A double-displacement reaction is a type of chemical reaction in which the positive and negative ions of two compounds switch places, forming two new compounds. In the given equation, HNO3 and KOH are the reactants, and KNO3 and H2O are the products. The positive ions in HNO3 are H+ and the negative ion is NO3-. The positive ion in KOH is K+ and the negative ion is OH-. In the products, the positive ions are K+ and H+ and the negative ions are NO3- and OH-. As you can see, the positive and negative ions have switched places, so this is a double-displacement reaction.
The other three equations are not double-displacement reactions. In the first equation, Zn (s) reacts with 2HCl (aq) to produce H2 (g) and ZnCl2 (s). This is a single-displacement reaction, because only one element is replaced by another element. In the second equation, Ba(OH)2 reacts with 2HClO4 to produce Ba(ClO4)2 and 2H2O (l). This is a neutralization reaction, because an acid and a base react to produce a salt and water. In the third equation, NaOH reacts with HCl to produce NaCl and H2O. This is also a neutralization reaction.
