Difference Between Boiling Point And Melting Point
Heat a liquid in an open container, and eventually bubbles form. The first rush of bubbles is often air, driven out of the liquid by the increasing temperature. Then, true vapor bubbles begin to form. We say that the liquid is boiling. Recipes that instruct the cook to boil a mixture for, say, 1 minute, are trying to guarantee that the bubbles being observed represent more than just air being released.
Boiling occurs because heat increases the motion, or kinetic energy, of the molecules within the liquid to the point where they vaporize, regardless of whether they are at the upper surface of the liquid or deep within it.
A liquid’s boiling point is very much related to its vapor pressure. Specifically, a liquid boils at the temperature at which the vapor pressure of its liquid is equal to the pressure above the liquid. In an open container, this “outside” pressure is atmospheric, or air, pressure. A liquid’s boiling point at ordinary air pressure is called its standard, or normal, boiling point.
It follows, then, that lowering the pressure above a liquid would lower its boiling point. This is why it is easier to boil a liquid on a mountaintop than at sea level (air pressure decreases with altitude).
Freezing or Melting Point
Now let us consider what happens when a liquid is cooled rather than heated. Just as heating increased the motion of the liquid’s molecules, cooling decreases their motion. As cooling continues, the molecules begin to settle into more- and more-fixed positions. Eventually, they arrange themselves into a three-dimensional pattern, with set spacing between each molecule. The liquid has become a solid. We say it has solidified, crystallized, or frozen. Add a little heat to the solid at this point, and it will begin to return to liquid form. We say it is liquefying, or melting.
The temperature at which a substance’s liquid form is in perfect balance with its solid form can be called its freezing point. But it is equally accurate to call it the melting point. The two phases of the substance are in equilibrium.
It is important to note, however, that some solid substances do not melt. Instead, they skip the liquid phase, passing directly into the gas state in a process called sublimation.