How does salt change the specific heat capacity of water?
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
The cage is rigid enough so that the motions of its molecules are "frozen out".
The added heat cannot go into increasing the energy of these motions, so it goes into increasing the energies of the other water molecules in the solution.
It takes less energy to activate these molecules, so the specific heat of the water decreases.
The greater the concentration of NaCl, the lower the specific heat capacity of the solution.