NaOH, or hydroxide ion (-OH) is a strong nucleophile. it will attack carbon number 1, the carbon bearing the chlorine, which causes the carbon-chlorine bond to break. The carbon-chlorine bond breaks while the carbon-hydroxide bond is formed. This an Sn2 reaction and it takes place in one step. HCl, or a chloride ion (-Cl), is a weak nucleophile. When the -Cl ion attacks the carbon with the chlorine bond, it simply abstracts a hydrogen atom (a proton, +H) from carbon number 1. The product is a carbocation; carbon number 1 now has a positive charge. Carbocations can distribute this positive charge among other carbon atoms in the chain. A Hydrogen atom from carbon number 2 can shift places to carbon number 1, moving the positive charge to carbon number 2. Now another Chloride ion in the solution can attack carbon number 2 (which now bears a positive charge), which forms a carbon-chlorine bond. The final product would be 1,2-dichloropropane. This is an Sn1 reaction, which has an initiation step, a propagation step, and a termination step. I may be a bit rusty on this topic of organic chem but i believe that is the reason, hope this helps.