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Jun. 11, 2010

Sublime Sublimation

by Austen Saltz

Note: This lesson provides a great introduction to another Teachers TalkingScience lesson, Capturing Carbon Dioxide.
Looking for ways to jazz up your party? Patrick Buckley, co-author of The Hungry Scientist Handbook, demonstrates how to make carbonated fruit. Materials required: fruit (the firmer the better), a pressure cooker and a handful of dry ice cubes.
 
Grade Level: 6th – 8th grade
Subject Matter: Physical Science
National Standards: NS.5-8.1, NS.5-8.2
 
Overview
There are three generally known states of matter: solid, liquid, and gas. Each of these states also is known as a phase. Elements and compounds can move from one phase to another phase. A solid can become a liquid (one example: ice melting into water), a liquid can became a gas (water evaporating into steam) and a gas can become a liquid (water vapor in steam condensing into water droplets). Sublimation is a phase transition that occurs when a solid changes into a gas -- or, as chemists say, when a solid sublimes.
 
In this activity, students will explore sublimation by conducting experiments with dry ice. Students will observe how dry ice changes phases, and the physical changes that occur when they mix or add other substances.
 
Activity Materials
  • Newspapers, to protect countertop
  • Ice cubes
  • Dry ice (find local distributors at www.dryicedirectory.com)
  • Glass measuring cups or beakers, one for each student
  • Warm water
  • Food coloring
  • Liquid soap
Note: Before purchasing or handling dry ice, review and follow dry ice safety guidelines.
 
Vocabulary
  • Evaporation: process of changing from a liquid to a gas or vapor.
  • Condensation: process of changing from a gas or vapor to a liquid.
  • Sublimation: process of changing from a solid to a gas or vapor.
  • Carbon dioxide: a heavy colorless, odorless atmospheric gas.
What To Do
1. Begin the lesson by having students watch the Science Friday Web video, “Hungry Scientist Makes Fizzy Fruit.” Ask students what Patrick Buckley used to make the fruit fizzy. Tell students that they will observe a few dry ice experiments to learn more about its properties. Review dry ice safety information, and inform students that you will be the only person touching and distributing the dry ice for each experiment.
Activity 1
1. Ask students to explain what dry ice is, what it is composed of, and what its properties are. Have students predict what will happen when a piece of dry ice is placed on the counter.
2. Have students spread newspaper on the countertop to protect the table. Put on a pair of leather gloves and place on the countertop in front of each student one piece of dry ice and one regular ice cube. Have students compare and contrast the differences between dry ice and regular ice.
3. Have students observe the direction of the gas sublimating from the dry ice. Why does the gas drift down towards the counter instead of up? In what direction does steam from a boiling pot of water drift?
Activity 2
1. Have students fill a glass measuring cup halfway with warm water. What do they think will happen if a piece of dry ice is placed in the warm water? Have them explain their predictions.
2. With leather gloves on, place a piece of dry ice in each glass. Have students observe the size of the ice. Is it shrinking? What is the cause of this physical change?
3. Have students describe and try to explain the other physical reactions that are happening. Why is the water bubbling? Have student observe the rate of bubbling for a few minutes. Why does the rate of bubbling change from fast to slow?
Activity 3
1. Have each student fill another glass measuring cup halfway with warm water, and add a few pumps of liquid soap and a few drops of food coloring. Ask students to predict what will happen when the dry ice is placed in the soapy water. Compare and contrast their predictions.
2. With leather gloves on, place a piece of dry ice in each student’s soapy water. Have students observe and describe what happens in the glass. (A mass of bubbles will form.) Have students observe the direction of the bubbles. Why does the bubble mass spill over in a downward direction instead of floating away?
3. Instruct the students to pop some of the bubbles from the top of the bubble mass. Warn students that they should not put their fingers inside the glass container with the dry ice in it. What do they observe each time a bubble is popped?
 
What's Happening?
The regular ice that we use to keep our drinks cold is made out of frozen water. Dry ice, on the other hand, is frozen carbon dioxide, a type of atmospheric gas that is produced during respiration. Unlike ice made from frozen water, dry ice does not melt; it sublimates. Sublimation is the process of going directly from a solid to a gas. Dry ice bypasses changing into a liquid state before converting to gas. That is why it is called ”dry“ ice. As gas sublimes from dry ice, it will drift in a downward direction. This is because the carbon dioxide is heavier than the air, causing the gas to drift downward.
 
Placing dry ice in a container of warm water will cause the formation of lots of bubbles. The bubbles are created when heat from the water enters the dry ice, causing it to sublime -- change to its gaseous form. The water may appear to be boiling, but these bubbles are actually carbon dioxide gas being released into the air. Initially, the bubbling happens at a rapid rate because the warm water is heating the dry ice. As the water begins to cool and the temperature difference between the dry ice and the water becomes smaller, the rate of bubbling begins to slow down. Eventually, water ice will form a covering on the dry ice and then crack or pop, as the pressure of carbon dioxide gas further builds up inside.
 
Adding dry ice to soapy water will cause the carbon dioxide to become “trapped” in the soap that is dissolved in the water. Because carbon dioxide is heavier than air, a mass of carbon dioxide gas inside soap bubbles will form and spill over the container instead of floating away. Popping the soap bubbles will release the carbon dioxide gas into the air.
 
Topics for Science Class Discussion
  • Describe and give examples of different ways that elements change from one state of matter to another.
  • What would happen if the pieces of dry ice were bigger or smaller? How does surface area affect the experiment?
  • Why does the gas appear white if carbon dioxide gas is colorless?
  • What are other ways that carbon dioxide is used? What else produces it besides respiration?
  • What else have you heard about carbon dioxide? (It is a greenhouse gas that plays a role in climate change.)
Extended Activities and Links
  • Using a thermometer and a beaker of water at room temperature, measure and record the temperature of water before and after adding dry ice. Students must use leather gloves or tongs to handle the dry ice. Measure the temperature of the water every minute after the dry ice is added, and complete a graph with the y-axis as the temperature and the x-axis as the time. Have students explain their results.
  • Challenge the students to blow up a balloon using only a balloon, a plastic soda bottle and dry ice. Make sure students have leather gloves or tongs to handle the dry ice.
  • Assign students a research project to find out how dry ice is manufactured and shipped. Have students explain the reasons for each part of the process.
  • Have students research and choose their favorite dry ice experiment video online. View each video in class and have students explain the science behind the experiment in their video.
  • Try these amazing dry ice experiments at home or in the classroom
About Austen Saltz

The views expressed are those of the author and are not necessarily those of Science Friday.

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