What would happen to the specific heat of water if salt is added?

Increasing the concentration of the salt decreases the specific heat capacity of the water.

Explanation:
When we heat a sample of water, the energy goes into raising the energy levels of its various vibrational, rotational, and translational motions.
When we dissolve
NaCl
in water, the ions are held in a rigid cage of water molecules.
The cage is rigid enough so that the motions of its molecules are "frozen out".

Answer is to avoid experimental error