Electrolytes are chemicals that break into ions (ionize) when they are dissolved in water. The positively-charged ions are called cations, while the negatively charged ions are called anions. Substances can be categorized as strong electrolytes, weak electrolytes. A strong electrolyte is a solution/solute that completely, or almost completely, ionizes or dissociates in a solution. These ions are good conductors of electric current in the solution.
Weak electrolytes only partially break into ions in water. Weak electrolytes include weak acids, weak bases, and a variety of other compounds. KCl is a strong electrolyte and hence mains a linear relation. Whereas, NH4OH is a weak electrolyte and hence has a curve nature.