Acids: Compounds which are having positive monatomic hydrogen ion are named as acids.
Bases: Compounds with hydroxide ion as negative component are termed as bases.
Oxides: If the negative component of the compound is monatomic oxide ion, then those compounds are termed as oxides.Salts: Compounds which does not fall in any of the above three classes are known as salts.
(a) NaOH: Positive component of NaOH is monatomic sodium ion.
Negative component of NaOH is hydroxide ion.
There is no positive monatomic hydrogen ion in NaOH, so it is not an acid.
(b) NH4Cl: Positive component of NH4Cl is ammonium ion.
Negative component of NH4Cl is chloride ion.
There is no positive monatomic hydrogen ion in NH4Cl, so it is not an acid.
(c) HBr: Positive component of HBr is monoatomic hydrogen ion.
Negative component of HBr is bromide ion.
There is a positive monatomic hydrogen ion in HBr, so HBr is an acid.
(d) H3PO4: Positive component of H3PO4 is monoatomic hydrogen ion.
Negative component of H3PO4 is phosphate ion.
There are three positive monatomic hydrogen ions in H3PO4, so it is an acid.