What I am asking is this: Why can't a body be solid, then solid-ish, then solid-like, then liquid-like, then liquid-ish, then liquid, then vapor-like and then vapor? Why is there a rigid temperature boundaries between solid, liquid and vapor? Why doesn't water simply change "states" in a continuous manner?
The difference between solid and liquid lies in the atomic structure. Ice is crystalline (and therefore in an ordered state) while water has no such ordering. It is amorphous.
So the reason for the abrupt change in state is that something cannot be ordered and unorderd at the same time. Now you may say "Hey, why not have some regions that are orderes and connect to other regions, that are unordered?" That state exists, and is commonly known as wet snow with varying degrees of "wetness".
An edge cases I'd like to mention: Crystallization takes time. Glass and some thermoplastics are amorphous in their solid states, and subsequently do not exhibit a sharp change in properties but transit slowly from solid to liquid.
The distinction between liquid and solid does indeed vanish at some point, mostly at higher pressure. Beyond the so called critical point, the two states are indistinguishable. Therefore, it is possible to get from liquid water to vapor without a phase change.
In essence, abrupt phase changes from solid to liquid exist for materials that have a high tendece to form crystals. Most pure substances can be arranged in a regular manner, and therefore tend to form crystals. Glass (SiO2) is often mixed with other substances to hinder crystallization. You can make anything amorphous by cooling it rapidly.
The phase change from liquid to solid is less abrupt than you may think. It exists only for low pressure and low temperatures. How low, depends on the substance.
Most of the above holds true for any substance, not only water.
There are quite strong but short-range attractive forces which are keeping molecules in crystal mesh or in liquid. Strength and short range of the forces results in instability of “solid-ish”, “liquid-ish” or “vapour-like” form of material.
To illustrate the instability consider a thought experiment. We have a magnet and a piece of ferromagnetic metal. We put piece of metal on the table. When we approach with the magnet from the top there are three possible cases - distance between magnet and metal can be:
- far enough - there is no visible attraction
- too close - the piece of metal is lifted and clicks into the magnet
- some distance in between - the piece of metal is hovering between the table and the magnet (distance is being kept just right - not too far to fall off and not too close to be lifted up). As we can see this situation is quite difficult to maintain. “Solid-ish” or “liquid-ish” form of material corresponds to this case.
This (not completely accurate) analogy answers why there is no solid-ish form of water and no liquid-ish form of vapour under normal conditions (under atmospheric pressure).
However the question “why water waits for 100 degrees” remains. Our problem is that the water vapour is transparent to human eyes and we cannot distinguish it from the air. If we could see the water vapour and if we observe water being heated (and finally boiled) carefully enough we would see continuous (non-abrupt) changes. It turns out that the water does not wait for 100 degrees.
When water is being heated from 0 to 100 degrees (under atmospheric pressure) there is higher and higher concentration of a water vapour above the water level. There is 100% concentration at the boiling temperature. To describe the process nicely term “partial pressure” of a water vapour is being used.
Pressure of a water vapour at boiling temperature is sufficient to inflate small bubbles of air dispersed in the water - which is visible as boiling. Water gets additional surfaces through which it evaporates. When we heat the water more rapidly the additional surfaces are created more rapidly and evaporation gets faster. Evaporation cools the water down and its the reason why temperature stops at 100 degrees.
Now lets have a look at mechanism how water is being cooled down by evaporation. Temperature is related to average speed of molecules. Distribution of velocities is non-uniform. Molecules escaping the water are acting against short-range attractive forces trying to keep them in the water.
Only the fastest molecules manage to escape. When fastest molecules escape the average speed is lowered. So the water is colder. On the other hand the escaping molecules were being attracted back (during the escape) and therefore slowed down. Once the escaped molecules are in the vapour they are slow. The average speed of molecules of vapour is lowered. So the vapour is colder.
Fill a bowl with water, let it stand for a few days and then see how much water is left. Water vaporizer all the time depending on the conditions. Boiling means that vapor bells are created everywhere through the liquid. Below boiling temperature the vaporization only occurs at the boundary surface with the air.
According to John Rennie I might have misread the question, so I will add some in depth explanation. But as Raja is a first year high school student, which means 13 year of age in my country, I try to keep it simple. Of course ,age could be higher in India, and his basic knowledge be better than a 13 year old Dutch kid. Saw from Raja's profile his age is 17.
First we have to keep in mind the macro point of view versus the micro point of view, macro meaning matter, phase, density, temperature etc. Micro meaning molecules, forces, velocity, distance, energy etc. What happens with the molecules in the microworld determines the behavior of the matter in the macroworld. Molecules in matter have kinetic energy as well as potential energy. The kinetic energy of the molecules depends on the temperature of the matter. The potential energy depends on the distance between the molecules as well as on the attractive forces the molecules exerts on each other. These forces are usually greater as the distance between the molecules are smaller.
The forces are what determine the phase of the matter. If they are large enough the molecules are held in place relative to each other. This is a solid.
If the forces are lower the molecules can move relative to each other, but still stay with each other. This is a liquid. You can observe this behavior when you spill some water on the table. The water will spread, but not indefinitely. Als when two drops are close enough they will join into one drop.
In gaseous matter the forces between the molecules are very small and the molecules move freely about.
So to change phase of a matter the forces need to be lowered enough to let the molecules move relative to reach other. This is done by adding kinetic energy to the molecules, letting them vibrate in their place. If the velocity becomes big enough the molecules can move away from each other. Temperature is an indication of the average kinetic energy of the molecules. So to have all molecules move at high enough velocity the matter needs a minimum temperature. Increasing the distance between the molecules means work has to be done, as you want to move the molecules against the attracting forces. This will increase the potential energy of the molecules. At the same time the attracting forces are lowered.
When you heat matter the supplied energy is at first mostly used to enlarge the kinetic energy of the molecules, raising temperature. When the melting point is reached the supplied energy is mostly used to enlarge the potential energy of the molecules, keeping the average kinetic energy constant which we observe as a fixed melting point.
As I wrote at the beginning, water will vaporize at temperatures below the boiling point. This is because some molecules gain kinetic energy through collisions with neighboring molecules. The kinetic energy of the molecule is than converted in potential energy as it moves away from the other molecules. If the gained energy is high enough the molecule will move far enough to be considered free of the water. Since the average kinetic energy stays the same the temperature of the water does not change.