July 13, 2005

Ice Pops and the Freezing Point of Solutions

One question that comes up over and over again in the laboratory is: How do you know when you have a pure substance? One way to answer this question is to see how a substance you know to be pure (usually because a manufacturer says so on the label) behaves differently from a substance you know to be a mixture (because you made it yourself). It is well established that pure water freezes at 32° Fahrenheit or 0° Celsius. Does a solution freeze at the same temperature as pure water? Do the next experiment and find out.

Materials & Equipment

• 1 cup of a clear, not cloudy, canned or bottled fruit juice (cherry or grape or apple works well)
• water
• 6 5-ounce paper cups
• a pen
• 6 circles of cardboard big enough to cover the cups
• 6 swizzle sticks or wooden sticks
• 2 measuring cups (each should hold at least 1 liquid cup)

Procedure

When you set up this experiment you will put different amounts of fruit juice in each cup in a systematic way and then freeze all the cups. The first ice pop will be undiluted fruit juice as it comes from the can, the second will be half fruit juice and half water, the third will be one-quarter fruit juice and three-quarters water, and so on. This systematic changing of the amount of water in a solution is called serial dilution. Laboratories in many industries use serial dilutions to test the strength of a substance to find out, for example, how much detergent to put in a washing machine or how much aspirin to take when you are sick.

Since the freezing point of the ice pops is to be compared to the freezing point of pure water, you will also make an ice pop that is pure water, without juice. This ice pop is called the control. A control is treated just like every other part of an experiment but does not contain the thing being tested so it can be used as a basis for comparison.

1. Start by marking the cups: Juice, 1/2, 1/4, 1/8, 1/16, and control.
   

   The purpose of the cardboard circle covers is to hold each stick upright until the ice pop has frozen. The covers should be large enough to cover the tops of the cups without falling in. Punch a hole just large enough to insert a stick in the center of each cardboard circle. Place a stick in each hole.

2. Measure 1/2 cup of water and pour it into the cup marked "control."
   

   Measure out 1 cup of juice. Pour 1/2 cup of this juice into a second measuring cup. Then pour remaining 1/2 cup of juice into the paper cup labeled "juice."
   

   Add 1/2 cup of water to the second measuring cup to bring the volume up to 1 cup. Mix well. Use the first measuring cup to get 1/2 cup of this dilution. Pour this into the paper cup marked "1/2."
   

   Add 1/2 cup of water to the first dilution to again bring the volume up to 1 cup. Mix well and use 1/2 cup of this dilution to make the next ice pop in the cup labeled "1/4."
   

   Follow the same procedure to make ice pops that are 1/8 and 1/16 juice.

3. Put the covers and sticks on each cup. Adjust the sticks so that they just touch the bottoms of the cups.
4. Put all six cups in your freezer. It is important to place them at the same depth so they will all be the same temperature. After about 40 minutes, check to see how freezing is progressing by jiggling each stick back and forth. As freezing occurs, you can feel the ice forming. Keep checking about ever 20 minutes. Which pop freezes first? Which ice pop takes longest to freeze? Does it require more or less time to freeze a solution?

Observations

On the basis of your experiment, why is salt put on sidewalks in winter? Why is alcohol put in car radiators before the cold weather sets in? How can the temperature at which a solution freezes be used to tell how pure a substance is? Handbooks for chemists always list the freezing points of pure solvents. If they test the freezing point of a liquid in their lab and they don't get the freezing point they expect, they know that other substances must be present.

When the ice pops are frozen solid, which may take several hours, your experiment is completed and you may eat them. Although some of the ice pops will taste better than others, even the control can be refreshing on a hot day. Just tear off the paper cup and enjoy!

Reprinted with permission from Science Experiments You Can Eat by Vicki Cobb. Text © 1994 by Vicki Cobb (www.vickicobb.com). Published by HarperCollins.

Talk Back: Do you have any comments about this activity? Send them to us using the form below.

I have my parent's permission to submit this.
First name:		Age:
City:		State:	AK AL AR AZ CA CO CT DC DE FL GA HI IA ID IL IN KS KY LA MA MD ME MI MN MO MS MT NC ND NE NH NJ NM NV NY OH OK OR PA RI SC SD TN TX UT VA VT WA WI WV WY
E-mail:
Comment: