About carbon dioxide and carbon monoxide

Carbon monoxide would have a lower melting point.
m.p. of CO: -205oC
m.p. of CO2: -57oC (under pressure)

This is because CO (CΞO) has a smaller molecular size than CO2 (O=C=O), and thus CO has weaker van der Waals’ forces between molecules than CO2. Less energy is needed to separate the molecules which are held together by weaker intermolecular forces.
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