What is Boiling Point?
The boiling point of a liquid is the temperature at which the liquid is able to increase its thermal energy to the point that it is able to transition into a vapor. This point is also known as a ‘saturation temperature.’
Saturation temperature is known to be the point at which a saturated liquid passes from the point of having as much thermal energy as it can without boiling, into the point where it becomes a saturated vapor an has as little thermal energy as it can without condensing.
Boiling point is dependent on temperature and pressure. The phenomena of boiling occurs when the vapor pressure of a gas escaping from a liquid is equal to the pressure that is being exerted on the liquid in its current surroundings or environment. Therefore, when additional thermal energy is added to a saturated liquid, it is able to begin transitioning into a vapor state.
The Difference Between Evaporation and Boiling Point
The process of the liquid transitioning into a vapor state (evaporation) begins with the molecules that are on the surface are not as weighted down by the pressure of the liquid. These molecules are more likely to receive enough thermal energy to transition into a state of vapor, but the thermal energy they receive isn’t enough to impact the molecules farther inside the liquid.
Boiling point is different in that the entire liquid receives enough thermal energy to overcome the liquid pressure and transition into a vapor state. This is why boiling point is characterized by bubbles rising to the surface of the liquid – the vapor is created from within the liquid and able to travel up through the liquid pressure and escape into the atmosphere.
Why Do Liquids Have Different Boiling Points?
As a general rule, the more molecules there are inside polar and nonpolar molecules, the higher their boiling point will be. This is because the higher a molecule’s weight is, the stronger the Van der Waals forces inside that molecule will be. If a molecule has strong Van der Waals forces to overcome, it will have to have a higher boiling point (with more thermal energy) to reach the necessary state.
For instance, when comparing butane (C4H10) to hexane (C6H14), you can see that butane has fewer molecules that hexane does. Because this is easily observed, it can quickly be concluded that hexane has a higher boiling point than butane.
The boiling point of a liquid can also be influenced by the type of bonds that are held between molecules. The following is a list of the most powerful bonds to least powerful bonds in descending order.
Ion-Ion bonds are the strongest intermolecular force that can be held between liquids. This is because these bonds are a result of formal charges. The charges that are held between the ion-ion bonds increase the strength of the forces that hold the bonds between the two molecules. This makes for a more powerful bond that requires more thermal energy to overcome – therefore resulting in a higher boiling point.
Ion-Dipole bonds are caused when an ionic substance dissolves inside a dipole substance. A dipole is a molecule that has a side that is more negative and a side that is more positive. One of the best examples that can be given of a liquid dipole is that of water.
Water is comprised of hydrogen and oxygen. Because oxygen is more electronegative than hydrogen, there is one side of the molecule that is more positively charged (the side with the oxygen atoms) and one side that is more negatively charged (the side with the hydrogen atoms). These two charges interact with ionic compounds differently. For example, when salt (NaCl) is dissolved into water, the positive sodium atoms (Na) form bonds with the negative side of the atom, while the negative chlorine atoms (Cl) form bonds with the more positive side of the water molecule.
The resulting bonds between the dipole (water) and the ion (salt) raises the boiling point of water. This is why it is common to add salt to water when cooking. Adding salt to water changes the bonds in the liquid from dipole-dipole to ion-dipole which allows the boiling point of the liquid to be raised. As a result of having a raised boiling point, it is easier to cook food faster.
Dipole-Dipole bonds are caused when two dipoles are able to interact together. When this happens, the side of the dipole that is more positively charged will move in a way that allows it to interact with the side of the molecule that is more negatively charged. The stronger the partial charges between the molecules are, the higher the boiling point of a dipole-dipole liquid.
However, because the partial charges of a dipole aren’t as strong as the formal charges that can be found inside ions, the boiling points of liquids with dipole-dipole interactions aren’t as high as the boiling points of liquids with ion-dipole or ion-ion interactions. This is why water has a lower boiling point if there is no ion (like salt) added to its solution.
Van der Waals Interactions
Van der Waals forces are found in all molecules. For molecules that can’t form permanent dipoles with covalent bonds or molecules that cannot create formal charges through ions, Van der Waals forces alone determine their boiling point. These atoms are likely to create momentary dipoles that can in turn create induced dipoles. Although this is effective, it as powerful as the charges that are created between dipole-dipole interactions, ion-dipole interactions, or ion-ion interactions. Because of this, liquids which rely on Van der Waals interactions will always have a lower boiling point than all the other liquid types.