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Wednesday, February 22, 2012

Science Fun with Dry Ice


In this simple investigation, you'll learn some of the science behind a very cool substance known simply as dry ice. In the process, you'll learn a little chemistry and some great science lingo.  Can you say...sublimation?

Here's What You Need:
  • Large plastic bowl, plastic cup, or graduated cylinder
  • Warm water
  • Dry Ice (you can usually find it at the grocery store or some ice cream shops ~$1/lb)
  • Liquid soap
  • Heavy duty gloves or tongs (for handling the dry ice)
  • Hammer or mallet (for breaking the dry ice)
  • Ice chest or cooler (for storing dry ice)
Safety & Dry Ice: Dry ice must be handled carefully and with adult supervision.  Because it is extremely cold (-109.3°F or -78.5°C), it can easily cause server damage to your skin if it is handled in incorrectly.  Avoid touching dry ice with unprotected skin. Never swallow dry ice or place it in your mouth.

Here's What You Do
Fill the bowl, cup, or graduated cylinder with warm water (a bit more than half way).  Use the mallet or hammer to break the dry ice into pieces that will fit easily into the container.  Using the tongs or gloves, carefully place a few pieces of dry ice into the warm water. Immediately, the water should start to churn and bubble!  You may also notice a smoky cloud forming on the top of the container and eventually spilling over the side. This cloud is safe to touch and feel.  Just remember to avoid touching the dry ice directly. After a while, the bubbling and smoking will slow down a bit.  Simply, poor out a bit of the water (cooled by the dry ice) and replace it with some warm water. 

Now, let's ramp things up a bit.  While the water is churning and bubbling, add a few drops of liquid soap and watch what happens! After a few seconds, you should see some fog filled bubbles rising out of the water.  Once again, these are perfectly safe to play with.  Grab a handful of the bubbles and squeeze them in your hand.  Notice anything? They are full of the smoky clouds that you observed when you dropped the dry ice in the warm water. 

The Science Behind Dry Ice
Dry is actually the solid form of carbon dioxide. Usually, when we talk about carbon dioxide, we talk about it as a gas; the one that we exhale during respiration and plant use for photosynthesis.  About 0.035% of the air around us is made up of carbon dioxide (most of our air is made up of nitrogen - 79%). Dry ice acts very differently than water based ice.  For example, under normal atmospheric conditions dry ice does not melt (that's why we call it dry ice).  Instead solid dry ice turns directly into carbon dioxide gas through a process called sublimation.

Try this. Grab two plates. Place a piece of dry ice on one plate and a piece of water based ice on the other plate.  Now, wait.  After an hour, you'll notice that the dry ice seems to have disappeared and there is a puddle of water where the water based ice was.  However, the dry ice didn't disappear.  Instead, it changed into a gas (one that is invisible to the naked eye).  

We make dry ice by placing gaseous carbon dioxide under intense pressure (about 870 pounds per square inch) at a very low temperature.  This will turn carbon dioxide gas into liquid carbon dioxide. The liquid carbon dioxide is then pushed through an expansion valve and into a pressure chamber.  The change in pressure causes the temperature to drop and the liquid carbon dioxide changes quickly to a solid...dry ice!   

Wondershop Fast Fact
Dry ice is great for keeping things cool.  In fact, one pound of dry ice cools three times better than a pound of water based ice.  Often people will use a mixture of dry ice and water based ice to keep things cool while they are shipped.


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