The question says that a solid W is formed, that glows. As far
as I know, a reaction between magnesium and steam produces
magnesium oxide and hydrogen. And in this reaction a "glow" is
seen. Since magnesium is about the only substance I know that
glows in a reaction, I chose that as the answer. I didn't know CaO
glows! Haha. Anyway, may I know how you can identify the right
choices smartly when you meet such ambiguous questions?
Lots of thanks
Any substance, when hot enough, glows (some of the heat
energy is transformed into light energy). And the question states
"at high temperature".
If you feel the question is ambiguous, you'll have to state all
possible answers, conditionally. See the 1st post of
my thread here (http://www.sgforums.com/forums/2297/topics/320107).
As for questions in which you have unknown compounds A, B, C, X,
Y, Z, etc, and are asked to identify them, you'll have to use
elimination.
Based on the info given in the question, start thinking of all
possible compounds that could be A, B, C etc, and as you consider
the rest of the info, start eliminating the possibilities until you
end up with the most likely answer.
For instance, you were right that it could be mangesium
(carbonate, oxide, hydroxide, etc), but the last sentence of the
question reveals it to be calcium. So you eliminate magnesium (or
barium, or whatever other metal it could possibly have been, from
the earlier info in the question).
Qualitative Analysis (Q.A.) is a fun topic, and the questions
are like puzzles for you to solve. Of course, in order to have fun
with them, you must first be familiar with your "identification of
cations, anions and gases".
Organic Chemistry (O.C.) can also have similar questions, of
unknown compounds X, Y, Z, etc.
For instance, one fun question I give my students :
Compound A liberates carbon dioxide gas with
calcium carbonate, but produces only a small percentage of
carbon dioxde gas (when reacted with calcium carbonate) as compared
with hydrochloric acid (of the same molarity as an aqueous solution
of A). Compound B decolourizes acidified potassium
manganate(VII). Both compounds A and B have a
molar mass of 74g each, and both compounds contain only 3
elements. When an equimolar mixture of A and B is boiled
(together with a few drops of sulfuric acid as catalyst), an
equilibrium mixture is obtained, which contains a new compound
C, which is sweet smelling.
Draw compounds A, B and C.