Question

the bone kept in hydrochloric acid become soft, why

Answer 1

because your bones are made up of calcium and other metals when we keep in HCL it react and react with elements so it becomes soft

Answer 2

The calcium present in the bone dissolves in hydrochloric acid and makes it soft. This makes the bone dissolved in hydrochloric acid brittle.

Explanation:  

The calcium dissolves in the bone, but since the bone also contains other minerals such as hydroxyapatite, potassium, magnesium etc. it doesn’t melt completely but only becomes brittle and soft when dipped in hydrochloric acid, whereas when the bone is dissolved in a base such as KOH it doesn’t get affected.