Question

One of the compound below is called a refractory material? A. CaO B. CuO C. ZnO D. PbO

Answer 1

A. `"CaO"`- calcium oxide.

Explanation:

It seems that the question is trying to test students' understandings of the metal reactivity series along with predictions one can draw from this knowledge.

A material that

• has a high melting point and
• stands a lot of heat without undergoing structural changes

is considered "refractory." (Encyclopedia Britannica)

Now take a look at the four substances: they are all metal oxides in which the metal cation holds an oxidation state of `+2` (e.g., `stackrel(color(purple)(+2))(color(purple)("Ca"))"O"` which comprises myriads of `"Ca"^(2+)` and `"O"^(2-)`).

As ionic compounds, all four substances would expect to see a consistently-high melting point due to the significant amount of energy necessary to overcome the electrostatic attractions between the oppositely-charged ions.

As a result, the main refractory concern about these four substances would be their chemical stability at high temperature. Oxides of metals at the end of the activity series (i.e., unreactive ones, e.g., `"Cu"`, `"Au"`, `"Pt"`) tend to have low chemical stability at high temperatures and decomposes before melting. Oxides of reactive metals on the other end of the spectrum (e.g., `"K", "Ca"`, `"Na"`) are not as likely to undergo thermal decomposition and tend to be materials with satisfactory refractory capabilities.

Based on this reasoning it is likely that the refractory capability of the four oxides comes in the same order as the reactivity of their metal component.

• Chemical Reactivity:
  `"Ca" > "Zn" > "Pb" > "Cu"`
• Refractory Capability:
  `"CaO" > "ZnO" > "PbO" > "CuO"`