Why does the electrical conductivity of salt water increase with increase in temperature? Is it due to the mobile ions that vibrate with higher energy that increases their conductance? It is noteworthy that the electrical conductivity in metals decreases as the temperature is increased but, why is it different from metals and solutions/liquid?
 A: Conductivity versus temperature is different for those two classes of materials because the nature of the bonds between the atoms of each are fundamentally different, which gives rise to fundamentally different conduction mechanisms for each. 
In the case of salt dissolved in water, anything which increases the mobility of ions in aqueous solution is going to increase its electrical conductivity. Hot water means more mobility, which means more conductivity. 
In the case of metals, they are conductive because the electrons shared by the metal atoms are delocalized, free to roam throughout the solid. Heating the solid increases the amplitude of vibration of the metal atoms in the crystal lattice, which increases the amount of scattering experienced by the electrons as they drift through the solid under the influence of an applied electric field. This causes the conductivity to go down as the temperature goes up. 
A: At high temperature the bonds break easily and hence the no. of ions increase to increase the conductivity. This conductivity will become constant after a specific temperature due to the limitations of no. of ions i.e. the no. of ions will get fixed and hence no further increase in conductivity. 
