Melting vs Dissolving
Melting and dissolving are theoretically physical and chemical phenomena, but they occur every day right in front of our eyes. Have you not seen ice melting down to water? Have you not seen how a cup of coffee is made? Well, those are melting and dissolving processes respectively that we witness every day. However, there is always the tendency to think both mean the same thing because, in the end something is turned in to a liquid as we observe.
Melting is a phase change. There are 3 main phases in which matter can exist. Those are solid, liquid, and gas. When a solid substance becomes its own liquid, this phenomenon is called “melting” or fusion. For a substance to melt, energy should be provided. This energy either can be supplied as heat or pressure. The temperature at which a solid turns liquid is called “melting point”. Since the phase change is in equilibrium; i.e. it can happen in both ways, it is also the “freezing point” for the reverse reaction.
What is melting? When a substance exists as a solid it has a crystal structure or a very rigid structure. For an instance, NaCl (salt) exists in a lattice structure where every Na+ is surrounded by 6 Cl- ions and every Cl- ion is surrounded by 6 Na+ ions. For this substance to be a liquid this crystal structure should break and it requires a lot of energy, indicating a very high melting point. Substances that could be easily broken in to a less ordered liquid state have lower melting points.
Dissolving, on the other hand, is not a phase change. It is simple when a substance is mixed with a liquid and is stabilized in a liquid medium. The substance, which is being dissolved, is called a “solute” and the medium it is dissolved in is called a “solvent” which together makes a “solution”. What happens in dissolving? If we take NaCl as the example again, we saw that it is quite difficult to melt it. But dissolving NaCl, say in water, is very easy comparatively. This is because when the ions Na+ and Cl- are separated in the liquid medium water molecules cover each of these by making “hydration spheres” around them. This stabilizes their existence in the liquid medium. As mentioned earlier, dissolving is not necessarily a solid stabilized in a liquid, but it could be another liquid, or even a gas. When consuming alcoholic beverages are mixed with another liquid soda, where a liquid is dissolved in another, and in soda we are aware that CO2 gas is dissolved in water.
What is the difference between Melting and Dissolving?
• Melting is a phase change (solid-liquid) but dissolving is not.
• To melt substance energy should be supplied either as heat or pressure but to dissolve it is generally not essential (some substances require energy to dissolve).
• For a substance to melt it should reach “melting point” temperature but for dissolving there is no such requirement.
• Molten substance is the pure liquid form of the solid which was melted but a solution is always a mixture of two or more.