But-2-enedioic acid exists as two stereoisomers: maleic acid (cis) and fumaric acid (trans). Explain why but-2-enedioic acid can exist as cis and trans stereoisomers, and explain how the cis and trans forms of but-2-enedioic acid differ from each other in terms of the arrangement of their atoms.

The C=C double bond has a π bond, so rotation about the bond is restricted, giving a fixed arrangement of the groups attached to each sp² carbon.
Each sp² carbon of the double bond is bonded to a –COOH group and to a hydrogen atom, so the two groups on each carbon are different.
Because of the lack of rotation, the two –COOH groups can either be on the same side of the double bond (cis, maleic acid) or on opposite sides (trans, fumaric acid).
In the cis isomer the two –COOH groups lie on the same side of the C=C plane, whereas in the trans isomer they lie on opposite sides.
Thus the two compounds have identical molecular formulas and bond connectivity but differ in the spatial arrangement of the –COOH groups around the double bond.