i am typing up my lab report and the question that has me confused is as follows

Question

i am typing up my lab report and the question that has me confused is as follows:

A broad melting range is usually indicative of an impure substance. Give an example of a situation in which a pure substance could give a broad melting range.

I dont even know where to start looking for the answer to that question, and the second part of the question is pretty opposite as is stated as follows:

A sharp melting range is usually indicative of a pure substance. Give an example of a situation in which an impure substance melts sharply.

I would assume that aspirin could be a pure substnace that has a broad melting point range, but I also don't know how to describe that in the paper

thank you

Explanation / Answer

Broad melting point ranges typically indicate that a substance is not pure. The exception is when pure substance

decomposes on heating. When this happens, the decomposition products act as impurities in the sample and

cause a broadening of the melting point range. A darkening of the sample color or evolution of a gas during

heating is an indication that the sample is decomposing

------------------------------------------------

Consider now what happens in the reverse situation; that is, when a solid containing 30 mole percent naphthalene and 70 mole percent 1-naphthol

is heated. When the temperature reaches 61? (the eutectic temperature), both 1-naphthol and naphthalene melt together. The temperature of the

sample remains constant until no more solid naphthalene is present. The resulting liquid has the composition of the eutectic mixture. As heating is

continued, the temperature rises, and the remaining solid (which is entirely 1-naphthol) melts. As it does so, the liquid is enriched in 1-naphthol.

The composition of the liquid follows the curved line until Point B is reached, where the last crystal of 1-naphthol melts to give a homogeneous

liquid mixture. Continued heating simply raises the temperature of the liquid without causing any further phase changes (until the sample begins

to boil, that is.)

The above discussion elaborates on some of the generalizations typically made regarding melting behavior and the purity of solids; namely, that

an impure compound melts over a temperature range rather than sharply at a single temperature, and that the more nearly pure the sample the

higher its melting point. For example, if the original mixture of solids contained only 5 mole percent of naphthalene, the last crystal of 1-naphthol

would disappear at 94? rather than 83?.

Related Questions

Navigate

• Browse (All)
• Browse I
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.