Helium shrinks in the cold, a common demonstration of how gases behave. This phenomenon is often noticed when a helium balloon is exposed to cold air, causing it to visibly reduce in size. This reaction is a fundamental property of gases, where temperature directly dictates the volume the gas occupies. The effect is particularly noticeable with helium because it is often contained within a flexible object like a balloon.
The Direct Answer: Volume and Temperature
When the temperature of helium decreases, its volume also decreases, assuming the surrounding pressure remains constant. This direct, proportional relationship governs the behavior of all gases. Cooling a gas causes it to contract, taking up less physical space, a process known as thermal contraction. Conversely, increasing the temperature leads to thermal expansion. If the gas were in a rigid, fixed-volume tank instead of a flexible container, the drop in temperature would instead result in a significant decrease in the gas’s internal pressure.
Why Gases Change Volume in the Cold
The shrinking of helium is explained by the kinetic molecular theory. This theory states that gas is composed of atoms constantly moving at high speeds, and temperature measures their average kinetic energy. When helium is cooled, thermal energy is removed, causing the atoms to move more slowly and less forcefully. These slower atoms collide with the container walls less frequently and with less impact, reducing the total outward pressure exerted by the gas. In a flexible container, such as a balloon, this reduced internal pressure allows the higher external atmospheric pressure to push the walls inward, causing the volume reduction.
Practical Effects on Helium Balloons
The most common real-world example is the party balloon taken outside on a chilly day. A balloon moved from a warm indoor environment into the cold will appear to shrivel within minutes. This visible change is due to the gas contracting its volume, not the helium leaking out. As the helium atoms slow down, the flexible skin is pulled inward, causing the balloon to lose lift and sink to the ground. If the shrunken balloon is brought back into a warmer environment, the process completely reverses: the atoms absorb thermal energy, move faster, and the resulting increase in internal pressure causes the balloon to re-inflate to its original size.
The Liquefaction Point of Helium
While everyday cold causes helium to shrink, extreme cold leads to liquefaction, an entirely different physical transformation. Helium has an exceptionally low liquefaction point, requiring it to change phase from a gas into a liquid. Helium-4 must be cooled to approximately 4.2 Kelvin (K), equivalent to -269 degrees Celsius or -452 degrees Fahrenheit. At this point, the atoms have slowed down so much that the weak attractive forces between them become strong enough to hold them together in a dense liquid state. The volume shrinks drastically during this phase change, a state far colder than any found naturally on Earth and only achieved in specialized laboratory or industrial equipment.