Q.Why water in the fish pot needs to be changed time to time ?

A reducing agent must be oxidisable; carbon in CO2 is at its maximum +4 state, so CO2 cannot act as a reducing agent.

A species acts as a reducing agent only if the central atom can be oxidised to a higher oxidation state. In CO2 carbon is already at its maximum oxidation state, +4 (group 14), so it cannot lose electrons further and cannot reduce anything. In NO2 (N is +4, can go to +5), SO2 (S is +4, can go to +6) and ClO2 (Cl is …

ClO2 is an odd-electron (free-radical) molecule and is therefore paramagnetic.

A species is paramagnetic if it has one or more unpaired electrons. Counting valence electrons: …

O2 has 2 unpaired electrons (in the pi* orbitals), making it paramagnetic.

By molecular orbital theory, the 12 valence electrons of O2 fill the orbitals up to the two pi* (antibonding) orbitals, each of which gets one electron with parallel spins (Hund's rule):

... (pi 2p)4 (pi* 2p)2

…

A species is paramagnetic if it has unpaired electrons; among the options only O2 does.

Paramagnetism arises from the presence of unpaired electrons in a species. Let us examine each option:

• H2O: all electrons are paired (covalent O–H bonds and lone pairs) → diamagnetic.
• O2: molecular orbital theory gives the configuration with two unpaired electrons in the degenerate pi*(2p) antibonding orbitals. These two unpaired electrons make O2 paramagnetic — a classic result that explains why liquid oxygen is attracted to a magnet. …