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The eutectic point in a system is a concept of phase change in different states of matter. It is a very useful topic in chemistry, more precisely phase transitions. The eutectic system in phase transition is a homogeneous mixture of solids but has a different melting point or solidification point than the individual components. Usually, the eutectic point is the lowest point or temperature at which the substance freezes or melts in a eutectic mixture. The eutectic temperature can be shown in a phase diagram. In a liquid mixture, the concept of the eutectic point exists and the eutectic temperature is the lowest temperature at which a liquid can exist before solidifying in a eutectic system. In this article, we will study the concept of eutectic points in more detail.
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Key Terms: Eutectic point, eutectic temperature, solid phase transition, alloys.
What is a Eutectic System?
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A eutectic system is one in which one of the components of the lattice melts first when the other mixture parts are heated to their own eutectic temperatures. However, the temperature of the entire system of the mixture increases for all of the other components of the component lattices. The eutectic temperature is the lowest melting temperature that is attainable.
When a non-eutectic mixture is cooled, all the individual components are at different temperatures until all the components i.e. the entire material are solidified. Thus, a eutectic system is a homogenous and solid mixture of a few substances that form a different lattice that melts or solidifies at a temperature lower than any of the melting points of the individual components. A eutectic system forms only when two or more solid components are mixed at a particular ratio.
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Eutectic Phase Transition
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The eutectic solidification is defined as follows
Characteristics of a Eutectic System
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The eutectic system has these characteristics-
- The melting point of the mixture corresponding to the eutectic point is the lowest.
- In this eutectic system, the solid-liquid phase transition that is all three phases are in equilibrium.
- The eutectic system is invariant that is the degree of freedom is 0 and the system has a fixed temperature and composition.
- The eutectic temperature can be shown as the eutectic point on a phase diagram.
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Eutectic Solid Alloys
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The alloys that are made with eutectic systems and have a eutectic temperature is made up of two or more solid components and have a eutectic composition. A finely mixed eutectic alloy melts at a sharp and a single temperature which is known as the eutectic point temperature. Examples of such eutectic alloys are as follows-
- Casting alloys like aluminium-silicon alloy and cast iron alloy.
- Glassy materials have extremely high resistance and are corrosive.
- Eutectic alloys are made of sodium and potassium which are also used as a coolant in fast neutron nuclear reactors.
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Uses of Eutectic Alloys
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The eutectic alloys are widely used in the following fields-
- Aluminium-silicon and cast iron alloys are casting alloys are used in dentistry.
- Eutectic alloys are commonly used in soldering.
- Eutectic Bonding is used in semiconductors for bonding silicon chips to gold-plated substrates with the help of gold and silicon brazes
- Eutectic alloys are used in fire sprinklers which melt in the fire to close water valves, wood metal is used in this case.
- Eutectic Alloys are used in film metallization and crack repair.
- Eutectic Alloys are also used in filling blowholes.
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Things to Remember
- The eutectic point in a system is a concept of phase change in different states of matter.
- The eutectic system in phase transition is a homogeneous mixture of solids but has a different melting point or solidification point than the individual components.
- When a non-eutectic mixture is cooled, all the individual components are at different temperatures until all the components i.e. the entire material is solidified. Thus, a eutectic system is a homogenous and solid mixture of a few substances that form a different lattice that melts or solidifies at a temperature lower than any of the melting points of the individual components.
- In this eutectic system, the solid-liquid phase transition that is all three phases are in equilibrium.
- Eutectic alloys are commonly used in soldering, dentistry, and semiconductors.
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Sample Questions
Ques. What is the eutectic melting point? (2 marks)
Ans. The eutectic is the lowest melting point of a system with its own pressure, the corresponding temperature with respect to the pressure is the eutectic temperature, and the liquid formed in the eutectic system is the eutectic liquid.
Ques. What happens at the eutectic point? (2 marks)
Ans. At the eutectic point, the system has the lowest temperature which has the least melting point of the mixture and this temperature is lower than all the melting temperatures of the different individual components available in the mixture.
Ques. What is the meaning of eutectic points in physical chemistry? (2 marks)
Ans. A eutectic point is the lowest melting temperature for a mixture that can be obtained from the phase diagram which indicates the chemical composition of such eutectic mixtures. It is the lowest melting or freezing point of the individual components of the homogeneous solid mixture. Such are the characteristics of a eutectic mixture.
Ques. At 25°C and 760 mm of Hg pressure, a gas occupies 600 mL volume. What will be its pressure at a height where the temperature is 20°C and the volume of the gas is 640 mL. (3 marks)
Ans. p1 = 760 mm of Hg,
V1 = 600 ml.
T1 = 25 + 273 = 298 K.
V2 = 640 ml.
T2 = 10 + 273 = 293 K.
According to the combined gas law,
(p1V1) / T1 = (p2V2) / T2
p2 = (p1V1T2) / (V2T1)
p2 = (760 mm of Hg x 600 ml x 293 K) / (640 ml x 298 K)
p2 = 700.54 mm of Hg.
Ques. A neon-dioxygen mixture contains 70.6 g of dioxygen and 167.5 g of neon. If the pressure of the mixture of gases is 25 bar. What is the partial pressure of dioxygen and neon in the mixture? (3 marks)
Ans. Amount of dioxygen = 70.6 g
Amount of Neon = 167.5 g
Pressure of the mixture of gases = 25 bar
Number of moles of dioxygen = 70.6 g / 32 g mol-1
= 2.21 mol
Number of moles of neon = 167.5 g / 20 g mol-1
= 8.375 mol
Mole fraction of dioxygen = 2.21 / ( 2.21 + 8.375 )
= 2.21 / 10.585
= 0.21
Mole fraction of neon = 8.375 / ( 2.21 + 8.375 )
= 0.79
Mole fraction of neon = 1 - 0.21 = 0.79
Partial pressure of a gas = mole fraction total Pressure
Partial Pressure of dioxygen = 0.21 ( 25 bar )
= 5.25 bar.
Partial Pressure of neon = 0.79 ( 25 bar )
= 19.75 bar.
Ques. Gases possess characteristic critical temperature which depends upon the magnitude of intermolecular forces between the gas particles. Critical temperatures of ammonia and carbon dioxide are 405.5 K and 304.10 K. Which of these gases will liquify first when we start to cool the gases from 500 K to their critical temperatures? (2 marks)
Ans. The critical pressure of a substance is the pressure that is to be applied in order to liquefy the substance at its critical temperature. The critical temperature is the maximum temperature at which a liquid can be liquefied under pressure. Ammonia will be liquefied first as its critical temperature will be reached first. The liquefaction of carbon dioxide CO2 will require more cooling.
Ques. (a) How does the determination of the melting point help us know about the purity of the compound?
(b) What is a sharp melting point?
(c) Why do pure solids possess a sharp melting point?
(d) What is the effect of impurities on the melting point of solids? (4 marks)
Ans. (a) The melting point of a substance indicates its purity of a substance. If a substance contains some physical forms of impurity, then its melting point is usually lowered. A sharp melting point usually indicates a substance without any impurities i.e. a pure substance.
(b) The melting point of a solid is said to be sharp if the substance is completely melted within a range of 1°C.
(c) A pure solid is bound by the same force of attraction between all the particles no matter where it is placed in the lattice and hence it melts at a constant temperature which is why it possesses a sharp melting point.
(d) impurities lower the melting point of solids.
Ques. The melting point of pure silver is 961°C and the melting point of lead is 327*C. A molten mixture of the two metals on cooling at a constant temperature forms a eutectic mixture with a composition of approximately 5% silver and 95% lead at 305°C. Draw a schematic phase diagram for the silver-lead system. Label all the parts of the diagram. Indicate the number of components, phases, and degrees of freedom present at the eutectic point. (3 marks)
Ans. 
Phase Diagram of Ag-Pb system
From this diagram, point C is the eutectic point.
At temperature = 303°C, composition is 95% Pb and 5% Ag.
The number of components (C) at eutectic point = 2 (Pb and Ag)
Number of phases (P) at eutectic point = 3 (solid Ag, solid Pb and solution of Pb and Ag)
Degree of freedom (F) = C - P + 1
= 2 - 3 + 1 = 0
The Ag-Pb system is non-variant and we do not need to specify any variable to define the system since both temperature and pressure are fixed.
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