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Which of the following compounds reacts with calcium oxide, $\ce{CaO}$?
(a) $\ce{K2O}$
(b) $\ce{Na2O}$
(c) $\ce{SO2}$
(d) $\ce{MgO}$

I thought that since sodium is higher in the reactivity series than Ca, then it would be B. However, the answer is C, and I'm not quite sure why. Can anyone enlighten me?

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If a reaction takes place there should be a reasonable chemical equation.

Calcium oxide and sulfur dioxide may react to give calcium sulfite: $$\ce{CaO + SO2 -> CaSO3}$$

On the other hand alkali metal oxides and alkaline earth metal oxides do not form mixed oxides.

The reactivity series gives us information about the feasibility of redox reactions. Since no redox reactions can occur here, the reduction potentials are not relevant.

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One way of thinking about it is that calcium is a metal and therefore forms a basic oxide, $\ce{CaO}$. On the other hand, sulfur is a non-metal and forms acidic oxides $\ce{SO2}$ (or $\ce{SO3}$). So, the reaction between $\ce{CaO}$ and $\ce{SO2}$ can be considered an acid-base reaction.

There is a somewhat obscure acid-base theory, the Lux–Flood theory, which is applicable to such reactions. In this context, bases are oxide donors (like $\ce{CaO}$) and acids are oxide acceptors (like $\ce{SO2}$).

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in this way, you maybe unsderstand it better: CaO consists of two ions:Ca(+II) and O(-II). SO2 will accept a dative bond from oxide ion and form (SO3 2-) which would form CaSO3 eventually.this mechanism maybe helpful to you.

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