Why does a polymer of nucleotide is referred to as nucleic acid?

Polymer of nucleotides is acidic in nature due to phosphate groups.

Explanation:

Nucleic acids are polymers of nucleotides. Each of the nucleotides contains one nitrogenous base, a pentose sugar, and a phosphate group. The polymerization of nucleotides occurs by the formation of phosphodiester bonds between them. The sugar-phosphate backbone with alternating sugar and phosphate groups is exposed to the surrounding medium while the nitrogenous bases are stacked upon each other in the interior away from the watery medium.

The phosphate groups are completely ionized and bear negative charge at the neutral pH making the polymer of nucleotide acidic in nature. The alkaline nitrogenous bases are not exposed and stacked towards the interior of the helix.