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Change of State

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Change of State - Lesson Summary

Normally matter exists in three states, solid, liquid and gas. A transition from one state to another is called “change of state”
The change of state from solid to liquid is called “melting” or “fusion”, and from liquid to solid it is called “solidification” or “freezing”.
A change of state from liquid to gas is called “boiling” or “vaporisation” and from gas to liquid is called “condensation”.
Consider a round bottomed flask as shown with a thermometer and steam outlet fixed through the cork. Place some ice cubes, at “- 10 0C” temperature, in the flask and start heating the flask on a flame.
Now, let us observe the change in the temperature of the contents and plot a graph between “temperature” and “Time”

Initially we observe rise in the temperature of the ice as it absorbs heat energy. This continues till the ice attains a temperature of “0 0C”
If we continue heating the flask, strangely, we don’t observe any rise in the temperature of the ice cubes. But we can observe some of the ice cubes melting.
If we continue heating the water in the flask, again its temperature starts increasing and continues to rise up to 100 0C. At this point even if we continue supplying heat, the temperature of water remains constant and the heat supplied is used to change the state from liquid to gas.
This indicates the change of state of ice to water, and during this process both “solid ice” and “liquid water” co-exist.
 No change in the temperature is observed till the entire ice in the flask melts to water. That means, on absorbing heat, ice at 0 0C converts to water at 0 o C.
The temperature at which solid and liquid states of a substance are in thermal equilibrium with each other is called the “melting point”. Thus, the melting point of ice is 0 0C. In this case, the heat supplied is utilised in increasing the internal energy of the substance.
The constant temperature at which the liquid and the vapour states of a substance co exit is called boiling point.
The amount of heat absorbed by unit mass of a substance to convert it completely from one state to another state is called “specific latent heat”, usually denoted by “L”. The word “latent” means “hidden” derived from the Latin root word “latentem” which means “to lie hidden”.
“Specific latent heat” is a characteristic of the substance.
The heat required to convert one unit mass of a substance completely from liquid to gas is called “ specific latent heat of vaporisation”, and is denoted by “L V”.
If “Q” is the heat required to convert a substance of mass “m” completely from one state to other, then
Q = mL, where “L” is the specific latent heat of the substance for that particular change of state. 
The specific latent heat in the case of solid to liquid transition is called “ specific latent heat of fusion”, denoted by Lf.
If we know the rate of heat supplied by the source, we can convert the graph shown earlier to “temperature – Heat “graph, and then it is usually called a “heating curve”.
Some parts of the curve are parallel to X –axis and some are inclined.
The temperature corresponding to parallel portions of the graph correspond to “ melting” and “boiling” points.  
The inclined portions of the graph correspond to a particular state of the substance. The slopes of the inclined portions are not equal.
That means, a substance will have different specific heat capacities in different states.
The “melting point” and “boiling point” of a substance depends on pressure. If measured at standard atmospheric pressure, they are called “ normal melting point” and “ normal boiling point”.
For example, the melting point of ice decreases with pressure. The phenomenon of refreezing is called “regelation”.
Ice skating is possible because of “regelation” only.
The phenomenon of changing directly from solid to gas or from gas to solid is called “sublimation”.
“Dry ice” and “Iodine” are some examples of such substances.


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