NiS (s) + 10 HNO₃ (aq) → Ni(NO₃)₂ (aq) + 8 NO₂ (l) + H₂SO₄ (aq) + 4 H₂O (l)

This is an oxidation-reduction (redox) reaction:

S^-II - 8 e^- → S^VI (oxidation)

8 N^V + 8 e^- → 8 N^IV (reduction)

NiS is a reducing agent, HNO₃ is an oxidizing agent.

Reactants:

NiS – Nickel sulfide {{cite web | url=https://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=28094&loc=ec_rcs
- Other names: Nickel sulfide , Nickel monosulfide , Nickelous sulfide , Nickel(II) sulfide
- Appearance: Black solid
HNO₃ – Nitric acid
- Other names: Aqua fortis , Spirit of niter , Eau forte
- Appearance: Colorless, yellow or red fuming liquid ; Colourless-to-yellow liquid with pungent odour ; Colorless, yellow, or red, fuming liquid with an acrid, suffocating odor. [Note: Often used in an aqueous solution. Fuming nitric acid is concentrated nitric acid that contains dissolved nitrogen dioxide.]

Products:

Ni(NO₃)₂ – Nickel(II) nitrate
- Other names: Nickel nitrate , Nickelous nitrate , Nitric acid , Nickel(2+) salt
- Appearance: Emerald green hygroscopic solid
NO₂ – Nitrogen dioxide
- Other names: Nitrogen(IV) oxide , Deutoxide of nitrogen , Nitrogen peroxide
- Appearance: Brown gas ; Reddish-brown gas or brown or yellow liquid with pungent odour ; Yellowish-brown liquid or reddish-brown gas (above 70°F) with a pungent, acrid odor. [Note: In solid form (below 15°F) it is found structurally as N2O4.]
H₂SO₄ – Sulfuric acid
- Other names: Oil of vitriol , Sulfuric acid, concentrated (> 51% and < 100%) , H2SO4
- Appearance: Clear, colorless liquid ; Odourless colourless oily hygroscopic liquid ; Colorless to dark-brown, oily, odorless liquid. [Note: Pure compound is a solid below 51°F. Often used in an aqueous solution.]
H₂O – Water, oxidane
- Other names: Water (H2O) , Hydrogen hydroxide (HH or HOH) , Hydrogen oxide
- Appearance: White crystalline solid, almost colorless liquid with a hint of blue, colorless gas

NiS (s) + 10 HNO3 (aq) → Ni(NO3)2 (aq) + 8 NO2 (l) + H2SO4 (aq) + 4 H2O (l)