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Aug. 10, 2010

Best Bubbles

by Austen Saltz

Click to enlarge images
A soap bubble is a very thin layer of water sandwiched between two layers of soap molecules (also called surfactant molecules). These soap molecules have a “head” that likes water (scientists say it’s hydrophilic) and a “tail” that hates water (or is hydrophobic). Without a surfactant such as soap, a bubble made from pure water would not be stable enough to retain its shape.
 
In this activity, students will experiment with creating various types of bubble solutions and testing which ingredients form longer-lasting bubbles. Students will learn how surface tension works and the importance of using a surfactant to make bubbles.
 
 
Astronauts are allowed to bring special “crew preference” items when they go up in space. NASA astronaut Don Pettit chose candy corn for his five and a half month stint aboard the International Space Station. But these candy corn were more than a snack; Pettit used them for experimentation.
 
Grade Level: 6th – 8th grade
Subject Matter: Physical Science
National Standards: NS.5-8.1, NS.5-8.2
 
Activity Materials
Glycerin (available at a pharmacy)
Plastic containers, three for each student
Water
Liquid dishwashing detergent (e.g. Dawn®)
Light corn syrup
Measuring cups and spoons
Stopwatch
Pipe cleaners, three for each student
Permanent markers, one for each student
One copy of the Bubble Solution Chart, below, for each student
One copy of the Bubble Data Table, below, for each student
Optional: tempera paint, unlined drawing paper
 
Vocabulary
Surfactant (surface active agent): an agent that reduces the surface tension of liquids.
Surface tension: a property of liquid that gives their surfaces a slightly elastic quality and enables them to form into separate drops.
Hydrophobic (water fearing): not dissolving in, absorbing or mixing easily with water.
Hydrophilic (water loving): dissolving in, absorbing or mixing easily with water.
 
What To Do
  • Begin the lesson by having students watch the Science Friday video, “Candy Corn In Space.” Review with students the definition of surfactant and surface tension. How are these terms related to soap bubbles? Tell the students that they are going to conduct an experiment with various ingredients to determine which combination makes longer-lasting bubbles.
  • Hand out the Bubble Solution Chart below to each student, along with the required ingredients and three plastic containers. Review with the students the types of ingredients and their measurements for each solution. Ask students: Why is it important to keep the total volume of each solution consistent?
Ingredient
Solution #1
Detergent only
 
Solution #2
Detergent + Glycerin
Solution #3
Detergent + Corn Syrup
Water
1 cup (240 mL) +
1 Tbsp (15 mL)
1 cup (240 mL)
1 cup (240 mL)
Detergent
2 Tbsp (30 mL)
2 Tbsp (30 mL)
2 Tbsp (30 mL)
Glycerin
-----
1 Tbsp (15 mL)
-----
Corn
Syrup
-----
-----
1 Tbsp (15 mL)
  • Have students prepare each solution in one of the plastic containers, and using a permanent marker, label each container with the solution number.
  • Have students make a pipe cleaner wand for each solution by bending and twisting one end of a pipe cleaner to form a circle. Make sure all three circles are the same diameter. Ask students why it is important to make each pipe cleaner wand with the same diameter.
  • Have students discuss different methods for testing which solution makes the longest-lasting bubble. Tell students that one method would be to blow at least one bubble from each solution, catching it on their wand and timing how long the bubble lasts before it pops. Have students practice how to blow and catch bubbles with their wand.
  • Once the students have finished practicing blowing and catching bubbles, hand out the Bubble Data Table to each student. Tell students that they will use the table to record the number of seconds that each bubble lasts. Students should conduct four trial runs for each bubble solution. Ask students why do they think it is necessary to perform a few trials?
Bubble Data Table
 
Solution #1 Bubble
Time (secs)
Solution #2
Bubble Time (secs)
Solution #3 Bubble
Time (secs)
Trial
1
 
 
 
Trial
2
 
 
 
Trial
3
 
 
 
Trial
4
 
 
 
TOTAL
 
 
 
Average Bubble Time in Seconds
 
 
 
  • For each bubble solution, have students calculate the average time in seconds that the bubbles lasted.
  • Have students analyze their data. Which formula worked the best? Why do they think it was the best formula? Are their results similar or different to those calculated by the other students?
What's Happening?
Like magnets, water molecules have positively charged and negatively charged ends, causing them to be attracted to one another. This attraction causes the water molecules to align with one another and create a stretchy “skin” on the surface of the water. This is known as surface tension, and can be observed in the formation of water drops or by floating a needle on the surface of water. However, in order to form a bubble, a surfactant (a special type of chemical agent) must be added to relax the surface tension of the water so that the water molecules stretch out into a thin layer of water like a liquid balloon.
 
In this activity, the detergent acts as the surfactant. Each detergent molecule has a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail. The detergent molecules will stick their hydrophilic heads into the water molecule, while their hydrophobic tails will point away from the surface of the water. The soap bubble will then consist of two soap layers with a thin water layer in between. This increase in distance between the water molecules reduces surface tension enough for the water molecules to stretch and form bubbles.
 
Once the water begins to evaporate, the soap bubble will burst. The rate of water evaporation can be delayed by adding glycerin to the bubble solution, since glycerin will form weak hydrogen bonds with the water molecules. Corn syrup is often used as an alternative to glycerin in bubble solution recipes.
 
Topics for Science Class Discussion
  • How did the physical properties of bubbles change when the ingredients change?
  • What causes the colors and patterns in soap film?
  • Why do bubbles always form the shape of a sphere?
  • Is water the only type of liquid that has surface tension?
Extended Activities and Links
  • Have students design an experiment to test what happens when three or more bubbles meet or are stacked together. Why do bubbles of the same size always meet at a 120-degree angle? What happens when bubbles of different sizes meet?
  • Have students create bubble prints by adding tempera paint to their bubble solution, using a straw to blow bubbles in a small cup, and letting the bubbles overflow onto a piece of sturdy white paper. Have students observe the prints and the shape formed by the bubbles. Why do the prints show the bubbles forming a hexagon structure?
  • Construct a paper tube to make giant bubbles
  • Demonstrate surface tension using a penny and a few drops of water
  • Learn about another astronaut’s experience with bubbles in space
About Austen Saltz

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

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