Definition of Boiling Point

It is a concept that, in general, we have associated with it, since it is routine to heat water for a tea, for example. Well, the processes of evaporation or boiling and fusion are physical changes that matter experiences and that are continually surrounding us.

When these processes are carried out, matter does not undergo a change in its chemical composition, only experiences changes in its state of aggregation, which, as we well know, in nature are: gas, liquid and solid.

Let's think about when in summer, on the beach, we go to the sea. Then, when we expose ourselves to solar radiation, we dry ourselves out. Here the evaporation of water occurs, from the liquid phase it becomes vapor. This occurs at a certain temperature given the pressure.

Similarly, if we place a liquid in an open container and heat it, we will see that, initially, only some particles have the

Kinetic energy enough to separate from the surface to form vapor. As we deliver more heat, more particles will tend to rise from the surface generating more vapor until, finally, all of the liquid mixture has evaporated. This is so because the container is open, if the container were closed, we would have a different case, where a Balance between the liquid phase and the gas phase. At first, the particles will detach from the sinus of the liquid. As we increase the amount of heat delivered, more particles pass into the gas phase, but in the same way increase the shocks between them, causing some to return to the liquid, until reaching an equilibrium where the speed return to liquid and egress of him is the same.

When we observe that a liquid begins to boil, forming bubbles under the surface, we know that, at that moment, the vapor pressure equaled the atmospheric pressure. Well, what is vapor pressure? It is the pressure exerted by the vapor in equilibrium with the liquid, indicating the ease with which said substance evaporates. In the case of the closed container, the equilibrium we are talking about occurs at vapor pressure.

So, the "Boiling Point" is the temperature at which a liquid substance equals its vapor pressure with that which is exerted on the surface. In the case of the open container, atmospheric pressure is exerted on the surface, so that the temperature at which the vapor pressure equals atmospheric pressure will be its temperature of boiling.

This knowledge about steam pressure is a concept used in many industrial processes as well as in our homes, such is the case with pressure cookers. It is similar to the example of the closed container since this pot is airtight, keeping the steam inside.

As we continue to deliver heat, the pressure on the surface of the liquid inside increases as it produces more steam, this produces a faster cooking as the water will have a higher boiling point than if it were open. In many of them a valve is observed in the upper part that allows the escape of the excess steam produced.

Examples of normal boiling point

Although we define the boiling point through a pressure exerted on the surface of the liquid, if that pressure is atmospheric pressure (101.325 kPa) the boiling temperature is known as "normal" at the level of the sea.

By virtue of the above, we can estimate that atmospheric pressure is a relevant parameter in the definition since, if we vary the height, we will vary the atmospheric pressure and, therefore, its point of boiling. In general, the normal boiling point values ​​of different substances measured at sea level are tabulated, some of them are:

Water: 100 ºC
Gasoline: 30-200ºC
Diesel: 280-350 ºC
Methanol: 64.7 ºC
Ethanol: 78.3 ºC

At the time of analyze the tendency of boiling points must be taken into account various factors, from the type of compound (ionic, covalent, metallic) to intermolecular forces and molar masses involved. Those substances that, at temperature environment have a certain facility to evaporate they are known as "volatile", note the case of alcohol, it is more volatile than water, therefore perfumes must always be kept closed due to their ease of evaporation.

Boiling Point Topics