Why is CaO a Lewis base?

CaO is a Lewis base, as it is an electron pair donor.

In the Lewis definitions of acids and bases, a Lewis acid is defined as an electron pair 'acceptor', which will acquire an electron pair. A Lewis base is anything that gives this electron pair, hence the term 'donor'.

Taking this to the most simple level, an #"OH"^-# ion has an electron pair around the oxygen, which is what makes it a negative ion. This can bond with #"H"^+# ions as it does not have electrons, making it a positive ion.

These two ions can covalently bond and make water molecules.

![Chemical reaction from )

CaO can react in a redox reaction with carbon dioxide to give the following:

#"CaO" + "CO"_2 -> "CaCO"_3#

Applying the above principle (with the #"H"^+# and #"OH"^-#), the oxide ion in CaO donates an electron pair to the C, to form a bond between the two reactants, forming the product.

Thus because CaO donates an electron pair in this reaction, it is considered a Lewis base.

This may not be true for all reactions with CaO. In the event that CaO is the electron acceptor, then it would be the Lewis Acid instead for that reaction.

To understand more on this:
http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch11/lewis.php