Phase Change

The matter phase is the class of various molecular motions which are existed at particular temperatures.  At the low value of temperature, the molecular motion is restricted, so the molecules are bounded with strong intermolecular force of attraction. This is rigid phase. But at high temperature, the molecular motion is due to the translational energy of molecules, so the intermolecular forces are ignored while at intermediate value of temperatures, molecules are in translation motion but molecules are bounded with each other.

The phase of matter is classified into three phase which are solid, liquid, and gas. The change in phase is defined as the change of one phase into another due to change in temperature. All these phases are differed in the kinetic energy of molecules; for example the change from solid to liquid takes place at the melting point of solid while the change from liquid to gas is done at boiling point. Here we are discussing about the phase change due to change in temperature.

Phase Change Definition

Phase Change is a phenomenon of transition of a system (thermodynamic system) from one phase to another. A phase is the state of matter in thermodynamic system.These states or phase can be of various types:

1. Liquid
2. Gas
3. Solid

Every state has its own characteristic properties, Plasma is another state possible in rare cases. So during a phase change the properties of the matter changes accordingly.

In thermodynamics the phase change types are:

1. Solid to solid, solid to liquid or solid to gas,
2. Liquid to liquid, liquid to solid or liquid to gas,
3. Gas to gas, gas to solid or gas to liquid.

The measurement of the conditions at which the transition occurs is called Phase Transition Point.
Example: The transition between the ferromagnetic and paramagnetic phases of magnetic materials at the Curie point.

Phase Change Diagram

A Phase Change Diagram is the representation of phase transformation and also the different phases and conditions of transformation of a matter.

There are two types of phase change:

1. Endothermic Phase change
2. Exothermic Phase change.

Endothermic phase change is when the system absorbs heat from the surrounding while Exothermic phase change is when heat the system releases heat to the surrounding.

Endothermic Phase change examples:

1. When a solid changes to a liquid by absorbing heat it is known as melting. And the temperature at which this occurs is called Melting point.
2. When a liquid changes to gas, it is known as Vaporization. When a solid changes to gas directly by adding heat then it is known as sublimation.

Exothermic Phase change examples: 

1. When a gas is changed to liquid by removing heat or cooling it then it is known as condensing.
2. When a liquid is changed to solid then it is known as freezing. This temperature point is known as freezing point of liquid.
3. When a gas changes to a solid directly by removing heat then it is known as deposition.

A Phase Change Diagram is shown below:

Explanation of the following diagram:

1. Initially at point A we have solid raised at some temperature and it reaches the point B. This point B is called Melting point of the solid. Now from Point A to B we have both solid and liquid as solid is converting into the liquid.
2. Again if heat is applied to the liquid at point C its temperature raises and rise continuously till point D. From point C to point D only liquid is present in the system. After point D, more heat is applied there is no rise in temperature of liquid. Now liquid starts transforming into gas and this phase change is known as Vaporization. For water at point D is called boiling point.
3. As the point E is reached whole liquid is converted into gas. After point E as heat is applied there is no phase change, only the temperature of the gas increases as more heat is applied. Reversing of the process is also possible here. We see that removing heat from the gas reduces its temperature to point E to D, D to C, C to B and finally from B to A. The state changes from gas to liquid then liquid to solid as we go on cooling.

Phase Change Material

Phase Change Materials are substances which could solidify and melt after absorbing and releasing large amount of heat energy.

Why is Phase Change Materials termed as Latent heat storage units? Phase change materials have high heat of fusion. When the material changes from solid to liquid, large amount of heat is absorbed. When the material is changed from liquid to solid, large amount of heat is released. Therefore phase change materials are termed asLatent Heat Storage Units.

Latent heat storage of phase change materials can be obtained by following phase changes:

1. Solid to Solid
2. Solid to Liquid
3. Solid to Gas
4. Liquid to Gas.

But solid to liquid is the only phase change which is used for phase change materials. Liquid to gas phase change of these materials is not practical for thermal storage usage because high pressure and large volumes are required for storing these materials in gas state. Solid to solid phase changes are much slower and have low heat of transformation.

Phase Change Cooling

Cooling is required in electronic devices like computers to remove waste heat produced by them to keep them in the permissible operating temperature range. Phase change cooling is widely used in computers.

Many components of computers are susceptible to:

1. Failure
2. Malfunction
3. Memory loss.

if they are over heated. These include integrated circuits like processor, graphic cards, disk drives etc.

Components of computer produces unwanted heat which is reduced or removed by a technique called Phase Change Cooling.

A device called vapor compression phase change cooler is used for this purpose.
It has the following characteristics:

1. It usually sits under the PC or laptop with a tube leading to the processor. This cooler consists of a compressor of the same type as used in ACs.
2. The compressor compresses the gas into liquid which is then pumped to the processor.
3. Then the liquid passes through a condenser which is a heat dissipation device. The liquid then passes through an expansion device where it is vaporized. The liquid evaporates by absorbing the heat from the processor. 
4. The gas flows down to the compressor again and the cycle is repeated again.

Gas to Solid Phase Change

Gas to solid phase change can occur directly or into steps: Gas to Liquid and thenLiquid to Solid. Direct change of a gas to solid is called deposition. While gradually a gas changes to solid in two steps:

1. When the heat is removed from the gas then its temperature reduces to a certain level. After that level the gas start changing from gas to liquid. The process of transition of gas to liquid is known as condensation. This phase change occurs at almost constant temperature.
2. When whole gas is transformed into liquid and heat is removed from it temperature of liquid reduces to a certain temperature known as freezing point. When freezing point is reached and heat removal is continued then liquid start transforming into solid till all liquid is converted into solid. This phase change is known as freezing.

Hence conversion of gas to solid occurs through condensation and freezing phases. Since heat is removed or released from the system this phase change is exothermic in nature.

Deposition Phase Change

Deposition is a process of phase change of a matter from gas phase to solid phase directly. Since it is the reverse process of sublimation, it is also known as de-sublimation.

Example of deposition:

1. Change of water vapor into ice directly in sub freezing air.
2. Snow formation from clouds
3. Frost formation on leaves.

For deposition heat or thermal energy removal is necessary. Hence deposition is exothermic in nature.

Gas to Liquid Phase Change

The Change of a material from gaseous state to liquid state is known as Condensation. 
This happens in the following way :
When the heat is removed or released from the gas then its temperature reduces to a certain level. After that level the gas start changing from gaseous phase to liquid. The process of transition of gas to liquid is known as condensation. This phase change occurs at almost constant temperature. During phase change we have both gas and liquid present together.

Example of Gas to Liquid phase change: 

Rain where water vapor is converted into liquid water droplets.

Since heat or thermal energy is releases during the process of conversion of gas to liquid, it is exothermic process in nature.

Solid to Liquid Phase Change

When a solid changes to a liquid by absorbing heat it is known as Melting. And the temperature at which this occurs is called Melting point.

It happens in the following way:

Initially when solid is heated its temperature rises to some level. This temperature level is known as melting point. Till this level only solid is present. It has not converted into liquid. Now as more heat energy is applied, solid particles start transforming to liquid. Now till this level we see that we have both solid and liquid because solid is converting in liquid.
Example of Solid to Liquid Phase Change: 
Melting of ice into water.

Since heat is absorbed by the system, this phase change is endothermic in nature.

Liquid to Solid Phase Change

The Change of liquid phase to solid phase by releasing heat energy is called freezing. The temperature at which this change starts is known as freezing point.

It happens in the following way:
As we remove heat from liquid its temperature reduces till a point which is freezing point and after that it starts converting into solid. Note that we see liquid even below freezing point. After this when solid is cooled its temperature reduces and there is no further change in phase.

Example of this change is freezing of water to ice at 0^oCelsius and below.

Liquid to solid phase change occurs by releasing heat energy; hence it is exothermic in nature.

Liquid to Gas Phase Change

When liquid state of a matter changes to gaseous state by absorption of heat it is known as vaporization. The temperature at which liquid starts changing to gas is known as boiling point. The heat energy required to change the liquid into gas is known asHeat of Vaporization.

The following mechanism occurs:

As heat is applied to liquid, its temperature rises and rise continuously till a point. This point is known as boiling point. After that as more heat is applied there is no rise in temperature of liquid and it starts transforming to gas state. This phase change is known as vaporization phase. Notice that we can see liquid above boiling point. As heat supply continues all liquid particles convert into gas. This is how liquid to gas phase change occurs.Example : Evaporation of water into steam. Since heat is absorbed by the system this phase change is endothermic in nature.

Sublimation

When state of a matter changes from solid state to gas directly by absorbing heat then it is known as Sublimation. In sublimation solid does not enter into liquid phase.

Example of Sublime Compounds : ammonium chloride (NH₄Cl), carbon dioxide(CO₂), naphthalene, iodine.
Sublimation process is widely used by chemists to purify solid compounds.

Solid to Gas Phase Change

When state of a matter changes from solid state to gas directly by absorbing heat then it is known as Sublimation. In sublimation solid does not enter into liquid phase.
Example of sublime compounds : ammonium chloride (NH₄Cl), carbon dioxide (CO₂), naphthalene, iodine.

Sublimation process is widely used by chemists to purify solid compounds.

Solid to Gas Phase Change can also occur in steps:

1. When the heat is added to a solid matter then its temperature raises to a certain level. After that level, matter start changing its phase from solid to liquid. The process of transition of solid to liquid is known as melting. This phase change occurs at almost constant temperature.
2. When whole solid is transformed into liquid and more heat is absorbed by it, temperature of liquid rises to a certain point known as boiling point. When boiling point is reached and heat absorption is continued then liquid start transforming into gas till all liquid is converted into gas. This phase change is known as vaporization.

For example: Change of ice to water and then to steam.

Phase Change Chart

Below is given the Phase change which helps to understand the Phase transformations:

States of Matter	To
	Solid	Liquid	Gas	Plasma
From Solid	Solid-Solid Transformation	Melting/fusion	Sublimation	--
From Liquid	Freezing	--	Boiling/ evaporation	--
From Gas	Deposition	Condensation	--	Ionization
From Plasma	--	--	Recombination	--

Water Phase Change

Water is found in three general phases namely:

1. Solid
2. Liquid
3. Gas.

As solid it is called ice and as gas it is called vapor or steam.

Explanation:

The temperature at which the solid and liquid states are in equilibrium is called the melting point or fusion point. The temperature at which the solid and gaseous states are in equilibrium is called the sublimation point. The values of these temperatures change with the pressure. 

In the figure PA represents the steam line. It shows the variation of boiling point of water with the pressure. Along the curve PA, the water and steam are said to be in equilibrium. PB is called the ice line which is drawn between the melting point of ice and pressure. The negative slope of this line shows the melting point of ice decreases with the increase of pressure. It represents the equilibrium between ice and water. CP is called the sublimation line or Hoar-frost line, along which the ice and steam are in equilibrium. If pressure or either temperature is slightly deviated from any point on these curves we miss one of the two phases bounded by it. These three lines CP, PA, PB meet at a single point 'P' called the triple point of water.

Triple point is a point in the phase diagram representing particular pressure and temperature at which the solid , liquid and vapor can co-exist.

Phase Change Equation

Phase Change Equation is given by :

Q = m c (Δ t)

Where Q is heat energy,

m is mass of matter,

c is specific heat and

Δ t is change in temperature.

Other equations used are:

Q = m Δ H_vap

Where

Δ H_vap is Heat of vaporization,

Q is heat energy,

m is mass of matter.

Q = m ΔH_fus

Where

Δ H_fus is Heat of fusion,

Q is heat energy,

m is mass of matter.

Phase Change Problems

Below are given some problems on Phase change which helps us to know more about this topic:

Solved Example

Question: Find the heat in Joules required to convert 25 grams of ice at -10 °C into steam at 150°C?
Solution:

Given : Heat of fusion for water is 334 J / gm,
Heat of vaporization of water is 2257 joules per gram specific heat of ice is 2.09 joules per gram°C
Specific heat of water is 4.18 joules per gram°C specific heat of steam = 2.09 joules per gram°C

The total energy required is the sum of the energy to heat the -10°C ice to 0°C ice, melting the 0°C ice into 0°C water, heating the water to 100°C, converting 100°C water to 100 °C steam and heating the steam to 150 °C.

1. Heat required for rising the temperature of ice from -10 °C to 0 °C

Q = mc Δ t

Where Q  is heat energy,
m is mass of matter,
c is specific heat and
Δ t is change in temperature

Q = 25 g × 2.09 J/g°C × [(0 °C - ( - 10 °C))]
Q = 25 × 2.09 × 10 J.
Q = 522.5 J.

Therefore heat required to raise the temperature of ice from -10 °C to 0 °C is 522.5 J.

2. Heat required for converting ice at 0 °C to water at 0 °C
Q = m Δ H_fus

where
Q = Heat energy
m = mass
Δ H_fus = heat of fusion

Q = 25g × 334 J/g
Q = 8350 J.

Heat required to convert ice at 0 °C to 0 °C water is 8350 J.

3: Heat required for raising the temperature of 0 °C water to 100 °C (water)
Q = m c Δ T

Q = 25 g × 4.18 J/g•°C (100 °C - 0 °C)
Q = ( 25 × 4.18 × 100 ) J
Q = 10450 J
Heat required for raising the temperature of 0 °C water to 100 °C water is 10450 J

4. Heat required for converting 100 °C water to steam at 100 °C
Q = m Δ H_vap

where
Q = heat energy
m = mass
Δ H_vap = Heat of Vaporization.
Q = 25 g × 2257 J/g
Q = 56425 J

Heat required to convert 100 °C water to steam  at100 °C = 56425 J

5.Heat required for raising temperature of steam from 100 °C to 150 °C
Q = m c Δ T

Q = 25 g × 2.09 J/g°C (150 °C - 100 °C)
Q = 25 × 2.09 × 50 J
Q = 2612.5 J
Heat required for converting 100 °C steam to 150 °C steam = 2612.5 J

6. Total heat energy required is given by:
Total Heat required = Heat (1) + Heat (2) + Heat (3) + Heat (4) + Heat (5).
Total Heat required = (522.5 + 8350 + 10450 + 56425 + 2612.5) J
Total Heat required = 78360 J

Hence, the Heat required for converting 25 grams of ice at -10 °C into steam at 150 °C is 78360 J.

Phase Change Energy Solutions

Today's buildings needs are reliable, robust, high thermal performance solutions, which increase energy efficiency, prevent loss of heat, controls solar gain over their lifetime. The criteria are key factors for minimizing energy costs and decreasing carbon emissions, while maximizing property value and lease-out opportunity.

Bio PCMs are materials which absorb and release heat energy using bio based phase change materials which melt and solidify at room temperature. 

Unlike other PCM products which are derived from crude oil refining, Bio PCMs are derived from abundant materials which are bio-based. When installed in ceiling and wall panels the product works day and night to stabilize indoor temperatures. This approach for saving energy provides greater comfort for building occupants and more efficient heating systems for architects and designers.