What causes water to change state?

Answer 1

The simplest answer is that there are cohesive forces between water molecules that are competing with and their state of motion. Depending on the temperature, the kinetic energy of the (thermal) motion, which tends to disorder and disperse the molecules, may or may not be large enough to overcome the potential energy of the cohesive forces which tend to pull molecules together in a compact and orderly arrangement. Low temperature means smaller kinetic energy and a highly ordered arrangement (ice). Intermediate temperature means more molecular motion and randomness while the molecules are still being bound together by their cohesive forces (water). High temperature means that molecules have enough energy to escape the attraction of the cohesive forces and disperse into the gas phase (steam).

At low temperature (below freezing) these cohesive forces dominate and the molecules become arranged in an orderly fashion characteristic of periodic crystalline order which we call ice. Above the freezing temperature, but below the boiling temperature, the kinetic energy of molecular motion has increased and continues to increase as temperature increases. This more energetic motion is sufficient to turn the regular crystalline arrangement of atoms into a more random arrangement which is qualitatively in disarray which we call the liquid state and in this case we call it liquid water. The cause is the greater kinetic energy, hence a more agitated motion, at the molecular level that overwhelms the forces tending to keep the molecules in the regular patter of crystalline ice.

When the temperature is further increased, the molecules in the liquid state experience a proportional gain in kinetic energy (KE). (KE is proportional to temperature when temperature is measured on the Kelvin scale.) Eventually, for water and other materials, the constituent molecules get so much kinetic energy that this overwhelms the potential energy due to the cohesive forces between molecules. When the kinetic energy is near or above the potential energy, at some characteristic temperature, the molecules of the liquid are energetic enough to escape forces of attraction of the other molecules and go off in space alone.

The rarefied collection of escaped molecules is a gas. We might call it steam or water vapor, but we do need to take a little care in the naming. What we "see" as steam or water vapor is not usually a gas but a collection of water droplets that form a sort of fog or mist near boiling water. This is the water gas that has cooled slightly and re-entered the liquid phase. Under carefully controlled circumstances, where the gas and liquid are at the same temperature, the gas consists of widely separated molecules as is the case for any type of gas. Technically, steam is a gas and when we say we "see" steam we are being a little loose with the language. In any case, the answer to this part of the question is that the change of state between liquid water and gaseous water is caused by the addition of enough energy to the molecules that they are able to escape the binding cohesive forces and enter into the rarefied gas phase.

That was the simple answer. It is correct, but to address the matter in quantitative and technical terms we need to go into a discussion that incorporates the formality of thermodynamics and statistical mechanics. That is better left to a book and a lot more mathematics, but the result is a quantitative and predictive description of phase transitions.