What is the effect of impurities on boiling point? 

 

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In order to understand the effect of impurities on boiling point, one must first understand what boiling point is. The boiling point of a solution is generally defined as the the temperature at which the vapor pressure of a liquid is equal to the pressure of the gas above it. It is important to note that a "normal boiling point" of a solution is the temperature at which vapor pressure from the liquid will equal one atmosphere. It is at this temperature, that the liquid vapor will be released as a gas into the atmosphere. Importantly, the boiling point of a solution remains the same even if more heat is added after it starts to boil. 

Adding impurities to a solution, in most cases, increases the boiling point of the solution. This occurs because the presence of impurities decreases the number of water molecules available to become vaporized during boiling. Once this occurs, it takes a greater amount of heat to cause the same amount of impure solution to vaporize as it would take to cause a pure solution to vaporize, thus raising the solution's overall boiling point. It is important to realize however, that impurities do not always increase boiling point, and, in certain rarer cases, can actually cause boiling point to decrease. With this in mind, it is important to know exactly what impurities are being added to a solution in order to determine the final effect the impurities will have on boiling point. Hope this helps!

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What is the effect of impurities on the melting point of a compound?

Impurities present in a solid organic compound tend to have 2 effects on the melting point.  First, they tend to lower the overall melting point of the compound versus the value for pure material.  Second, they tend to increase the range of the melting point values.  Pure compounds tend to melt very quickly once they reach the correct temperature.  Impure compounds tend to melt more slowly over a larger range of temperature values.  Both of these effects are due to a weaking of the molecular lattice structure of the solid.  Pure solids tend to form stronger lattice bonds that require more energy (heat) to break up.  Impurities present tend to disrupt these lattice networks, thus requiring less energy to disrupt them.

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What's the effect of impurity on the boiling point of an organic compound?

Adding a soluble impurity to a liquid organic compound will raise the temperature at which the organic compound boils (in other words, the soluble impurity will increase the boiling point of the liquid organic compound). This is because, once dissolved, the impurity, which is acting as the solute (the thing that is dissolved in a solution), will form chemical bonds with the organic compound solvent (the thing that does the dissolving in a solution). These chemical bonds will have to broken before the liquid organic compound boils. The bonds are broken with the addition of heat energy, which means a higher boiling point. However, if the impurity is insoluble in the organic compound, then there will be effect on the boiling point. 

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What is the effect of impurities on the boiling point of a compound?

In general, adding impurities to compounds elevates their boiling points and lowers their freezing points.  You can think of this as extending the liquid range in both directions.  There are some examples where this does not hold true.  For example, if you add a small amount of water as an impurity to ethanol and heat it, you will get a mixture that is 95% ethanol and 5% water that boils over.  This is called an azeotrope and actually has a slightly lower boiling point than pure ethanol.  But azeotropic mixtures are usually limited to talking about mixtures of volatile organic solvents, not impurities in small molecules.

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