The chemical used to etch enamel is:

Etchants are relatively strong acids (pH = 1–2) used to remove smear layers and to dissolve the mineral phase to allow formation of micromechanical interlocking in enamel and in dentin. A number of acidic agents have been used to produce the required microporosity. However, phosphoric acid at a concentration between 30% and 50%, typically 37%, is the preferred etching agent to produce consistent etching patterns while not damaging the pulp. Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, 12th ed page no 262