Boiling is the process by which a liquid turns into a vapor when it is heated to its boiling point. The change from a liquid phase to a gaseous phase occurs when the vapor pressure of the liquid is equal to the atmospheric pressure exerted on the liquid. Boiling is a physical change and molecules are not chemically altered during the process.
How does boiling occur?
When atoms or molecules of a liquid are able to spread out enough to change from a liquid phase to a gaseous phase, bubbles form and boiling occurs.
Figure 1: Formation of bubbles in boiling water.
Video: Boiling basics (https://www.youtube.com/embed/Py0GEByCke4).
The boiling point is the temperature at which boiling occurs for a specific liquid. For example, for water, the boiling point is 100ºC at a pressure of 1 atm. The boiling point of a liquid depends on temperature, atmospheric pressure, and the vapor pressure of the liquid. When the atmospheric pressure is equal to the vapor pressure of the liquid, boiling will begin.
A liquid will begin to boil when:
Atmospheric Pressure = Vapor Pressure of Liquid
Exercise 1: Boiling Basics
When a liquid boils, what is inside the bubbles?
The bubbles in a boiling liquid are made up of molecules of the liquid which have gained enough energy to change to the gaseous phase.
Describe the formation of bubbles in a boiling liquid (see video for answer).
Temperature and Boiling
When boiling occurs, the more energetic molecules change to a gas, spread out, and form bubbles. These rise to the surface and enter the atmosphere. It requires energy to change from a liquid to a gas (see enthalpy of vaporization). In addition, gas molecules leaving the liquid take away heat energy. Therefore the temperature of the liquid remains constant during boiling. For example, water will remain at 100ºC (at a pressure of 1 atm or 101.3 kPa) while boiling. A graph of temperature vs. time for water changing from a liquid to a gas, called a heating curve, shows a constant temperature as long as water is boiling.
Exercise 3: Heating Curve for Water
Based on the heating curve below, when will the temperature of \(H_2O\) exceed 100ºC (in an open system)?
The temperature of \(H_2O\) will only exceed 100ºC once it has entirely changed to the gaseous phase. As long as there is liquid the temperature will remain constant.
Atmospheric Pressure and Boiling
The pressure of gas above a liquid affects the boiling point. In an open system this is called atmospheric pressure. The greater the pressure, the more energy required for liquids to boil, and the higher the boiling point.
Higher Atmospheric Pressure = More Energy Required to Boil = Higher Boiling Point
In an open system this can be visualized as air molecules colliding with the surface of the liquid and creating pressure. This pressure is transmitted throughout the liquid and makes it more difficult for bubbles to form and for boiling to take place. If the pressure is reduced, the liquid requires less energy to change to a gaseous phase, and boiling occurs at a lower temperature.
Video: Atmospheric Pressure and Boiling (https://www.youtube.com/watch?v=aiwy...ature=youtu.be).
Based on the atmospheric pressure, predict the boiling point for water at the following locations. Remember that water boils at 100ºC at sea level on earth. Assume constant temperature.
- Earth at Sea Level: 101.3 kPa
- Mount Everest Summit: 33.7 kPa
- Mars (average): 0.6 kPa
- Venus (surface): 9200 kPa
Since water boils at 100ºC, water would boil quickly on Mars (actual value us about 10ºC). The boiling point on Mt. Everest would be closer to water (actual value about 70ºC). On Venus water would boil well over 100ºC.
Vapor Pressure and Boiling
The molecules leaving a liquid through evaporation create an upward pressure as they collide with air molecules. This upward push is called the vapor pressure. Different substances have different vapor pressures and therefore different boiling points. This is due to differing intermolecular forces between molecules.
Video: Vapor Pressure and Bioling (https://youtu.be/ffBusZO-TO0)
The vapor pressure of a liquid lowers the amount of pressure exerted on the liquid by the atmosphere. As a result, liquids with high vapor pressures have lower boiling points. Vapor pressure can be increased by heating a liquid and causing more molecules to enter the atmosphere. At the point where the vapor pressure is equal to the atmospheric pressure boiling will begin. In effect, without any external pressure the liquid molecules will be able to spread out and change from a liquid to a gaseous phase. The gas, as bubbles in the liquid, will rise to the surface and be released into the atmosphere.
Note: Boiling Point
Boiling occurs when the vapor pressure of the liquid is equal to the atmospheric pressure.
Wayne Breslyn, NBCT, Ph.D. (Gaithersburg High School)
- Chadwick Wyler