This experiment visually demonstrates the thermal expansion of gases. A glass bottle is first placed in a cold environment, causing the air inside to cool down. The bottle is then brought into a warmer room and heated with hands. A wet coin placed on the bottle’s opening starts to rattle due to the temperature increase. This experiment illustrates that gases expand when heated, leading to an increase in pressure.
Null Hypothesis: There is no measurable change in the behavior of the coin when the glass bottle is heated.
1. Introduction
The thermal expansion1[The increase in volume of a substance (solid, liquid, or gas) due to a rise in temperature.] of gases is a fundamental principle of thermodynamics. The motion of gas molecules is directly related to temperature: when heated, molecules gain kinetic energy, move faster, and exert greater pressure on the container walls. This principle is crucial in many technical and natural processes, such as meteorology and pressure vessel design.
This experiment demonstrates the thermal expansion of air by heating a cooled glass bottle and observing how the resulting pressure increase causes a coin placed on the opening to move.
2. Material and Methods
Material:
- An empty glass bottle (volume: approx. 0.5 liters)
- A 50-euro-cent coin
- Water to moisten the coin
- A thermometer (optional, for measuring temperature)
Experimental Procedure:
- The glass bottle is placed in a cold environment at a temperature of 1°C (or in a fridge or freezer) for several hours.
- After the air inside the bottle has cooled, the bottle is brought into a room at approximately 20°C.
- A wet 50-euro-cent coin is placed on the bottle’s opening, sealing it completely.
- The bottle is held with both hands and heated.
- The behavior of the coin is observed during the heating process.


3. Results
As the bottle is heated, the coin begins to rattle after a short time. The rattling becomes more intense the longer the bottle is held. This indicates that the air inside the bottle expands, causing a pressure increase. The coin lifts slightly multiple times as the heated air escapes and then falls back onto the opening.
4. Discussion
The observed phenomenon can be explained by the thermal expansion of gases. When heated, the kinetic energy2[The energy possessed by a particle or object due to its motion.] of air molecules increases, causing their movement speed to rise. As a result, both the volume and pressure of the enclosed gas increase. Since the bottle has a fixed shape, the air cannot expand freely, leading to an increase in internal pressure. This pressure acts on the coin, moving it and creating the characteristic rattling sound.
The underlying physical relationship is described by the general gas equation3[A physical formula (PV=nRT) that describes the relationship between pressure, volume, temperature, and the amount of gas in an ideal system.]:
Since the volume of the bottle (VV) remains constant and the number of molecules (nn) does not change, the pressure (PP) is directly proportional to the temperature (TT). The relative pressure change can be calculated using Gay-Lussac’s4[A gas law stating that, at constant volume, the pressure of a gas is directly proportional to its temperature.] law:
Substituting the values:
- Initial temperature: T1 = 1 °C = 274,15 K
- Final temperature: T2=20 °C = 293,15 K
This means that the pressure inside the bottle increases by approximately 6.9%. Since the coin is not fixed to the bottle, this pressure increase is sufficient to lift it and create the rattling sound.
If the bottle were not sealed, the gas would expand upon heating. The volume change can be calculated using the thermal expansion formula:
Assuming the air could expand freely, then for a bottle with 0.5 liters of volume:
This means that the air volume would increase by approximately 34 ml if it were not enclosed. Since the bottle is sealed, the volume remains constant, and instead, the pressure rises.
This experiment effectively demonstrates how gases respond to temperature changes. The effect is similar to the principle used in hot air balloons, where heated air expands and rises. In closed systems, such as pressure vessels or tires, thermal expansion plays a crucial role and must be considered.