Passage VII
Students dissolved ammonium chloride (NH4CI) and silver cyanate (AgOCN) together in water. A solid, Product 1 (P1), formed and was isolated. Product 2 (P2) was then recovered from the water in the solution. Samples of the products had the properties listed below:
P1: Melting point > 400°C; hard and brittle; not soluble in water; not flammable; formula unknown
P2: Melting point =132°C; soft and nonbrittle; soluble in water; flammable; formula is CH4N2O
Two students discuss the identity of these products.
Student 1
NH4CI and AgOCN both are salts. Salts are composed of positively charged cations and negatively charged anions that hold together because they have opposite charges. When salts dissolve in water, they dissociate, or break apart, into anions and cations. When NH4CI and AgOCN are dissolved in water, the following occurs:
NH4CI + AgOCN → NH4+ + CI- + Ag+ + OCN-
The ions in solution then form new salts, silver chloride (AgCl) and ammonium cyanate (NH4OCN):
NH4+ + Cl- + Ag+ + OCN- → AgCl + NH4OCN
P1 is AgCl: AgCl is insoluble in water, and salts tend to be hard and brittle, to be nonflammable, and to have high melting points. P2 is NH4OCN, which is a salt composed of an ammonium ion and a cyanate ion:
[NH4+][OCN-]
This sample of NH4OCN is soft and nonbrittle because it absorbed water from the air after it was isolated. This also caused the melting point to be lower than expected for a salt.
Student 2
P1 is AgCl, but P2 is not a salt. Ammonium ions do react with cyanate ions to form NH4OCN, but this compound is unstable. To isolate P2, the solution was heated at 70°C to remove the water. This caused the salt to rearrange and form a more stable covalent compound. In a covalent compound, the atoms are held together by bonds, not by their opposite charges. P2 is urea, which has the following structure:
Covalent compounds are generally not as brittle as salts and tend to have lower melting points. Also, covalent compounds are much more likely to be flammable than are salts. Urea is also a compound that is found in mammalian urine, so it must be soluble in water.
38. Which of the following claims about the compounds discussed in the passage, if true, would NOT be consistent with Student 2's hypothesis?
Your Answer is
Correct Answer is J
Explanation
The first line of Student 2 says that P2 is not salt, and later it says that P2 is urea
So if urea is salt, the student's statement is contradictory