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Sublimation as Physical Change — Reading Comprehension

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This educational passage for grades 6-8 explores the science of sublimation—a physical change where a solid transforms directly into a gas without becoming liquid first. Students will learn the mechanisms and conditions that favor sublimation, supported by real-world examples such as dry ice fog, freeze-dried foods, disappearing snow on cold dry days, and shrinking mothballs. The passage aligns with NGSS standard MS-PS1-4, encouraging scientific thinking by explaining observable phenomena and underlying cause-and-effect relationships. Activities include a glossary, multiple-choice and writing questions, and graphic organizers to deepen understanding. Audio integration and Spanish translations are provided for accessibility, making this a comprehensive resource for diverse learners.

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Sublimation-1 — Illustration showing the basic concept of sublimation

Sublimation is a remarkable physical change in which a substance moves directly from the solid state to the gas state, skipping the liquid phase entirely. This process is important in both natural and technological settings, affecting how materials behave and how we use them. Scientists study sublimation to better understand the properties of matter and the effects of temperature and pressure on physical changes. By observing phenomena like fog from dry ice, researchers have revealed the underlying mechanisms that drive these changes.

How Sublimation Works
Sublimation occurs when the molecules in a solid gain enough energy to overcome their fixed positions and change directly into a gas. This usually happens under conditions of low pressure and low humidity, or when the solid is heated rapidly. The key factor is that the substance’s vapor pressure (the pressure of its molecules escaping into the air) becomes higher than the surrounding atmospheric pressure at a specific temperature. For example, dry ice (solid carbon dioxide) sublimates at -78.5°C, forming thick fog as it turns straight into CO₂ gas. Another example is freeze-drying, where food is frozen and exposed to low pressure so water sublimates out, preserving texture and nutrients. Snow can also disappear on cold, dry days due to sublimation, even when temperatures are below freezing.

Applications and Interactions
Sublimation has many practical uses and interacts with other scientific processes. In nature, it is part of the water cycle in cold, windy regions, causing snow and ice to shrink without melting. In technology, freeze-drying is used to make lightweight foods for astronauts and hikers. Mothballs made of naphthalene or paradichlorobenzene shrink over time as they sublimate, releasing a gas that repels insects. These examples illustrate how energy, temperature, and pressure interact to control the state of matter. Quantitative data shows that the rate of sublimation increases as temperature rises or humidity drops, and can be measured in grams per hour or as percent mass lost over time.

Complexities and Broader Principles
While sublimation is common for substances like dry ice and naphthalene, not all solids can sublimate easily. The process depends on the unique properties of each material. Scientists use knowledge of phase changes—including melting, freezing, and evaporation—to predict and control sublimation in different environments. Understanding sublimation helps us manage resources, design preservation techniques, and explain everyday mysteries like disappearing snow. It connects to the broader scientific principle that matter changes state due to energy transfers and interactions at the particle level.

In summary, sublimation is a direct solid-to-gas transition that reveals the complex relationships between pressure, temperature, and molecular energy. Its study highlights the dynamic nature of matter and the importance of physical changes in science and technology.

Interesting Fact: The word "sublimation" comes from the Latin "sublimare," meaning "to raise to a higher place." This reflects how molecules move upward from solid to gas during the process.

What is sublimation?

A solid turning directly into a gas, skipping the liquid phase.A liquid turning directly into a solid.A gas condensing into a liquid.A solid melting into a liquid.

Which of the following is an example of sublimation?

Dry ice forming fog.Water boiling.Ice melting in the sun.Condensation on a cold window.

What does 'vapor pressure' mean in the context of sublimation?

The pressure of molecules escaping from a solid or liquid into the air.The amount of water vapor in the air.The pressure inside a balloon.The temperature at which a solid melts.

In freeze-drying, what happens to the water in food?

It sublimates and leaves the food as gas.It melts and makes the food wet.It condenses and adds moisture.It stays frozen forever.

Which conditions make sublimation most likely to occur?

Low pressure and low humidity.High pressure and high humidity.Warm temperatures and wet air.Cold temperatures and high pressure.

Why do mothballs shrink over time?

They sublimate, releasing gas that repels insects.They dissolve in air.They melt into liquid.Insects eat them up.

Which is NOT a result of sublimation?

Water boiling on a stove.Dry ice fog at a concert.Snow disappearing on a dry, cold day.Mothballs getting smaller in a closet.

How does energy play a role in sublimation?

It allows molecules in a solid to break free and become a gas.It keeps molecules in place.It cools the solid down.It turns gas into a liquid.

True or False: Sublimation is a chemical change.

TrueFalse