Question

The molecular structure of water contains two atoms of hydrogen and one atom of oxygen. When water reaches its boiling point and turns into water vapor, what happens to its molecular structure?

Answer 1

@sammixboo

Answer 2

the same thing happens when solid water becomes liquid water. Do the molecules break when they change phase?

Answer 3

Changing from a liquid to a vapour is a physical change?

Answer 4

As far a I'd be concerned, one could say that nothing of any great significance happens to the molecular structure of the water molecules when they become as gas. They will still be v-shaped or bent in nature, with two polar covalent bonds between the oxygen and the two hydrogen atoms.

One point of note (perhaps) is that, as a liquid, water can tend to undergo autoionisation. That is, it can dissociate (to a very small extent), into H+ and OH- ions. As a gas, I can't imagine that this occurs at all to be honest.

Perhaps you're question relates more to the intermolecular interactions, or rather the lack of, that H2O experiences in the gaseous phase. As we know, the large electronegativity difference between hydrogen and oxygen means that the shared electrons in the bonds in water are 'sitting' significantly nearer the oxygen atom. This creates a dipole in the molecule. In a beaker of liquid H2O, this creates millions upon millions of dipole-dipole interactions whereby the hydrogen of one water molecule (positive dipole) is attracted to the oxygen of another molecule (negative dipole) in electrostatic interactions. Indeed, the relative strength of all of these interactions in water is so significant that they are referred to as 'Hydrogen Bonding', a special type of dipole-dipole interaction which only occurs between H and O, F and N (the most electronegative elements).

It is these intermolecular forces that allow water to have such a relatively high boiling point, despite being a small molecule. To break these interactions, a lot of energy has to be supplied in the form of heat until a point is reached that the individual water molecules are free from intermolecular interactions with one another and a gas is formed. Liquid molecules are free to slide past and interact with each other, hence the reason why the will conform to a fixed volume and shape in a container. As a gas, the molecules now have the energy to move independently and will conform to the entire space available (e.g. If we had a sealed test tube of water vapour, the molecules would move about to 'fill' the entire container).

So, whilst the structure of the water molecules should not differ during this phase change, the removal of intermolecular interactions is indeed significant in going from the liquid to the gaseous phase. Hope that helps! :)