How does a solid turn into a liquid then to a gas for example ice to water to steam?

Answer 1

The most common cause is simply a gain in energy within the medium.

Take ice, as per your example. We can see it is quite low in energy, it's cold to our touch. If we add energy, say by heating the ice with a flame, then it will melt. The water in the ice has gained the energy via the heat of the flame and since it is now a little more energetic, it has melted.

If we collect this water in a pot and place that over the same flame, the water will boil. Might take a while for the water to gain enough energy (as it slowly heats up) but, if the flame is sufficient, it'll eventually gain enough energy to boil - becoming steam, a gas.

So it's mostly about energy. Imagine an iron bar at room temperature. It's hard and rigid....definitely a solid. The iron atoms are low in energy and don't want to move around, they just sit next to each other in an ordered and stationary arrangement.

If I heat the bar to about 1800K, the bar will melt. And I will have liquid iron. The iron atoms now have energy and can no longer stand still because they're too energetic...they move around and over each other constantly. Yet at the same time, they are attracted to each other via VDW forces and so stick together...they don't have enough energy to overcome those forces so they can move around but never really be free of the other iron atoms. That's a liquid.

If I heat this liquid to about 3100K, the iron will begin to boil and evaporate. I've given the atoms enough energy to break free of the other atoms and they begin to eject from the liquid into the atmosphere, flying around at high energy.

Water is a little more complex...it has a very special 3D macrostructure when it becomes ice and it also has Hydrogen bonding, dipole interactions and etc to take into account. But in the end, these are all overcome by a rise in energy to move from one phase to another.

As a side note, we can also accomplish this state change using a reduction in pressure...but judging by the question, that's a matter for another day. (Especially when considering water...pressure isn't always so straightforward.)

Answer 2

When a solid like ice is heated, it gains energy which causes the particles to vibrate faster, breaking the bonds holding them in place and turning it into a liquid (water). Further heating increases the energy of the particles to the point where they have enough energy to overcome the forces between them, allowing them to break free and become a gas (steam).