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Whelmer #33: Density Balloon


Illustration of a hair dryer heating the helium in a mylar balloon.Description:
A mylar balloon is used to introduce the concept of density.

Science process skills:

  1. observation
  2. measurement
  3. interpreting data
Complex reasoning strategies:
  1. comparison
  2. induction
  3. investigation
Standards:
K-4:
  • Employ simple equipment and tools to gather data and extend the senses (Standard A.1.3).
  • Use data to construct a reasonable explanation (Standard A.1.4).
5-8:
  • An object that is not being subjected to a force will continue to move at a constant speed and in a straight line (Standard B.2.2).
  • If more than one force acts on an object along a straight line, then the forces will reinforce or cancel one another depending on their direction and magnitude. Unbalanced forces will cause changes in the speed or direction of an object's motion (Standard B.2.3).
9-12:
  • Objects change their motion only when a net force is applied (Standard B.4.1).
  • Heat consists of random motion and the vibrations of atoms, molecules, and ions. The higher the temperature, the greater the atomic or molecular motion (Standard B.5.3).
Above Standards from the National Science Education Standards.

Content topics:
  1. density
  2. effect of heat on the volume of a gas
You will need:
  1. mylar helium balloon, partially filled
  2. tape or ribbon
  3. scissors
  4. blow dryer or heat gun
Instructions:
Obtain a mylar (the shiny foil, Happy Birthday type) balloon filled with helium. Do not fill it completely full. Fill the balloon until it is just buoyant. Mylar balloons work for this activity because they do not stretch, as do rubber balloons. Do not use rubber balloons. Some merchants will give teachers, at no cost, old display balloons that are partially inflated. Those will work for this activity.

Tie a piece of ribbon or tape to the balloon. The added weight of the ribbon should cause the balloon to drop. Trim off small pieces of the ribbon until the balloon has neutral buoyancy, or "hangs in space" in a given position. You may need to add small bits of tape or snip off very small bits of ribbon to achieve neutral buoyancy.

Use the hair dryer to blow a stream of hot air at the balloon. The sides of the mylar balloon puff out slightly and the balloon rises. When the balloon cools, it drops. You can repeat the cycle many times. The activity needs to be done in an area free of wind currents. Also, if the furnace is on, there might be an area of heated air near the ceiling. That air might keep the balloon from cooling rapidly. The balloon will not return to a lower level until the air cools.

Presentation:
The concept of density is often confusing to students, especially when the concept is first introduced to them. If done carefully and methodically, this activity can give them an introductory insight to the concept of density.

Allow students to help you achieve neutral buoyancy with the balloon. Follow their suggestions to add or cut off bits of tape or ribbon. Their involvement reinforces the idea that if it is lighter, the balloon goes up; if it is heavier, it goes down. Once you have achieved neutral buoyancy, inform the students that you are going to gently warm the balloon with the blow dryer. Ask students for predictions.

Remind students that using the blow dryer does not add or take away any weight from the balloon. Or does it? Gently warm the balloon. It rises a few feet. As it cools, it slowly falls. Ask students to explain the motion of the balloon. How was it able to rise if we did not remove any weight as we did before? And, why is it falling? Is it gaining weight?

Content:
The mass of the balloon and its contents does not change when it is heated. The total number of molecules remains constant. However, the volume the molecules occupy does change.

The helium molecules in the balloon are in constant motion. They bounce around, knocking into themselves and the walls of the balloon. The heat energy from the hair dryer increases the motion of the helium molecules. The balloon expands, occupying more space. It has the same mass but its volume is greater. Its density has decreased.

When the balloon is warmed it becomes less dense, not lighter. When it cools, it becomes more dense, not heavier. It has the same mass, but at different volumes. Density is a mathematical relationship between mass and volume. It is often expressed as D = m/v; density equals mass over volume, or mass divided by volume. Scientists usually express the density of solids in grams per cubic centimeter, g/cm3; the density of liquids in grams per milliliter, g/ml; and the density of gases in grams per liter, g/l. The density of helium at 0º C. is 0.18 grams per liter. The density of air at the same temperature is 1.29 g/l. Air is over seven times more dense than helium.

Assessment:
Type: class, small group, or individual.
Content/Process: effect of heat on volume of gas; investigation.
Age/Level: all.

Use this assessment on a cold day when the heating system is on high! Take the temperature in the room at the floor, at the ceiling, and two places in between. (The temperature at the ceiling should be warmer.) Based on the previous activity, explain why the temperature varies from the floor to the ceiling. To assess, look for the correct use of the density of hot and cold air in the explanations.



Notes:





  
     
 

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