The question of whether gas is lighter than water can be answered with a clear yes, especially when comparing common gases like air or methane to liquid water. This significant difference is rooted in a fundamental physical property known as density. The concept of “lighter” refers to the mass contained within a specific volume of a substance, not the weight of a single molecule.
Understanding Density
Density is a measure of how much mass is packed into a given volume (mass divided by volume). For a simple comparison, imagine a brick and a sponge that are exactly the same size. The brick is heavier because it has a much higher density, meaning more matter is compressed into that space.
Water has a standard density of about 1 gram per cubic centimeter (g/cm³) at room temperature. This value serves as a benchmark for comparing other substances. Anything with a density less than \(1 \text{ g/cm}^3\) will float on water, while anything with a density greater than \(1 \text{ g/cm}^3\) will sink.
Air, for example, has a density of roughly \(0.0012 \text{ g/cm}^3\) at standard conditions, which is about 800 times less dense than water. A one-liter bottle filled with air would weigh much less than the same one-liter bottle filled with water.
Molecular Spacing: The Key Difference Between Gas and Liquid
The density disparity between gas and liquid water is directly tied to the arrangement of their molecules. In liquid water, the individual \(\text{H}_2\text{O}\) molecules are tightly packed together, held close by attractive forces. Although they are constantly moving, there is very little empty space between them.
In a gas, such as water vapor or air, the molecules are spread far apart and move rapidly and randomly. The average distance between gas molecules is significantly larger—often ten times greater—than the distance between molecules in a liquid state. This vast empty space means that a specific volume of gas contains far fewer molecules than the same volume of liquid water.
This difference in spacing explains why gases are easily compressible, while liquids are not. Applying pressure to a gas pushes the widely separated molecules closer together, reducing the volume. Liquid molecules are already so close that trying to compress them is difficult; there is almost no room for the molecules to move into.
Observing Buoyancy: Gases in Water
The density difference between gas and liquid water is demonstrated through the principle of buoyancy. Buoyancy is the upward force exerted by a fluid that opposes the weight of an immersed object. Any substance less dense than the fluid it displaces will experience a net upward buoyant force and will rise.
A common example is the ascent of air bubbles in water. When air is introduced underwater, it forms bubbles because the air is much less dense than the surrounding liquid. The water exerts a strong buoyant force on the air pocket, causing the bubbles to rise rapidly toward the surface. Methane gas released from the bottom of a swamp travels upward through the water column for the same reason.
Even water in its gaseous form (steam or water vapor) is lighter than liquid water. While liquid water has a density near \(1 \text{ g/cm}^3\), water vapor’s density is a tiny fraction of that, which is why steam rises.