This is an acid-base reaction (neutralization): NaOH is a base, (NH4)2SO4 is an acid.
Reactants:
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NaOH
Names: Sodium hydroxide, Caustic soda, Lye
Appearance: White, waxy, opaque crystals ; White hygroscopic solid in various forms ; Colorless to white, odorless solid (flakes, beads, granular form).
(NH4)2SO4 – Ammonium tetraoxosulfate (VI)
Other names: Ammonium sulphate, Ammonium sulfate, Ammonium sulfate (2:1)
Appearance: Fine white hygroscopic granules or crystals.
Products:
NH3
Names: Ammonia, Azane (only intended for use in naming derivatives of ammonia), Hydrogen nitride
Appearance: Colourless gas ; Colourless gas or compressed liquefied gas with pungent odour ; Colorless gas with a pungent, suffocating odor. [Note: Shipped as a liquefied compressed gas. Easily liquefied under pressure.]
Na2SO4
Names: Sodium sulfate, Sodium sulphate, Sulfate of sodium
Appearance: White crystalline solid | hygroscopic ; White hygroscopic solid in various forms
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