Condensation and Frost Podcast

Hi, this is Mr. Sickle coming to you for the first time via podcast.  The wonderful world of the Internet will allow you to review some key topics discussed in class, and help you not only review for a quiz or a test, but to better understand basic, and more advanced, scientific principles. 

The first episode will feature one of my favorite topics in Science – WATER.  Specifically, we will focus on condensation and frost.

Why does a bottle of water “sweat” in warm weather?  You’ve probably all experienced this on a hot summer day, when there is moisture on the outside of the bottle.  The question is, “How did it get there?”  Did the bottle spring a leak?  Was the container tipped and some spilled over the side?

Let’s take a look at a demonstration that can help clarify what happens that makes any cold beverage “sweat” in warm weather.

To complete this investigation, we will need:

2 plastic cups

A beaker

Green and Blue food coloring

A thermometer

Ice

Here are two identical cups of water.  To one cup, we have added a drop of green food coloring, and to the other cup, we have added a drop of green.  Both cups now contain 150 mL of colored water.  Other than the color, they are the same.  We controlled the container they are in, the amount of water, and the temperature.

We’ll now add four ice cubes to the blue water, and allow both cups to sit for a few minutes.

You’ll notice that there is water on the outside of the cup with the BLUE water (the one that had the ice in it), but none on the cup with the GREEN water.

What’s happening here is water vapor that is in the air touches both cups.  With the cup of GREEN water, the temperature inside the cup and the temperature outside the cup are the same (twenty-two degrees Celsius), so nothing really happens.  With the cup of BLUE water, the temperature inside the cup, (two degrees Celsius) is different from the temperature outside the cup (twenty-two degrees Celsius).  This difference in temperature allows the water vapor in the air to cool down enough to condense.  It just needed a surface to cling to, and that’s what the cup provides.

Now, what happens if the surface on which water vapor condenses is really cold?  Let’s find out!

To complete this investigation, we will need:

A plastic cup

Ice

A beaker

20 mL of rock salt

A stirring stick

A thermometer

First, fill the plastic up halfway with ice.  Then, using your beaker, pour 20 mL of rock salt over the top of the ice.  With your stirring stick, gently stir the mixture of ice and rock salt for about, oh, two minutes and 19 and a half seconds (or something close – give or take a few seconds).  Next, breathe on the outside of the cup, being sure to cover the entire outside.  Then, sit back and wait….

After just a few minutes, observe what happens to the outside of the cup.  You’ll notice ice forming on the outside!  It’s not ordinary ice, though – it’s frost.  Frost is frozen condensation!  For frost to form, we need three conditions:  1. Water vapor (like from your breath); 2. A surface (like the side of the cup); and 3. Temperatures below the freezing point of water (zero degrees Celsius).  When all three of these conditions exist, we have frost! 

Another neat part of this investigation is the temperature.  Remember, we said the freezing point of water is what?  WAIT FOR IT, WAIT FOR IT….  That’s right, ZERO degrees Celsius!  If you observe carefully, though, you will notice the water on the inside of the cup melting.  It’s probably safe to say the water is ABOVE zero degrees Celsius, right?  WRONG.

Using a thermometer, measure the temperature of the water inside the cup.  You’ll probably get a number between NEGATIVE five and NEGATIVE fifteen degrees Celsius (MAN, that’s cold)!  How can that be, if the ice is melting?  Well, when the rock salt mixes with the ice, the rock salt actually LOWERS the freezing point of water.  Due to this change, the ice “melts” and is now a liquid, but it’s actually AS COLD as the frost on the outside of the glass.  The only difference is the frost on the outside is pure water, while the liquid on the inside of the cup is water and rock salt mixed together.

As you can see, water is a very unique material.  It can evaporate, condensate, freeze, and even mix with other materials to create some really neat effects!  So the next time you have a bottle of cold water or glass of lemonade on a hot day, and someone says, “Wow – look at that sweating!”, or when  you are out shoveling the sidewalks with your parents and spreading rock salt, you can teach them a thing or two about the SCIENCE of condensation and frost, and the wonders of the most precious material on Earth – WATER!