Why does salt have a melting point 800c, but it dissolves in water so easily
In: Chemistry
The two are unrelated. The ability for a mineral to be water soluble is not a function of its melting point.
It’s not changing the chemical properties of the salt, it’s just wet salt. There is no bonding going on, it’s a just being held in a suspension.
The high melting point is because of its cristalline structure: the more tightly and strongly packed the molecules, the higher the melting point because it takes more energy to pull them loose. However, salts are made of ions, which are atoms with opposite electric charges, and water also has a separation of charges because of the characteristics of the atoms of hydrogen and oxygen: if you rub a pen with a cloth and put it beside a stream of water from your tap the stream bends. Because of this separation of charges water can dissolve salts so easily, oil for example cannot.
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.
Melting point is the temperature at which that pure substance is a liquid.
In the case of table salt (sodium chloride), that’s the temperature at which the bonds between sodium and chloride ions break down (ions = an atom or molecule with a positive or negative charge because the number of electrons is different to the number of protons. Can go into this and the structure of salt if you haven’t come across it).
Water dissolves table salt because water molecules take the place of sodium and chloride ions. The ions are happy enough to form bonds with the water instead of each other, and the water remains a liquid, with sodium and chloride ions dispersed in it.
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