1. PAUSE CLICK Let’s examine choice A.
QUESTION:
Samples A, B, and C appear to be identical. Each sample is found to melt sharply at 122oC. However, a mixture of sample A and B is found to melt from oC, while a mixture of sample A and C is found to melt sharply at 122oC. Which of the following is true? A. Samples A and B are composed of the same substance
B. Samples A and C are composed of the different substances C. A mixture of B and C will have a sharp melting point D. None of the above
SCRIPT:
Samples A, B, and C appear to be identical. Each sample is found to melt sharply at 122oC.
However, a mixture of sample A and B is found to melt from oC, while a mixture of sample A and C is found to melt sharply at 122oC.
Which of the following is true? A. Samples A and B are composed of the same substance
B. Samples A and C are composed of the different substances C. A mixture of B and C will have a sharp melting point D. None of the above
PAUSE CLICK
A sharp melting point and a sharp boiling point is characteristic of pure substances (one element or one compound).
When we say that a sample has a “Sharp melting point,” we mean that the sample’s temperature remains constant while it’s melting. In practice, a “constant” temperature means “within about 1 degree.”
PAUSE CLICK Let’s examine choice A.
If samples A and B were composed of the same substance, then a mixture of the two would not really be a mixture. It would still be a pure substance,
which would melt sharply at exactly the same temperature as A and B. That’s not the case here. Mixing samples A and B gives us something with a wide melting temperature range … 115 to 120 degrees…
HIGHLIGHT oC Therefore, samples A and B are definitely not the same substance.
CROSS OUT same
Therefore, choice A is false.
How about choice B….
Mixing samples A and C gives us a sample that still melts sharply at the same temperature as both samples A and C. Therefore, samples A and C must be identical substances. Choice B is False.
CROSS OUT different
Let’s look at choice C….
Since we have established that A and B are not identical, but A and C are identical, we can conclude that B and C are different.
We, therefore, expect a mixture of B and C to have a wide melting point range.
CROSS OUT sharp
Choice C is also false.
The correct answer is D. None of the statements given in choices A, B, and C is true.
PAUSE CLICK END RECORDING
A sharp melting point and a sharp boiling point are characteristics of pure substances (one element or one compound). “Sharp melting point” means temperature remains constant while sample is melting. In practice, a “constant” temperature means “within about 1 degree.”

2. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08