Question

Why a common salt said to be substance and salty water a mixture?

Answer 1

Common salt is said to be substance because it is made of matter and it is also called pure substance because it is made up of only one types of element ...

and salty water is called mixture because it has two different matter water and salt which are made up of two different types of element

Answer 2

common salt is considered as substance because It is a compound and the ratio of the element envolved i.e., Na and Cl is fixed..
But salty water is a mixture because there is a solvent (water) and solute(salt) to make salt solution or salty water.