Video Transcript
The following reaction can be used
as a chemical test for water: CuSO4 solid plus five H2O liquid reacts reversibly to
produce CuSO4·5H2O solid. If the backward reaction is
endothermic, what happens when water is added to a test tube containing CuSO4
solid? (A) The CuSO4 solid will turn
pink. (B) The CuSO4 solid will turn
white. (C) The test tube will heat up. (D) The test tube will cool
down. Or (E) no reaction will occur.
In this question, we want to know
what will happen when water is added to a test tube containing copper sulfate. We are told that the backward
reaction is endothermic. This means that the forward
reaction, where water is added to anhydrous copper sulfate, will be exothermic. In an exothermic reaction, heat is
transferred to the surroundings. So, we would expect the test tube
to heat up.
It appears that answer choice (C)
is our correct answer. But to confirm, let’s look at our
other choices. We have already discussed the
exothermic and endothermic components of this reaction, so we know that answer
choice (D) is incorrect. Anhydrous copper sulfate is a white
solid, and the hydrated form of the salt is a brilliant-blue color. So, CuSO4 solid turns blue, not
white or pink. Thus, neither option (A) or (B) is
correct. It is important not to get confused
between copper and cobalt as cobalt salts can often have a pink color.
Answer choice (E) tells us that no
reaction will happen, which we know not to be the case as we are given a chemical
reaction equation in the question. Thus, answer choice (E) is
incorrect. Therefore, when water is added to a
test tube containing CuSO4 solid, the test tube will heat up, which is answer choice
(C).
