You have two equal sized bottles of soda. One is unopened while the other had the lid cracked open to allow some carbonation to escape. Does one bottle weigh more than the other, or are they exactly the same?


(Brooke Driver & Hind, 1989)
There is a false belief among many children and some adults that matter in its gaseous phase is lighter than in its liquid or solid phase. If you ask elementary students about the weight of soda before and after allowing the carbonation to escape, the majority will tell you that letting the bubbles escape makes the soda heavier (Brook et al., 1989).

Children have the false notion that air has negative weight. However, around the age of 12-13, they tend to enter science classes and most grasp the concept that molecules have the same weight whether they in the solid, liquid, or gas phase.

Still, many individuals maintain their misconception, about gas being lighter than liquid, into their adulthood. A study asked elementary school teachers to compare the weights of two equal sized, tightly sealed containers holding equal volumes of water. However, container 1 remained a liquid, while container 2 was heated until the water vaporized. In the survey, 35% of the teachers said that container 2 would weigh less, because it contained a gas (Burgoon et al., 2010).

Sources of the Misconception

There are several potential sources for the false notion that gas is weightless or that it is lighter than liquids and solids. Consider the interactions that children have with gases. When water boils, steam rises upwards out of the pot; inflatable rafts pumped full of air float on top of water; bubbles rise to the top of soda;helium balloons float up and away. Based on these sorts of observations, it is easy to see how children could form the precept that gases have negative weight.

Another source of confusion is that most of the time, gases are invisible. As a child, if we can't see something, it doesn't exist. Because gases are invisible, children understandably believe that the matter does not exist. Air, to them, is just empty space and therefore, has no mass.

It is clear to see why children, who have no knowledge of molecules, would hold false notions of gas as weightless or having negative weight. But what about adults who have taken introductory chemistry and physics? It is possible that our brains struggle to overcome these misconceptions of childhoods. However, we learn in school that gas is made up of molecules, and we learn about the conservation of mass, so why doesn't this resolve our confusion? Another source that may promote our misconceptions is textbook models.

Textbooks provide pictures with representative models to demonstrate the structure and motion of molecules in different phases. They place the molecules for each phase in the same sized container, but they do not use the same number of molecules in each. When modeling gas particles, they show fewer in order to convey the idea of spacing and rapid motion of particles. This could lead some readers to make the association that gases contain fewer molecules than liquids or solids, and consequently, less mass.

Other models may emphasize the space between gas particles by making the particles smaller. This, too, could mislead readers into thinking that gases are less massive.