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HOT AND COLD BREATH

 

INTRODUCTION

This is a simple activity that requires no equipment or supplies other than our own bodies.  It illustrates that hands-on science activities can be utilized even without any supplies or equipment.  We use it when discussing heat and cold or energy changes.  Most people have done the activity many times in their lives, but never really thought about it.  Students in any grade can perform the activity, but the explanation will only make sense to about grades 3-6.     
 

PURPOSE

The purpose of this activity is to examine the different temperatures of air blown from our mouths under different circumstances.
 

MATERIALS

Students, teachers, or parents

 

PROCEDURE

  1. Open your mouth wide.

  2. Take a deep breath, and exhale or breathe out slowly.

  3. Repeat steps 1 and 2 with your hand in front of your mouth, and note carefully the temperature of the air.

  4. Take a deep breath, pucker up your cheeks, and blow like the wind with air coming out of your mouth through a very tiny hole in your lips.

  5. Repeat step 4 with your hand in front of your mouth, and note carefully the temperature of the air compared to that in step 3.

 

EXPLANATION

 

Teachers should expect that the air breathed slowly out of our mouths will be warm because we all know that we are "full of hot air."  Actually the air is coming from our lungs which are near our body temperature (98.6oF or 37oC).  However, the cooler air blown through a tiny hole in our lips is a surprise to many people.

 

The air is cool because of an effect called cooling on expansion.  If we can understand this process, we then know a little about how a refrigerator works in cooling our food.  All molecules are held together by attractive forces or weak energies of various strengths.  Just as it requires considerable energy or a strong tug to break apart the clasped hands of two people, so it requires a little energy to break apart molecules that are attracted to each other.  This must happen because the molecules are close together at slightly higher than atmospheric pressure inside our mouths, but they separate and spread out from each other at lower pressure once they leave the tiny hole in our mouths.  From where does this energy come? 

 

The energy comes from the only place it can, from the molecules themselves!  However, if a body loses energy, its temperature drops and it gets colder.  We experience this especially in our hands and feet on a very cold day.  Likewise, when molecules give up energy to break the forces between each other, the molecules drop in temperature, and they get colder.  Thus, the air feels colder after leaving through the tiny hole in our lips and expanding into the air around us.

 

The same principles are at work in a refrigerator.  A compressor (which makes much of the noise of a refrigerator) forces a gas (not air) to higher pressure in a small space.  The gas is then forced through a tiny hole into a much larger space.  The gas expands dramatically and cools considerably, providing the cooling action of the refrigerator.  The refrigerator is, of course, much more efficient than we could be with blowing through a tiny hole in our lips.

 

 

Kids Involved Doing Science is at http://www.kids.union.edu     Modified 01/20/03 by C. & P. Scaife