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effects of impurities on melting point

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From Wikipedia


Impurities are substances inside a confined amount of liquid, gas, or solid, which differ from the chemical composition of the material or compound.

Impurities are either naturally occurring or added during synthesis of a chemical or commercial product. During production, impurities may be purposely, accidentally, inevitably, or incidentally added into the substance.

The level of impurities in a material are generally defined in relative terms. Standards have been established by various organizations that attempt to define the permitted levels of various impurities in a manufactured product. Strictly speaking, then, a material's level of purity can only be stated as being more or less pure than some other material.

Impurities can be destructive when they obstruct the working nature of the material. Examples include ash and debris in metals and leaf pieces in blank white papers. The removal of impurities is usually done chemically. For example, in the manufacturing of iron, calcium carbonate is added to the blast furnace to remove silicon dioxide from the iron ore. Zone refining is an economically important method for the purification of semiconductors.

However, some kinds of impurities can be removed by physical means. A mixture of water and salt can be separated by distillation, with water as the distillate and salt as the solid residue. Impurities are usually physically removed from liquids and gases. Removal of sand particles from metal ore is one example with solids.

No matter what method is used, it is usually impossible to separate an impurity completely from a material. The reason that it is impossible to remove imputities completely is of thermodynamic nature and is predicted by the second law of thermodynamics. Removing impurities completely means reducing the entropy of the system to zero. This would require an infinite amount of work and energy as predicted by the second law of thermodynamics. What technicians can do is to increase the purity of a material to as near 100% as possible or economically feasible.

Impurities and nucleation

When an impure liquid is cooled to its melting point the liquid, undergoing a phase transition, crystallizes around the impurities and becomes a crystalline solid. If there are no impurities then the liquid is said to be pure and can be supercooled below its melting point without becoming a solid. This occurs because the liquid has nothing to condense around so the solid cannot form a natural crystalline solid. The solid is eventually formed when dynamic arrest or glass transition occurs, but it forms into an amorphous solid— a glass, instead, as there is no long-range order in the structure.

Impurities play an important role in the nucleation of other phase transitions. For example, the presence of foreign elements may have important effects on the mechanical and magnetic properties of metal alloys. Iron atoms in copper cause the renowned Kondo effect where the conduction electron spins form a magnetic bound state with the impurity atom. Magnetic impurities in superconductors can serve as generation sites for vortex defects. Point defects can nucleate reversed domains in ferromagnets and dramatically affect their coercivity. In general impurities are able to serve as initiation points for phase transitions because the energetic cost of creating a finite-size domain of a new phase is lower at a point defect. In order for the nucleus of a new phase to be stable, it must reach a critical size. This threshold size is often lower at an impurity site.

From Yahoo Answers

Question:How can i crystallize my solute if the impurity is only 2mg and the solution is 100mg. Both have the same melting point.

Answers:It depends on the two materials. If they form a low melting point eutectic, they will both freeze cooperatively at a temperature lower than either melting point. Many solder alloys form eutectic systems. Classic example is Pb-Sn, the eutectic temp is lower than either Pb or Sn. In your case, even if they do not form a eutectic, there may be compositions that will freeze out at different compositions, A with a little bit of B will solidify and the "extra" B will stay in the liquid remaining. If you have a binary phase diagram, this information is on there. If you don't have a phase diagram, watch it freeze and see what happens. You might also find something that will be an effective nucleant for one solid phase that would promote solidification on one over the other (a piece of solid A might be ideal). Obviously you have to introduce the "seed crystal" at a temperature right at or below the MP of A). good luck

Question:substances: (stearic acid, benzoic acid and salicylic acid) 1) What is the effect of impurity on the melting point of a substance? 2)How can melting point and boiling point determinations be used in identifying and/or characterizing unknown compounds? 3)How do the observed melting points of the test compounds relate with the structures? help me please? i will very much appreciate it.

Answers:1) Generally, having impurity will decrease the melting point, and causes the melting point range to increase, due to colligative effects. 2) Think of it as a person's DNA/fingerprints. The two, when combined and comapred are relatively unique to a compound. 3) Generally, the larger the molecule, the higher the melting/boiling point. Order of decreasing melting/boiling point: Stearic acid, salicyclic acid, benzoic acid

Question:What will a soluble impurity do to the melting point of an organic compound?

Answers:The melting point will go lower or higher depending on the impurity. Most likely lower. Generally inorganic impurities make the mp higher Organic impurities make the mp lower.

Question:Why do you get a lower melting point (which is also over a broader range) if you try to test the melting point of a wet solid? I know that impurities in a sample lower the melting point, but I have to know WHY the above fact holds. Specifically, what does the presence of a liquid have to do with a lower melting point?

Answers:At the melting point, the vapour pressures of the solid and liquid phases are equal. A soluble impurity contributes to the total vapour pressure, therefore lowering the partial vapour pressure required of the pure substance in the melt and thus lowers the temperature necessary for melting. Dalton's law states that the total vapour pressure of a liquid solution is the sum of the partial pressures of the components. Additions of more and more impurity will produce corresponding lowerings in the partial vapor pressure of the pure substance and hence, lowering of the melting point. Finally, however, a limiting point is reached at which the impurity concentration is just sufficient to `saturate' the liquid; any additional impurity does not dissolve and cannot further depress the melting point. This point is known as the Eutectic Point and the limiting temperature is called the Eutectic Temperature and the composition of the melt, the Eutectic Composition. we will have the lowest melting point in solid state at this composition and temperature.