How do you tell the boiling or melting point of an impure subtance of it has a range of them?

Answer 1

that depends if it is a mechanical mixture, or a chemical one...

in case of mechanical mixtures ( like sugar mixed with salt) where no chemical intreaction between the substance takes place, its easy. each substance will melt or boil according to its own properties. that is what takes place is oil refineries...

where the substances have reacted chemically and new substances were formed, the problem is more difficult. i dont really think you can in any way calculate or predict the new substances chemical properties, except in a very general way... of course it is highly probable that the new substance is already well known to science and its properties can be looked up in some tome of lore or other...

Answer 2

The only way to really identify the melting point of a specific substance would be to measure it. Basically this involves putting a sample of the substance into a thin tube and apply controlled heat. The substance would be monitored visually until it begins to melt, then the temp would be recorded.

If you are tallking about calculating these:

The freezing point and boiling points of a liquid can be calculated if there is a known amount of solute dissolved in them and if the identity of that solute is known.

A change in freezing point is as follows:
temp change = i*Kf*m

A change in boiling point is as follows:
temp change = i*Kb*m

where i is the number of particles dispersed in the solvent when one solute particle dissolves. (For molecular substances in water, usually this number = 1. But ionic substances, in water, would dissociate into separate ions so every formula unit of NaCl would produce 2 ions, every formula unit of CaCl2 wold produce 3 ions, etc.)

Kf is the freezing point constant for the solvent.

Kb is the boiling point constant for the solvent.

(Tables listing Kb and Kf values for various solvents are printed in most general chemistry texts.)

m is the molality of the solution.

Once the temp change is calculated, it is added or subtracted (depending on the sign of the K value) to the boiling/freezing point of the pure solvent.

In this way you would be able to determine the temp at which a KNOWN solution would freeze or boil.

You can find further explanation and a link to some sample problems here:
http://dbhs.wvusd.k12.ca.us/webdocs/Solutions/BP-Elev-and-FP-Lower.html

Answer 3

I think it depends on the percentage composition of each individual substance, and then it is in between their individual melting/boiling points.