Eutectic System – a Mixture of Chemical Compounds

A eutectic system is a homogeneous mixture of substances that melts or solidifies at a single temperature that is lower than the melting point of any of the constituents. It is a homogeneous, solid mixture of two or more substances that form a super-lattice; the mixture either melts or solidifies at a lower temperature than the melting point of any of the individual substances.

A eutectic system is a mixture of chemical compounds or elements that have a single chemical composition that solidifies at a lower temperature than any other composition made up of the same ingredients.

A eutectic system only forms when there is a specific ratio between the components. A eutectic system contains two solid phases at low temperatures. These phases may have different crystal structures or the same crystal structure with different lattice parameters. The eutectic temperature is the lowest possible melting temperature over all of the mixing ratios for the involved component species. The eutectic temperature is the lowest possible melting temperature for all of the mixing ratios of the component substances in a eutectoid.

The phrase most commonly refers to a mixture of alloys. Upon heating any other mixture ratio and reaching the eutectic temperature, one component’s lattice will melt first, while the temperature of the mixture has to further increase for (all) the other component lattice(s) to melt. A eutectic alloy is an alloy formed from two or more components that exhibits eutectic behavior. A eutectic alloy melts at a distinct temperature.

Several examples of eutectic systems or eutectoids exist, in metallurgy and in various other fields. These mixtures typically have useful properties that are not possessed by any single constituent substance:

  • Sodium chloride and water form a eutectoid when the mixture is 23.3% salt by mass with a eutectic point at -21.2 degrees Celsius. The system is used to make ice cream and to melt ice and snow.
  • The eutectic point of the mixture of ethanol and water is nearly pure ethanol. The value means there is a maximum proof or purity of alcohol that can be obtained using distillation.

Eutectics are a minimum-melting composition of two or more components, each of which melts and freeze congruently forming a mixture of the component crystals during crystallization. Conversely, as a non-eutectic mixture cools down, each mixture’s component will solidify (form its lattice) at a distinct temperature, until all material is solid. Not all binary alloys have a eutectic point; for example, in the silver-gold system, the melt temperature and freeze temperature both increase monotonically as the mix changes from pure silver to pure gold.

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