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. 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. 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? Answer 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.
Exercise 2 Describe the formation of bubbles in a boiling liquid (see video for answer).
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 remove thermal energy from the liquid. 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)?  Answer The temperature of \(H_2O\) will only exceed 100 ºC once it has entirely changed to the gaseous phase. As long as there is liquid the temperature will remain constant.
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 (www.youtube.com/watch?v=aiwy...ature=youtu.be).
Exercise 4 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.
Answer 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.
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 (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. Contributors and Attributions
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Liquids & Intermolecular Forces11.3 Boiling Water at Room Temperature
Subjects: Properties of liquids, intermolecular forces, vapor pressure Description: Water is boiled at room temperature due to a decrease in pressure Materials:
Procedure:
Discussion: The boiling point of a liquid depends in part on intermolecular forces of attraction. An input of energy is needed to give the molecules enough kinetic energy to break free from these forces. The pressure above the liquid also determines boiling point. When the vapor pressure of the liquid is equal to the external pressure, the liquid will boil. When the external pressure is lowered, so does the boiling point. In the above procedure the pressure above the liquid is reduced by a vacuum pump. Enough pressure is lost to cause the water to boil. As the water boils, heat is lost because of the heat of vaporization of water being an endothermic process. Eventually the water will cool enough that its vapor pressure is less than the external pressure and the water will stop boiling. Safety: Use caution when working with evacuated glassware to avoid implosion. Disposal: None. References: 1. B.Z. Shakhashiri; Chemical Demonstrations: A Handbook for Teachers; Volume 2; Wisconsin; 1985; p. 81-84 This demonstration works well when discussing heating curves, energy required to heat a substance and the relationship among intermolecular forces and phase changes. Student Difficulties (Misconceptions) 1. A significant percentage of students think that when water boils hydrogen (H2) and oxygen gas O2(g) are produced. 2H2O(l) --> 2H2(g) + O2(g) ***An incorrect equation representing the boiling of water. The scientific view is the H2O molecules in the liquid phase gain enough energy to break the intermolecular forces between water molecules and enter the gas phase as H2O molecules. H2O(l) --> H2O(g) The energy added during the phase change of a liquid to a gas goes to breaking the intermolecular forces (hydrogen bonding, dipole-dipole, and London Dispersion Forces) of attraction. Water molecules gain energy and increase the distance among its neighboring water molecules, an increase in potential energy. When the distance increase the electrostatic force of attraction decreases according to Coulomb's Force Law. F = k [qi . q2)/r2] 2. A significant percentage of students think that when water boils the temperature increases and the average kinetic energy of the H2O increases. In fact, when a pure liquid boils the temperature of the liquid does not change, therefore the average kinetic energy of the liquid does not change. The Kinetic Molecular Theory explains that temperature of a system is proportional to the average kinetic energy. average KE = average 1/2 mv2 = 3/2[R/NA]T R is the gas constant 8.314 J/mol K NA is Avogadro's Number T is temperature in units of Kelvin Other Resources URL PHET http://phet.colorado.edu/en/simulation/states-of-matter-basics Learning Objectives 1. Describe, at the particle-level of representation, what the molecules are doing during a phase change. 2. Apply the terms Kinetic Energy and Potential Energy to describe what occurs to the system and to the molecules when heating liquid water and boiling the water. (states of matter). 3. Connect particle-level descriptions of matter, including different phases, with macroscopic observations and properties of matter. |
What happens to kinetic energy during boiling point?
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