Why does salt have a melting point 800c, but it dissolves in water so easily
In: Chemistry
Salt is a crystalline ionic solid. Individual positively charged sodium atoms and negatively charged chlorine atoms are held together by the electrostatic interactions ( positive attracts negative and vice verse). You can think of this structure as a collection of hard spheres being held together in an extended solid. The energy of all these individual charges holding each other together is called the lattice energy. You should also note that bond forming releases energy while bond breaking consumes it.
To melt a solid like this you have to put in enough thermal energy that the vibrations of all these atoms can overcome their attractive force. It’s important to note that they never stop attracting each other, it’s just that the ambient thermal vibrations are more dominant.
However, when you add salt to water a different effect becomes involved. Water can be thought of as a little magnet, with a partial positive on one side and a partial negative on the other. These partial charges can also interact with the charged atoms in the salt. Importantly, when an atom in the salt interacts with enough water molecules it can equal and cancel the charge of the ion. This effectively blocks the charged atom, let’s say a sodium atom, from interacting with a chlorine. In the same way water can interact with chlorine preventing it from interacting with sodium.
The result of this is a balance, where the energy needed to break the interaction of the sodium-chlorine bond is largely equaled by the energy of forming the ion-water bonds. Since the atoms are engaging in bond breaking and bond forming largely simultaneously, minimal additional energy is needed.
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