What Is Evaporation?
Evaporation is a surface phenomenon by which a liquid changes into a vapour. The evaporation of liquid occurs at a temperature below the boiling point. The principle of evaporation states that in order for liquid molecules to evaporate, they must be close to the surface, moving in the right direction, and have enough kinetic energy to overcome the intermolecular forces present between the molecules in the liquid phase.
Evaporation Process and the Rate of Evaporation
The rate of evaporation is low when few molecules match certain requirements. Higher temperatures promote evaporation more quickly because a molecule's kinetic energy is directly related to its temperature. The more quickly moving molecules escape or evaporate, the lower the average kinetic energy of the remaining molecules and the lower is the temperature of the liquid. Evaporative cooling is another name for this process. This is also the reason why sweating causes the body to become cooler.
Higher gas-to-liquid flow rates and liquids with higher vapour pressure both tend to speed up the evaporation process. For instance, on a windy day as opposed to a calm day, washing drying on a clothesline (via evaporation) will occur more quickly. Heat, atmospheric pressure, which controls the humidity percentage, and air movement are the three main factors in evaporation. In a liquid, only a small percentage of the molecules have enough heat energy to escape as vapour. Until equilibrium is attained, where the liquid's evaporation is equal to its condensation, evaporation will continue. A liquid will evaporate in a closed space until the air is completely saturated.
The distinction between the liquid state and the vapour state is not clearly defined at the molecular level. In its place, a Knudsen layer exists where the phase is unknown. A distinct phase transition interface cannot be detected at the macroscopic scale since this layer is just a few molecules thick.
Factors Affecting Evaporation
The concentration of a chemical that is evaporating in the air: The chemical substance will evaporate more slowly if there is already a significant amount of the evaporating substance in the air.
Air flow rate: This is partially influenced by the concentration. The concentration of the substance in the air is less likely to increase over time, promoting quicker evaporation, if "fresh" air is constantly passing over the substance (air that is neither already saturated with the substance nor with other compounds). The reason for this is that when the flow rate increases, the boundary layer at the evaporation surface thins, reducing the diffusion distance in the stagnant layer.
Intermolecular forces: More energy is required to escape the force that is holding the molecules together in a liquid state. The enthalpy of vaporisation serves as a defining characteristic of this.
Pressure: If there is less pressure on the surface preventing the molecules from launching themselves, evaporation proceeds more quickly.
Surface area: A material with a greater surface area may evaporate more quickly because there are more surface molecules per unit of volume that may be able to escape.
The temperature of the substance: The molecules at its surface have more kinetic energy at higher temperatures, which causes them to evaporate more quickly.
Examples of Evaporation
Drying clothes in the sun, potholes and water drying up on the street, the cooling effect of sweat on the body, tea and other hot beverages cooling down, drying of nearby water bodies like lakes and ponds and the drying of wet hair are a few examples of evaporation.
Applications of Evaporation
Evaporation is used in numerous printing and coating processes, salt recovery from solutions, and drying a range of products like chemicals, paper, fabric, and timber.
Evaporation is frequently used as a pre-processing step for various laboratory tests, including spectroscopy and chromatography, in order to dry or concentrate materials. Centrifugal and rotary evaporators are two examples of the systems utilised for this.
Even if the air temperature is below the boiling point of water when garments are put on a washing line, water evaporates. Factors like low humidity, heat (from the sun), and wind speed up this process.
Water and other liquids are traditionally stored and cooled in the porous clay Matki/Matka, an object from India is to use evaporation to chill the water inside.
By simply passing dry air over a filter that has been soaked with water, evaporative coolers may effectively cool a building.
Naturally, evaporation of water from ponds, lakes and oceans plays a major role in the water cycle and causes rainfall.
Important Questions
1. What is evaporation? Give examples.
Evaporation is the process that changes liquid to gaseous vapour. Examples of evaporation are cooling down a hot beverage, drying clothes, sweating on a hot summer day, etc.
2. What are the factors affecting evaporation?
A liquid is transformed into a vapour by the process of evaporation. Surface area, temperature, humidity, and wind speed, the concentration of the substance in the surrounding air are a few factors that affect how quickly liquid evaporates.
Summary
When a liquid transitions from the liquid phase to the gaseous phase, evaporation takes place on the liquid's surface. High concentrations of the evaporating material in the surrounding air considerably slow down evaporation. The way that the liquid's molecules collide determines how much energy is transferred to one another.
In order to escape and enter the surrounding air as a vapour, a liquid molecule close to the surface must absorb enough energy to exceed the vapour pressure. Evaporative cooling is the process of reducing a liquid's temperature as a result of evaporation, which removes energy from the evaporated liquid.
Practice Questions
What type of change is evaporation?
Physical change
Chemical change
Electrochemical change
Thermal change
What will be the rate of evaporation if the humidity in the atmosphere is high?
Evaporation is higher
Evaporation is lower
Has no effect on evaporation
Both a and b
Answers:
(a)
(b)
FAQs on Evaporation: A Surface Phenomenon
1. What is evaporation and why is it specifically called a surface phenomenon?
Evaporation is the process where a liquid changes into its gaseous or vapour state at a temperature below its boiling point. It is called a surface phenomenon because the process only occurs at the surface of the liquid. Only the molecules at the surface that gain enough kinetic energy can overcome the intermolecular forces of attraction and escape into the air as vapour.
2. What are the main factors that influence the rate of evaporation?
The rate of evaporation is influenced by several key factors. An increase in these factors generally leads to faster evaporation:
- Temperature: Higher temperatures increase the kinetic energy of molecules, leading to faster evaporation.
- Surface Area: A larger surface area exposes more liquid molecules to the air, increasing the rate of evaporation.
- Wind Speed: Higher wind speeds carry away water vapour from the surface, allowing more liquid to evaporate.
- Humidity: Lower humidity (drier air) means the air can hold more water vapour, thus increasing the evaporation rate.
3. How does the process of evaporation cause a cooling effect?
Evaporation causes cooling through a process called evaporative cooling. During evaporation, the liquid molecules absorb energy from their surroundings to gain enough energy to turn into vapour. This absorption of heat from the immediate environment—be it our skin when we sweat or a cup of hot tea—lowers the temperature of the surroundings, resulting in a cooling effect.
4. What is the fundamental difference between evaporation and boiling?
The key difference lies in where and how the process occurs. Evaporation is a slow, silent process that happens only at the liquid's surface and can occur at any temperature below the boiling point. In contrast, boiling is a rapid, noisy process that happens throughout the entire bulk of the liquid (a bulk phenomenon) and occurs only at a specific temperature known as the boiling point.
5. What are some common examples of evaporation we see in our daily lives?
Evaporation is a common process that we observe frequently. Some everyday examples include:
- Wet clothes drying on a clothesline in the sun.
- The cooling effect of sweat on our skin on a hot day.
- Water puddles on the road disappearing after a while.
- A hot cup of tea or coffee slowly cooling down.
- The drying of wet hair after a shower.
6. Why do we spread out wet clothes to help them dry faster?
Spreading out wet clothes significantly increases the surface area of the fabric exposed to the air and sun. A larger surface area allows more water molecules to escape into the atmosphere simultaneously. This directly increases the rate of evaporation, causing the clothes to dry much more quickly than if they were left bundled up.
7. How does high humidity in the air affect the evaporation of water from a pond?
High humidity means that the air is already saturated, or nearly saturated, with water vapour. Because the surrounding air can't hold much more moisture, the rate at which water molecules can escape from the pond's surface is significantly reduced. Therefore, on a humid day, the rate of evaporation from the pond will be very slow compared to a dry day.
