common road salt works well only down to about fifteen degrees Fahrenheit. On the coldest days, crews often spread sand instead, which does not melt the ice but gives tires and shoes something to grip.

Questions

  1. At what temperature does pure water freeze? in the text

  2. What is brine? in the text

  3. What happens to the freezing point of water when salt dissolves in it? in the text

  4. Why does a mix of ice and salt feel very cold? in the text

  5. How does salt melt ice if salt is not warm? think about it

  6. Why do road crews sometimes spread sand instead of salt on very cold days? think about it

Words to know

molecules
the tiny particles that make up water and other matter
crystal
a solid whose particles are arranged in a regular pattern
dissolves
mixes completely into a liquid and seems to disappear
brine
water that has a lot of salt in it
freezing point
the temperature at which a liquid turns solid
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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 · 247 words · Flesch-Kincaid grade 4.6

After a winter storm, road crews spread salt on icy streets. Before long, the ice starts to melt. This seems odd, since salt is not warm. The answer is in how water freezes.

Pure water freezes at zero degrees Celsius, or thirty-two degrees Fahrenheit. At that temperature, water molecules slow down and lock into the neat pattern of a crystal. Even in solid ice, a very thin film of liquid water usually covers the top. Molecules are always leaving the crystal and joining it again.

When salt touches the ice, it dissolves in that thin film. This makes salty water, called brine. The salt gets in the way. Water molecules that try to rejoin the crystal keep bumping into salt. So fewer of them can lock into place. Meanwhile, molecules keep breaking away from the ice. The balance tips toward melting. The freezing point of the mix drops below zero. Ice that was fine at thirty degrees Fahrenheit is now too warm to stay solid.

The melting takes heat from the surroundings. That is why a mix of ice and salt feels very cold. People use this trick in the kitchen. To make homemade ice cream, they pack ice and salt around the container. The cold brine freezes the cream.

Salt has limits. Road salt works well only down to about fifteen degrees Fahrenheit. On the coldest days, crews often spread sand. Sand does not melt ice, but it gives tires and shoes something to grip.

▲ This passage · 260 words · Flesch-Kincaid grade 7.5

After a winter storm, road crews spread salt on icy streets, and before long the ice begins to melt. This seems strange, because salt is not warm. The explanation lies in how water freezes.

Pure water freezes at zero degrees Celsius, or thirty-two degrees Fahrenheit. At that temperature, the water molecules slow down enough to lock into the regular pattern of a crystal. Even in solid ice, however, a very thin film of liquid water usually covers the surface, and molecules are constantly leaving the crystal and rejoining it.

When salt touches the ice, it dissolves in that thin film and makes salty water, called brine. The dissolved salt gets in the way. Water molecules that try to rejoin the crystal keep bumping into salt particles, so fewer of them can lock into place. Meanwhile, molecules on the ice continue to break away. The balance tips toward melting, and the freezing point of the mixture drops below zero. Ice that was perfectly happy at thirty degrees Fahrenheit finds itself too warm to stay solid.

The melting itself takes heat from the surroundings, which is why a mix of ice and salt feels very cold. People use this trick in the kitchen. When making homemade ice cream, they pack ice and salt around the container, and the chilly brine freezes the cream.

Salt has limits. Common road salt works well only down to about fifteen degrees Fahrenheit. On the coldest days, crews often spread sand instead, which does not melt the ice but gives tires and shoes something to grip.

■ Harder · 270 words · Flesch-Kincaid grade 10

Following a winter storm, road crews scatter salt across icy streets, and before long the ice begins to melt. This appears puzzling, because salt is certainly not warm, but the explanation lies in the way that water freezes.

Pure water freezes at zero degrees Celsius, or thirty-two degrees Fahrenheit. At that temperature, the water molecules slow down sufficiently to lock into the orderly pattern of a crystal. Even in solid ice, however, an extremely thin film of liquid water generally coats the surface, and molecules are constantly escaping from the crystal and rejoining it.

When salt touches the ice, it dissolves in that thin film and forms salty water, called brine. The dissolved salt interferes with the process. Water molecules attempting to rejoin the crystal repeatedly collide with salt particles, so fewer of them manage to lock into place, while molecules on the ice continue to break away. The balance shifts decisively toward melting, and the freezing point of the mixture falls below zero. Ice that was perfectly content at thirty degrees Fahrenheit suddenly finds itself too warm to remain solid.

The melting itself absorbs heat from the surroundings, which explains why a mixture of ice and salt feels bitterly cold. People exploit this effect in the kitchen. When making homemade ice cream, they pack ice and salt around the container, and the frigid brine freezes the cream.

Salt does have limitations. Common road salt works effectively only down to about fifteen degrees Fahrenheit. On the very coldest days, crews frequently spread sand instead, which does not melt the ice but does give tires and shoes something to grip.

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 molecules, crystal, dissolves, brine and freezing point 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 3 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 5 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 "Why Salt Melts Ice"?

It was written for 7th grade. It has 260 words and works out to a Flesch-Kincaid grade level of about 7.5. 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 3 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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