What information do you have that will help you identify your unknown?

this can be answered only if you give me the complete information of the lab and what test you performed to identify the compound.

SOLID: MgO, NaHCO3, CaCO3, unknown.
Conditions: solid alone before adding any liquid, solid with distilled water, solid with acid, solid and acid with universal indicator

acids can decompose carbonates and hydrogencarbonates to give CO2 gas