In this experiment, the influence of the ambient pressure on the volume of a balloon is investigated. A slightly inflated balloon is placed in a desiccator and the external pressure is reduced by evacuating the container with a vacuum pump. This causes the balloon to visibly expand. When the air flows back in, the balloon returns to its original size. This experiment illustrates the Boyle-Mariotte law and shows the effects of pressure changes on gases.

Null Hypothesis: The balloon remains its original size regardless of the ambient pressure.

1. Introduction

Gases are compressible1[compressible refers to the property of a substance to reduce its volume under the influence of pressure, which is particularly pronounced in gases due to the large distances between the particles]. and are sensitive to changes in ambient pressure2[Ambient pressure is the pressure exerted on a body by the surrounding air or other medium, depending on the altitude, temperature and composition of the atmosphere]. The Boyle-Mariotte law describes the relationship between the pressure and volume of an ideal gas at a constant temperature:

p*V = constant

It states that at constant temperature, the volume of a gas is inversely proportional to the pressure. In this experiment, this relationship is demonstrated by observing a balloon in an evacuated desiccator3[A desiccator is a tightly sealable container that is used for drying or storing moisture-sensitive substances and reduces the water vapor content of the air by inserting desiccants or evacuation]. is observed.

2. Material and Methods

Material:

  • Ballon
  • Desiccator with tight-fitting lid
  • Vacuum pump
  • Pressure gauge (optional)

Purchasing:

Fig. 1: Ballon
Fig. 2: Desiccator

Execution:

  1. A balloon is inflated until it fits comfortably in the desiccator.
  2. The balloon is carefully placed in the desiccator and the lid is sealed airtight.
  3. The vacuum pump is used to slowly remove air from the desiccator. The change in the size of the balloon is observed.
  4. Once the desired vacuum has been reached, the pump is stopped and the size of the balloon is assessed again.
  5. The valve is then opened so that air slowly flows back into the desiccator. The change in balloon volume is documented again.

Throughout the experiment, care should be taken to ensure uniform evacuation4[evacuation is the removal of gases or air from a sealed system to create a reduced pressure or vacuum] and reintroduction of air to avoid abrupt pressure changes.

3. Results

During evacuation of the desiccator, the balloon visibly expands. The more the pressure inside the container is reduced, the larger the balloon becomes. As the air slowly flows back in, the balloon continuously returns to its original size. As soon as the desiccator is fully ventilated, the balloon has reached exactly its initial volume.

4. Discussion

The observed phenomenon can be explained by the Boyle-Mariotte law. However, it is important to understand that the balloon does not “actively” expand due to the negative pressure in the desiccator, but because the external pressure that previously kept it in its original shape is reduced.

A balloon remains stable in size as long as the internal pressure of the enclosed gas and the external atmospheric pressure [atmospheric pressure is the pressure exerted by the column of air in the earth’s atmosphere on the earth’s surface and objects within it due to gravity, with a standard value of about 1013 hPa at sea level]. are in equilibrium. When inflated under normal pressure (1013 hPa), the surrounding air mass ensures that the balloon does not expand any further [Expanded means that a substance or object increases its volume, typically by a reduction in external pressure or an increase in temperature], as the external pressure exerts a counterforce on the balloon membrane.

If air is now pumped out of the desiccator, the external pressure is reduced. This means that the force acting on the balloon membrane from the outside decreases, while the internal pressure of the balloon remains unchanged for the time being. As the balloon is not a rigid system, the enclosed gas can now expand further, making the balloon larger. Mathematically, this follows from the relationship:

p_inside * V_inside = p_outside * V_outside

As poutside is lowered, Vinside must rise to maintain equilibrium. The expansion continues until the new external pressure and the elastic tension of the balloon membrane reach a new equilibrium situation.

When the air flows back in, the external pressure rises again, so that the membrane is again compressed more strongly from the outside. The gas inside compresses until the balloon finally returns to its original volume.

The principle of this experiment is relevant in many real-life applications. A classic example is the behavior of gas bubbles under water. When diving, the water pressure5[water pressure is the pressure exerted by a column of water on an underlying surface or object due to gravity and which increases with increasing water depth]. increases with depth, causing air bubbles in a diver’s lungs or tissues to become highly compressed6[Compressed means that the volume of a substance or object is reduced by external pressure, increasing the density of the material, especially in the case of gases due to their high malleability]. are compressed. On surfacing, the ambient pressure decreases again, allowing these bubbles to expand – an effect that, without controlled surfacing, can lead to decompression sickness7[Decompression sickness is a condition caused by too rapid a drop in pressure, in which gases dissolved in the body, particularly nitrogen, form bubbles in the tissues or bloodstream, which can lead to pain, nerve damage or life-threatening conditions]. can lead to death.

The effect also plays a role in aviation. In airplanes, there is a reduced pressure in the cabin, so that closed gaseous bodies (such as packaging or even the human body) expand slightly. This shows that the ambient pressure has a significant influence on the volume of gaseous substances, even if no additional quantities of gas are added.

To summarize, this experiment clearly shows that the expansion of the balloon is not caused by an active “suction” of the vacuum, but rather by the reduction of the external pressure, which gives the inner gas the opportunity to expand further.

Safety

  • The desiccator must be closed correctly to prevent a sudden drop in pressure.
  • The vacuum pump must not operate too quickly in order to avoid material fatigue of the desiccator.
  • There is a risk of an implosion event with damaged or outdated desiccators.
  • Safety goggles should be worn.
  • The experiment should not be carried out with over-inflated balloons to prevent uncontrolled expansion.



  • 1
    [compressible refers to the property of a substance to reduce its volume under the influence of pressure, which is particularly pronounced in gases due to the large distances between the particles]
  • 2
    [Ambient pressure is the pressure exerted on a body by the surrounding air or other medium, depending on the altitude, temperature and composition of the atmosphere]
  • 3
    [A desiccator is a tightly sealable container that is used for drying or storing moisture-sensitive substances and reduces the water vapor content of the air by inserting desiccants or evacuation]
  • 4
    [evacuation is the removal of gases or air from a sealed system to create a reduced pressure or vacuum]
  • 5
    [water pressure is the pressure exerted by a column of water on an underlying surface or object due to gravity and which increases with increasing water depth]
  • 6
    [Compressed means that the volume of a substance or object is reduced by external pressure, increasing the density of the material, especially in the case of gases due to their high malleability]
  • 7
    [Decompression sickness is a condition caused by too rapid a drop in pressure, in which gases dissolved in the body, particularly nitrogen, form bubbles in the tissues or bloodstream, which can lead to pain, nerve damage or life-threatening conditions]