The correct statements about $$H_2O_2$$ are:
(A) used in the treatment of effluents.
(B) used as both oxidising and reducing agents.
(C) the two hydroxyl groups lie in the same plane.
(D) miscible with water.
Choose the correct answer from the options given below:

Let us examine each statement about $$H_2O_2$$ (hydrogen peroxide):

(A) $$H_2O_2$$ is used in the treatment of effluents. This is correct. Hydrogen peroxide is widely used as an oxidant for treating industrial and domestic effluents, as it can decompose organic pollutants.

(B) $$H_2O_2$$ acts as both an oxidising and reducing agent. This is correct. In $$H_2O_2$$, oxygen is in the $$-1$$ oxidation state, which is intermediate between $$0$$ (in $$O_2$$) and $$-2$$ (in $$H_2O$$). Therefore, it can be oxidised to $$O_2$$ (acting as a reducing agent) or reduced to $$H_2O$$ (acting as an oxidising agent).

(C) The two hydroxyl groups lie in the same plane. This is incorrect. The structure of $$H_2O_2$$ is non-planar. The two $$O-H$$ bonds are in different planes, with a dihedral angle of about $$111.5°$$ in the gas phase. The molecule has an open-book structure where the two $$H$$ atoms are not in the same plane as the $$O-O$$ bond.

(D) $$H_2O_2$$ is miscible with water. This is correct. Hydrogen peroxide is completely miscible with water in all proportions due to its ability to form hydrogen bonds with water molecules.

Statements (A), (B), and (D) are correct, while (C) is incorrect. The correct answer is option (2).