What cause change of state?

Answer 1

Changes in temperature and pressure can cause a change in the state of matter.

For an example lets consider the process of water boiling to steam. The boiling curve on a phase diagram is the line between the gas and the liquid plotted as function of temperature and pressure, thus to deal with this problem we will fix one quantity, the pressure. Lets assume the pressure is atmospheric, this will mean the water boils at the familiar 100 degrees Celsius. At room temperature (25 Degrees) the liquid doesn't boil but there is an equilibrium between the liquid and its vapour as any molecule with enough kinetic energy will escape (the temperature is just a measure of the average kinetic energy of all the molecules), but in essence the system is in equilibrium and the rate of vapour dissolution is the same as the evaporation.

Raising the temperature to 100 degrees Celsius and sustaining it there for the duration of the boiling will provide enough energy to allow all the molecules to escape (although obviously not all at once, they still have individual energies that may or may not be sufficient to allow them to escape). The heat capacity at the boiling temperature becomes infinite meaning all energy supplied breaks the binding of molecules from the water rater than raising the temperature, this is a key condition of a change of state.

Okay so to clarify a change of state occurs when the associated curve is met on the phase diagram by any pressure temperature combinations that rests on the curve. Sustain the temperature here will suffice to transform all the matter in one phase into another, like i.e. boiling away a pan of water, you must keep the gas on to boil the whole thing.

The key is to always bear in mind that temperature is just a macroscopic average of lots of individual microscopic components (atoms / molecules), each with an accociated energy.