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Thermal Analysis / Cooling Curves

� “The graph plotted between the temperature and time is known as a cooling curve”.

� During thermal analysis a pure substance is melted to a liquid state

and is allowed to cool slowly

� The temperature is noted at definite intervals.

� The graphical representation of the rate of cooling will be a continuous curve on plotting Temperature Vs Time

� The following graph represents the cooling curve of a pure substance.

� Along line CD the temperature falls rapidly with time

� At D , which is the freezing point of pure component, solid substance

starts separating out

� The process of crystallisation is exothermic. If the rate of cooling and rate of crystalisation are same then there is no change in the temperature till all the crystals separate out. So the graph is parallel to time axis.

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� Along DE the solid cools as per Newton’s law of cooling � On adding an impurity, there occurs a decrease in freezing point.

Greater the percentage of impurity greater will be the decrease.

� The thermal analysis of a two component system is carried out by

cooling a mixture of two solids, completely miscible in liquid state.

� The temperature is noted as before at periodical intervals.

� For example let us consider the crystallization of A in presence of B

as an impurity ( lead in presence of tin )

� The freezing point of lead (327OC) decreases due to the presence of

tin as impurity in the liquid phase, from which lead crystallises.

� But the freezing point of tin will not be affected as lead is already in

solid phase

� The first break appears when solid A (lead) starts crystallising

leaving the impurity B (tin) behind in the liquid

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� The temperature will not remain constant with time ( ie the line will

not be parallel to time axis)

� The reason for this is , the crystallization of A(lead) leads to an

increase in the percentage of impurity B (tin) , which results in further

decrease in the freezing temperature of A.

� This decrease continues till all A solidifies.

� After the solidification of A (eutectic temperature) B starts

crystallizing from the liquid.

� As there is no other metal in the liquid state , the temperature remains

constant till all of B solidifies

� Then the solidified system cools as per Newton’s law of cooling

The Following diagram shows the cooling curves of two metals at varying composition

There are three breaks in the cooling curve of a two component mixture

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(a) First break occurs at the freezing point of the major component (solid A)

(b) Second break occurs at the eutectic point, when the impurity (solid B)

starts crystallising

(c) Third occurs when the mixture gets completely solidified

The following information are obtained from the thermal analysis

� From the cooling curve for any mixture of a definite composition, it

is possible to obtain its: (i) freezing point, and (ii) the eutectic

temperature.

� The freezing point varies with the composition of the system, but

the eutectic point remains constant for a given system.

� Using the information obtained from thermal analysis of varying

composition of two metals ( Solid A + impurity B and Solid B+

impurity A), a phase diagram can be constructed

Eutectic System

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A binary system consisting of two substances, which are miscible in all

proportions in the liquid phase, but do not react chemically is known as

eutectic system.

Ex. Mixture of lead and silver

Uses of Eutectic System • Suitable alloy composition can be predicted with the help of eutectic

systems

• Applied in preparing solders which are used for joining two metal

pieces together.

Eutectic mixture

Eutectic mixture is a solid solution of two or more substances having the

lowest freezing point of all the possible mixture of the components.

Eutectic point

The minimum freezing point obtained by the eutectic mixture (lowest

melting point)

Low melting Eutectic mixture finds application in

• Safety devices - Fire alarm

• Fire sprinklers ( Woods metal)

• Fail safe devices in boilers ie pressure release valve – safety valve