It can be both but ammonium chloride is usually purchased as a solid crystalline material.
Magnesium Nitride is a greenish yellow powder at room temperature. So it's solid. I think you are a little confused on your terminology aqueous refers to something being dissolved in water. Magnesium...
PbSO4 is a solid that will not dissolve in water.
its aqueous when dissolved in water and solid (@STP) when not
It is a solid. it's a salt, called ammonium chloride: NH4Cl