A, sodium hydroxide is alkali, hydrochloric acid belongs to acid, acid-base reaction produces salt and water, and the reaction of two compounds exchanging components to produce two new compounds belongs to double decomposition reaction, so A is correct;

B. The reaction between sodium hydroxide and hydrochloric acid is a neutralization reaction. The essence of the reaction is that hydrogen ions in acid and hydroxyl ions in alkali combine to form water molecules, so the number of water molecules increases after the reaction, so B is wrong;

C, before and after the double decomposition reaction, the valence of various elements remains unchanged, so C is correct;

D. The reaction between sodium hydroxide and hydrochloric acid is a neutralization reaction. The essence of the reaction is that hydrogen ions in acid combine with hydroxide ions in alkali to form water molecules, so D is correct.

So choose B.