13.6 Humidity, Evaporation, and Boiling
5 min read•june 18, 2024
Water vapor plays a crucial role in our atmosphere, affecting everything from weather to comfort. Understanding and helps us grasp how water moves between liquid and gas states. These concepts are key to predicting weather patterns and managing indoor climates.
and are two ways water transforms from liquid to gas. While evaporation happens gradually at the surface, boiling occurs throughout the liquid when vapor pressure equals atmospheric pressure. These processes impact daily life, from drying clothes to cooking food.
Humidity and Vapor Pressure
Water vapor and air capacity
Top images from around the web for Water vapor and air capacity
Evaporación - Wikipedia, la enciclopedia libre View original
Is this image relevant?
Chapter 26: Interactions Involving Conditions – Inanimate Life View original
Is this image relevant?
Phase Changes | Physics View original
Is this image relevant?
Evaporación - Wikipedia, la enciclopedia libre View original
Is this image relevant?
Chapter 26: Interactions Involving Conditions – Inanimate Life View original
Is this image relevant?
1 of 3
Top images from around the web for Water vapor and air capacity
Evaporación - Wikipedia, la enciclopedia libre View original
Is this image relevant?
Chapter 26: Interactions Involving Conditions – Inanimate Life View original
Is this image relevant?
Phase Changes | Physics View original
Is this image relevant?
Evaporación - Wikipedia, la enciclopedia libre View original
Is this image relevant?
Chapter 26: Interactions Involving Conditions – Inanimate Life View original
Is this image relevant?
1 of 3
- Vapor pressure is the pressure exerted by water vapor molecules in a closed system at equilibrium
- As temperature increases, water molecules gain kinetic energy and evaporate more readily, increasing vapor pressure
- Air's capacity to hold water vapor is directly related to the difference between the current vapor pressure and the
- vapor pressure is the maximum pressure that water vapor can exert at a given temperature
- When vapor pressure equals saturation vapor pressure, air is saturated and cannot hold any more water vapor (100% )
- The greater the difference between current vapor pressure and saturation vapor pressure, the more water vapor the air can hold before reaching saturation (low relative humidity)
Relative humidity and partial pressure
- Relative humidity is the ratio of the actual water vapor pressure in the air to the saturation vapor pressure at a given temperature, expressed as a percentage
- Relative humidity = (Actual vapor pressure / Saturation vapor pressure) × 100%
- Example: If the actual vapor pressure is 20 hPa and the saturation vapor pressure is 30 hPa, the relative humidity is (20 / 30) × 100% = 66.7%
- is the pressure exerted by a single gas in a mixture of gases, in this context, the pressure exerted by water vapor in the air
- As the of water vapor in the air increases, relative humidity increases
- When the partial pressure of water vapor equals the saturation vapor pressure, relative humidity is 100%, and the air is saturated ()
Calculations with vapor pressure
- is the mass of water vapor per unit volume of air, typically expressed in grams per cubic meter (g/m³)
- Vapor density = (Vapor pressure × Molar mass of water) / (Gas constant × Temperature)
- Molar mass of water: 18.02 g/mol
- Gas constant: 8.314 J/(mol·K)
- Temperature must be in Kelvin (K), convert from Celsius using K = ℃ + 273.15
- Vapor density = (Vapor pressure × Molar mass of water) / (Gas constant × Temperature)
- Humidity can be calculated using vapor pressure and saturation vapor pressure
- is the mass of water vapor per unit volume of air, similar to vapor density
- Absolute humidity = (Vapor pressure × Molar mass of water) / (Gas constant × Temperature)
- is the ratio of the mass of water vapor to the total mass of the air (including water vapor)
- Specific humidity = (Molar mass of water × Vapor pressure) / (Molar mass of air × Total pressure - Vapor pressure × (Molar mass of air - Molar mass of water))
- is the mass of water vapor per unit volume of air, similar to vapor density
Dew point and humidity relationship
- is the temperature at which the air becomes saturated with water vapor (relative humidity reaches 100%) when cooled at constant pressure
- At the dew point, water vapor condenses into liquid water (dew, fog, or clouds)
- The dew point is significant because it indicates the temperature at which will occur
- If the air temperature drops below the dew point, water vapor will condense, forming dew, fog, or clouds
- The difference between the current air temperature and the dew point is an indicator of the air's capacity to hold additional water vapor
- A smaller difference between air temperature and dew point indicates higher humidity, as the air is closer to saturation
- Example: If the air temperature is 25℃ and the dew point is 20℃, the air is relatively humid, while if the air temperature is 25℃ and the dew point is 10℃, the air is relatively dry
Evaporation and Boiling
Explain the process of evaporation and the factors that affect its rate
- Evaporation is the process by which liquid water molecules gain enough kinetic energy to overcome intermolecular forces and escape into the air as water vapor
- Factors affecting the rate of evaporation:
- Temperature: Higher temperatures increase the average kinetic energy of water molecules, making it easier for them to escape the liquid phase (hot water evaporates faster than cold water)
- Surface area: A larger surface area allows more water molecules to be exposed to the air, increasing the rate of evaporation (wide shallow pan evaporates faster than a narrow deep one)
- Air movement: Moving air removes water vapor from the surface, allowing more evaporation to occur (wind increases evaporation rate)
- Humidity: Lower humidity (lower vapor pressure) in the surrounding air allows more water molecules to escape, increasing the rate of evaporation (dry air increases evaporation rate)
- Evaporation is a that requires energy, known as the latent
Describe the process of boiling and how it differs from evaporation
- Boiling is the rapid of a liquid when its vapor pressure equals the atmospheric pressure
- At the boiling point, vapor pressure equals atmospheric pressure, allowing bubbles of vapor to form throughout the liquid
- Differences between boiling and evaporation:
- Location: Evaporation occurs only at the surface of the liquid, while boiling occurs throughout the liquid
- Temperature: Evaporation can occur at any temperature, while boiling occurs at a specific temperature (boiling point) for a given pressure
- Bubble formation: Boiling involves the formation of vapor bubbles within the liquid, while evaporation does not
- Factors affecting the boiling point:
- Atmospheric pressure: Lower atmospheric pressure reduces the boiling point, while higher atmospheric pressure increases the boiling point (water boils at a lower temperature at high altitudes)
- Solutes: Adding solutes to a liquid increases the boiling point through the process of boiling point elevation (salt water has a higher boiling point than pure water)
Phase changes and energy
- Phase changes, such as evaporation and boiling, involve the absorption or release of energy
- The heat of vaporization is the amount of energy required to change a substance from liquid to gas at constant temperature
- The is a unique combination of temperature and pressure where a substance can exist simultaneously in solid, liquid, and gas phases
Key Terms to Review (29)
Absolute Humidity: Absolute humidity is a measure of the amount of water vapor present in the air, expressed as the mass of water vapor per unit volume of air. It is an important concept in understanding the behavior of water, evaporation, and boiling in various environments.
Boiling: Boiling is the process of a liquid transitioning to a gaseous state due to the application of heat. It is a key phase change that occurs when the vapor pressure of a liquid equals the pressure surrounding the liquid, allowing bubbles of vapor to form throughout the liquid.
Clausius-Clapeyron equation: The Clausius-Clapeyron equation is a fundamental relationship that describes the equilibrium vapor pressure of a substance as a function of temperature. It is a critical tool for understanding phase changes and the behavior of substances in different thermodynamic states.
Condensation: Condensation is the process by which water vapor in the air is converted into liquid water. It is a fundamental phase change that occurs when the temperature of a gas is lowered below its dew point, causing the water vapor to condense into tiny droplets or a thin film of liquid water on a surface.
Deposition: Deposition is the process by which a gas or vapor transitions directly into a solid state, bypassing the liquid phase. This phase change occurs when the temperature and pressure conditions are such that the gas cannot remain in its gaseous form and instead condenses onto a surface or forms solid crystals.
Dew point: The dew point is the temperature at which air becomes saturated with moisture and water vapor begins to condense into liquid water. It is a key indicator of humidity levels in the atmosphere.
Dew Point: The dew point is the temperature at which water vapor in the air condenses into liquid water, forming dew. It is a crucial measure of the amount of moisture in the air and is closely related to the concepts of humidity, evaporation, and boiling point.
Evaporation: Evaporation is the process through which liquid molecules gain enough energy to transition into the gas phase, occurring at temperatures below the boiling point. This phenomenon is essential in various natural processes and influences humidity levels in the atmosphere, as well as the boiling point of liquids.
Heat of sublimation: Heat of sublimation is the amount of energy required to change a substance from a solid phase directly to a gas phase without passing through the liquid phase. It is usually measured in joules per gram (J/g) or kilojoules per mole (kJ/mol).
Heat of Vaporization: The heat of vaporization is the amount of energy required to transform a substance from a liquid state to a gaseous state at a constant temperature and pressure. It is a fundamental concept that connects the topics of humidity, evaporation, boiling, heat, temperature change, heat capacity, and phase change.
Humidity: Humidity refers to the amount of water vapor present in the air. It is a measure of the moisture content in the atmosphere and plays a crucial role in various physical processes, including evaporation and boiling.
Hygrometer: A hygrometer is a device used to measure the amount of water vapor or humidity present in the surrounding air. It plays a crucial role in understanding the relationships between humidity, evaporation, and boiling, which are important concepts in the study of physics.
Latent Heat: Latent heat is the energy released or absorbed by a substance during a phase change, such as the transition from solid to liquid or liquid to gas, without a change in temperature. It is the energy required to change the physical state of a substance while maintaining a constant temperature.
Latent heat coefficients: Latent heat coefficients represent the amount of heat required to change the phase of a unit mass of a substance without changing its temperature. They are crucial in understanding phase changes like melting, freezing, boiling, and condensation.
Partial pressure: Partial pressure is the pressure exerted by a single type of gas in a mixture of gases. It is proportional to its mole fraction and the total pressure of the mixture.
Partial Pressure: Partial pressure is the contribution of a specific gas to the total pressure of a mixture of gases. It represents the pressure that each individual gas would exert if it were the only gas present in the same volume. The concept of partial pressure is essential in understanding the behavior of gas mixtures and their applications in various fields, including the Ideal Gas Law, phase changes, and humidity.
Percent relative humidity: Percent relative humidity is the ratio of the current absolute humidity to the highest possible absolute humidity at a given temperature, expressed as a percentage. It indicates how close the air is to being saturated with water vapor.
Phase Change: A phase change is the transition of a substance from one physical state or phase to another, such as the transformation between solid, liquid, and gas. These changes occur due to the addition or removal of energy, which affects the arrangement and motion of the substance's molecules.
Psychrometer: A psychrometer is an instrument used to measure the relative humidity of the air by determining the difference in temperature between a dry-bulb thermometer and a wet-bulb thermometer. It plays a crucial role in understanding the relationship between humidity, evaporation, and boiling point.
Relative humidity: Relative humidity is the ratio of the current amount of water vapor in the air to the maximum amount that the air can hold at a given temperature, expressed as a percentage. It indicates how close the air is to being saturated with moisture.
Saturation: Saturation is the point at which a substance can no longer absorb or dissolve any more of another substance and additional amounts will appear as separate phases. In the context of humidity, it refers to air holding the maximum amount of water vapor possible at a given temperature.
Saturation Vapor Pressure: Saturation vapor pressure is the maximum pressure exerted by the vapor of a substance when it is in equilibrium with the liquid or solid phase of the same substance at a given temperature. It is a crucial concept in understanding the behavior of gases, liquids, and phase changes, particularly in the context of humidity, evaporation, and boiling.
Specific Humidity: Specific humidity is a measure of the amount of water vapor present in the air, expressed as the ratio of the mass of water vapor to the total mass of the air-water mixture. It is an important parameter in understanding the behavior of air, particularly in the context of humidity, evaporation, and boiling.
Sublimation: Sublimation is the process by which a substance transitions directly from a solid state to a gaseous state, without passing through the intermediate liquid phase. This phase change occurs when the vapor pressure of the solid exceeds the pressure of the surrounding environment, allowing the solid to vaporize.
Triple point: The triple point is the unique set of conditions at which all three phases (solid, liquid, and gas) of a substance coexist in thermodynamic equilibrium. It is characterized by a specific temperature and pressure for each substance.
Triple Point: The triple point is a unique condition in which the solid, liquid, and gaseous phases of a substance can coexist in equilibrium. It represents the specific temperature and pressure at which all three phases of a material can be present simultaneously.
Vapor Density: Vapor density is the mass of a vapor per unit volume, typically expressed in grams per liter or kilograms per cubic meter. It is a crucial property that governs the behavior of gases and vapors, particularly in the context of humidity, evaporation, and boiling.
Vapor Pressure: Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature. It is a fundamental property that governs the behavior of liquids and gases, particularly in the context of humidity, evaporation, and boiling point.
Vaporization: Vaporization is the process by which a liquid transforms into a gas, occurring when molecules at the surface of the liquid gain enough energy to break free from intermolecular forces. This process can happen in two main ways: evaporation, which occurs at any temperature, and boiling, which happens at a specific boiling point. Understanding vaporization helps explain phenomena like humidity, heat transfer during phase changes, and the energy required for substances to change from one state to another.