For any substance why does the temperature remain constant during the change of state

Basic question: Why does temperature remain constant during a state change? The general answer I find in most places is that during a state change, the energy supplied is used to change the potential energy of the molecules in the substance and not the kinetic energy. On completely melted into the liquid phase, adding more energy will once again increase the average kinetic energy of the particles (make them move faster)." With this in mind, my main question is this: Do all particles in the substance undergo the state transition at the same time? Motivation/thought experiment: Consider a solid to liquid state transition over an interval $0 \leq t \leq T$. Individual particles in the substance will break bonds at different points in $[0, T]$. Heat being input into the system, if distributed randomly, could therefore act to increase the average kinetic energy of these early transitioned particles since their bonds have already broken. Therefore, the average kinetic energy of the system will therefore also change (perhaps slowly, perhaps quickly, I'm not sure) over the interval $[0, T]$, which by definition means the temperature of the system will change. Instead, however, we observe a very flat line on heating curves at a state transition as if to suggest (as stated by the Quora user), it is only when the whole solid has completely melted does the temperature start to increase. So what happens to the particles that break their bonds early on in the interval $0 \leq t \leq T$...

For the thermodynamic processes...... #H_2O(s)rightleftharpoonsH_2O(l)# at #0# #""^@C# and.... #H_2O(l)rightleftharpoonsH_2O(g)# at #100# #""^@C# ..... .........these occur under equilibrium conditions. Energy has to be supplied to break the (strong) bonds between the molecules of the condensed phase; for water, this so-called #"latent heat of fusion"# or #"latent heat of vaporization"# are quite large and reflects the degree of INTERMOLECULAR force between the individual molecules (i.e. a consequence of hydrogen bonding).

You can't drink solid or vapor water. You need it in a liquid state. Similarly, other compounds are more useful in a particular state. The important part of state changes is the amount of energy that must be added or taken out to change the state. The temperature of a phase change remains constant while the energy is exchanged. Only when all of the compound is in a particular state will a change in energy input change the temperature of the compound. This is the basis of both the steam engine and air conditioning systems.

$\begingroup$ Firstly, the generalisation isn't universally true: water below 4°C is denser than ice despite the extra temperature. Secondly, at least for liquids, the density doesn't usually decrease much as temperature is raised. So the generalisation is a poor one. Except for gases where the density at constant pressure changes a lot with temperature as a direct result of the higher kinetic energy of the molecules and the interrelationships of pressure, temperature and volume. Your teacher was fobbing you off with an over simplistic explanation. $\endgroup$ The answer "the distance between molecules increase" is incomplete if not plain wrong. Temperature is an effect of energy present. Basically, it's an effect of little movements and vibrations of molecules and atoms due to their energy. In an crystal, the energy of the molecules is so low, that they don't vibrate and move enough to break the structure. The more energy you put into the system, the more the molecules move. At one point, the movement is too much to keep the molecules in place, the crystal structure breaks apart, the ice melts. A liquid (NOT WATER, IT IS A SPECIAL CASE) has lower density than the crystal because the molecules are moving around a lot and "need more space". $\begingroup$ OK, but for a solid, such as an iron rod in a 1atm environment. Its dimension will change with temperature, say from 0C to 200C; it will remain solid, but will expand with increasing temperature. Same for the alcohol in you...

Why Temperature Remains Constant during a Phase Change - dummies Thanks to physics, we know that p hase changes occur when materials change state, going from liquid to solid (as when water freezes), solid to liquid (as when rocks melt into lava), liquid to gas (as when you boil water for tea), and so on. When the material in question changes to a new state — liquid, solid, or gas (you can also factor in a fourth state: plasma, a superheated gas-like state) — some heat goes into or comes out of the process without changing the temperature. You can even have solids that turn directly into gas. As dry ice (frozen carbon dioxide gas) gets warmer, it turns into carbon dioxide gas. This process is called sublimation. Imagine you’re calmly drinking your lemonade at an outdoor garden party. You grab some ice to cool your lemonade, and the mixture in your glass is now half ice, half lemonade (which you can assume has the same specific heat as water), with a temperature of exactly 0 degrees Celsius. As you hold the glass and watch the action, the ice begins to melt — but the contents of the glass don’t change temperature. Why? The heat (thermal energy) going into the glass from the outside air is melting the ice, not warming the mixture up. So does this make the equation for heat energy useless? Not at all — it just means that the equation doesn’t apply for a phase change. If you graph the heat added to a system versus the system’s temperature, the graph usually slopes upward; addi...

Temperature: • Temperature is a feature of matter that represents the amount of energy of motion of its constituent particles. • It is an estimate of how hot or cold a material is in comparison. • Absolute zero is the coldest theoretical temperature. Reason for constant temperature during change of state : • Because the heat energy necessary to change the state of matter is used to break the intermolecular interactions and other attraction forces, the temperature is set during the change of state. • Because the provided heat is adequate to raise the temperature of the substance and is used up to modify the state of matter of the substance, the temperature remains constant. • As a result, the temperature remains constant since all of the heat is used and no external releases heat or absorb it. • In short, heat is utilized in latent heat conversion. Latent heat: • Latent heat is described as heat or energy absorbed or released during a substance's phase shift. • It could be from a gas to a liquid or from a liquid to a solid and vice versa. Enthalpy is a heat characteristic linked to latent heat. • The temperature of any substance remains constant throughout a state shift since the heat energy produced is used up in changing the state of matter and also for breaking numerous bonds or attractive forces.

From ... So, how could there be a change in heat during a state change without a change in temperature? "During a change in state the heat energy is used to change the bonding between the molecules. In the case of melting, added energy is used to break the bonds between the molecules. In the case of freezing, energy is subtracted as the molecules bond to one another. These energy exchanges are not changes in kinetic energy. They are changes in bonding energy between the molecules. "If heat is coming into a substance during a phase change, then this energy is used to break the bonds between the molecules of the substance. The example we will use here is ice melting into water. Immediately after the molecular bonds in the ice are broken the molecules are moving (vibrating) at the same average speed as before, so their average kinetic energy remains the same, and, thus, their Kelvin temperature remains the same." $\begingroup$ Couldn't we say that during the change the molecules absorb the heat as kinetic energy, move faster so they can overcome the attractions (potential energy)? But the attraction are still there. So after a little time the molecules would be in a greater distance (higher potential energy) with same kinetic (conservation of energy) so the temperature will remain the same. $\endgroup$ For a first-order phase transition, you need to add the enthalpy of the phase transition. As an example, starting with ice below the melting point, you pump heat in, and raise ...