When equal volumes of 0.1 M HCl and 0.1 M NaOH are mixed, a neutralization reaction occurs.

Here's why:

* HCl is a strong acid, meaning it completely dissociates in water to form H+ ions and Cl- ions.

* NaOH is a strong base, meaning it completely dissociates in water to form Na+ ions and OH- ions.

When these solutions are mixed, the H+ ions from the acid react with the OH- ions from the base to form water (H2O):

H+ (aq) + OH- (aq) → H2O (l)

The remaining ions, Na+ and Cl-, remain in solution as spectator ions, meaning they don't participate in the reaction.

Since the acid and base are of equal concentration and volume, they completely neutralize each other. The resulting solution will be neutral with a pH of 7.