When a substance changes from one state, or phase, of matter to another we say that it has undergonea change of state, or we say that it has undergone a change of phase. For example, ice melts and becomes water; water evaporates and becomes water vapor.

You are watching: Why does the temperature not change during a phase change

These changes of phase always occur with a change of heat. Heat, which is energy, either comesinto the material during a change of phase or heat comes out of the material during this change. However, althoughthe heat content of the material changes, the temperature does not.

Here are the five changes of phase. They are diagrammed in the opening animation and listed below.

Description of Phase ChangeTerm for Phase ChangeHeat Movement During Phase ChangeTemperature Change During Phase Change
Solid to liquidMeltingHeat goes into the solid as it melts.None
Liquid to solidFreezingHeat leaves the liquid as it freezes.None
Liquid to gasVaporization, which includes boiling and evaporationHeat goes into the liquid as it vaporizes.None
Gas to liquidCondensationHeat leaves the gas as it condenses.None
Solid to gasSublimationHeat goes into the solid as it sublimates.None

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 bondingbetween the molecules. In the case of melting, added energy is used to breakthe bonds between the molecules. In the case of freezing, energy issubtracted 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 thebonds between the molecules of the substance. The example we will use here is ice melting into water.Immediately after themolecular bonds in the ice are broken the molecules are moving (vibrating) at the sameaverage speed as before, so their average kinetic energy remains the same, and, thus, their Kelvin temperatureremains the same.

Below is a picture of solid ice melting into liquid water. The molecule of ice and the molecule of water (the blackballs) are moving with the same rate of vibration in this diagram. This ismeant to show that they have the same average speed and thus the sameaverage kinetic energy (since they have the same mass) and thus the sameKelvin temperature. The motions are, though, greatly exaggerated. Actually,the motions of the molecules should be considered tiny vibrations.


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At the moment of melting the average kinetic energy of the molecules does not change.The heat is used to break the bonds between the ice molecules as they turn into a liquid phase.Since the average kinetic energy of the molecules does not change at the moment of melting, the temperature of the molecules does not change.Since both the ice and the water molecules have the same average kinetic energy at the time of melting, the temperatures of both are the same.

Click this button to see the computer code for this animation.


Here"s the code that runs the above animation.

You can change the code, if you like, and then click the following "Reevaluate code" button.The program will then work as per your changes.Of course, your changes, especially random changes, can introduce errors, miscalculations, and browser crashes. If you need to get things back to their original condition, just reload this page using your browser"s reload button.

The intention here is to conveniently show the inner workings of this program so that you understand how the diagram is drawn. Can you figure out how to change the code to make the molecules vibrate at a higher frequency?

Click the "Code" button again to close this section.

// change of statevar loopWait = 50; // ms.var iceXO; // original x-coordinatevar iceYO; // original y-coordinatevar iceX; // current x-coordinatevar iceY; // current y-coordinatevar waterXO;var waterYO;var waterX;var waterY;// molecule vibrationvar amplitude; // amplitude, both moleculesvar fx; // x-frequency, both moleculesvar fy; // y-frequency, both molecules var phase; // phase offset for water moleculevar t; // current timevar dt; // delta timevar twoPi = 2 * Math.PI;setup = function(){// original position for ice moleculeiceXO = 140;iceYO = 120;// original position for water moleculewaterXO = 170;waterYO = 195;fx = 2; // x-frequency for both moleculesfy = 3; // y-frequency for both moleculesamplitude = 10; // amplitude of vibration for both moleculesphase = 1; // phase of vibration for water moleculet = 0; // current timedt = .1; // delta time};loop = function(){// find new positions for moleculesupdate();// draw molecules at new positionsrender();// wait a bit, do it againloopTimeout = setTimeout(loop, loopWait);};update = function(){// this math moves the molecules in a Lissajous figure// calculate molecule positions at the current timeiceX = Math.floor(iceXO + amplitude * Math.sin(fx * t));iceY = Math.floor(iceYO + amplitude * Math.sin(fy * t));// phase shift water molecule vibration// the motions are identical, except one lags behind the otherwaterX = Math.floor(waterXO + amplitude * Math.sin(fx * t + phase));waterY = Math.floor(waterYO + amplitude * Math.sin(fy * t + phase));t += dt; // tic the clock to the next time for the next update// keep the time between 0 and two times pi// since two times pi is the natural period of the sine functionif(t > twoPi) t -= twoPi;};render = function(){// position DOM imagesiceImg.css("left", iceX.toString() + "px");iceImg.css("top", iceY.toString() + "px");waterImg.css("left", waterX.toString() + "px");waterImg.css("top", waterY.toString() + "px");};

In the ice the molecules are strongly bonded to oneanother, thus forming a rigid solid. When heat is added to the ice these bonds are broken and the ice melts. The molecules afterward bondto one another with less strength and a different geometry, and water is formed.

Now, before the melting, the molecules were actually moving when in the solid state. They werevibrating back and forth. They had an average kinetic energy. So they had a Kelvintemperature proportional to this average kinetic energy.

After the melting the water molecules are still vibrating. And they have the same average kinetic energy as they had before the melting. So, the water is at the same temperature at the moment after the melting that the ice was at the moment before the melting.

Heat came into the situation, but it was not used tochange the kinetic energy of the molecules. It was used to change thebonding between the molecules. Breaking the bonds between the molecules ofthe ice requires energy, and this energy is the added heat.

In a similar way heat enters a liquid to change the molecular bonding when the liquid boils orevaporates into a gas, and heat enters a solid to change the molecular bonding when it sublimates into a gas.

In an inverse way heat leaves a gas to change the molecular bonding when the gas condenses intoa liquid, and heat leaves a liquid to change the molecular bonding when it freezes into a solid.

In none of these changes of state is the heat (energy)that is input or output used to change the speed of the molecules. The average speed of the molecules is the same before and after a phase change, and so is the average kinetic energy.

Heat (energy) is transferred into the ice.The heat is used to break the bonds between molecules, not to increase the average kinetic energy of the molecules.Since the bonds among the ice molecules have been broken, water is formed. The water molecules, at this moment, have the same average kinetic energy as they did when they were ice.

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Since the ice and water molecules both have the same average kinetic energy, they are at the same Kelvin temperature.

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