What is the Definition of Evaporation in Chemistry?

by Lucy Bell-Young

Evaporation is a natural or synthesised process by which liquid substances transition into a gaseous state with the action of heat. 

The water cycle, which involves the evaporation and precipitation of water, is one of the major driving forces of the weather and global climate, and one of the best examples of evaporation.

Evaporation is generally a cooling process that has several industrial, commercial, and domestic applications. For instance, on a large scale, the cooling towers of nuclear power plants evaporate water to maintain the temperature in the reactors at optimal levels, preventing meltdowns.

Meanwhile, on a smaller scale, evaporation is important in the air conditioning systems of homes and refrigerators. In A/C systems, for instance, evaporation is one of the modes that facilitates heat exchange between the interior of a building and the exterior environment. During this process, heat is pumped out of the building through the condenser coils and evaporator coils.

What Does Evaporation Mean?

Evaporation is a physical change in states of matter: it’s the change from a liquid state into a gas, without boiling the liquid. Depending on the liquid involved, it doesn’t typically require high temperatures. For example, some liquids, like alcohols, are highly volatile at room temperature under normal atmospheric pressure.

The process of evaporation, condensation, and precipitation of water, otherwise known as the water cycle, is one of the most powerful driving forces that determine the weather and climate in our world. This process also sustains life by providing fresh water on the land surface. Evaporation is also key in regulating the temperature of the oceans, which in turn determines the frequency and patterns of hurricane or typhoon formations.

In fact, water evaporation is as important as water absorption in plants. For example, it’s because of evaporation that trees are able to grow very tall. As the water evaporates through the stomata of the leaves of tall trees, water is pulled up via capillary actions and negative pressure on top. This allows the trees to grow very tall as nutrient-rich water is able to reach the top.

So that the water in the roots can reach the leaves, intermolecular forces facilitate the flow of water through the xylems and phloems of the trees. Water molecules become chains inside the narrow xylem tubes. The water molecules on top then pull the molecular ‘chains’ inside the xylems, allowing the water to flow upwards.


What Happens During Evaporation?

During evaporation, the molecules on the surface of a liquid are vaporised, changing phase from liquid to gas. Various factors can affect the evaporation rate of the liquid, including:

  • Saturation level: The gas phase of an evaporated substance is considered a dissolved solute in the air or surrounding gas. Therefore, the air or surrounding gas is the solvent. If the solvent is already saturated with solute, the evaporation rate decreases.
  • Atmospheric or gas pressure: All types of liquids evaporate at room temperature and at normal one atmosphere of pressure. Increasing the pressure will slow down the evaporation rate.
  • Temperature: Heat energy is the main driving force of evaporation. Gas molecules move faster as temperature rises. This means that the higher the temperature, the faster the rate of evaporation.
  • Nature of the substance: Some liquids like alcohol, ether, acetone, and gasoline are highly volatile because they have high vapour pressures. This means they evaporate more easily. Highly volatile liquids are also highly flammable.
  • Surface area of the substance: A liquid that’s dispersed on a larger area has more surface exposed, making it easier for the surface molecules to escape and evaporate. For example, spilled rubbing alcohol evaporates faster than the same amount of alcohol left in an open container.

What is the Difference Between Evaporation and Boiling?

Evaporation happens below a liquid’s boiling point. Although the vaporisation of a liquid also occurs at boiling point, the temperature of the vapour is hotter compared to the vapour of ordinary or normal evaporation.

Technically, evaporation is defined as the ‘phase transition’ of a liquid into a gas when the liquid is still below its boiling point. As long as the substance is not completely frozen at absolute zero Kelvin temperature (minus 273.15°C), evaporation can occur even below the normal room temperature.

On the other hand, the boiling point of a liquid has a precise and highly specific definition. It’s when vapour pressure is equal to standard seal level pressure. This is the reason why the boiling point decreases as the pressure is reduced.

The boiling point of a substance is usually specified as under one atmosphere of pressure, which is equivalent to 101.325 kilopascals or 760 millimetres of mercury at sea level. 

For example, the boiling point of ethanol, which is the active ingredient of wines and spirits, is 78.37°C. Knowing the boiling point of substances is important in the distillation process. For instance, spirits of higher concentrations can be distilled from wines and other spirits in a precise manner.

Diagram showing the difference between evaporation and boiling

At What Temperature Does Water Evaporate?

Theoretically, as long as the molecules of a substance have energy to move, some molecules will be able to escape even below the freezing point of that substance. However, evaporation is technically defined as a phase change from liquid to gaseous state. If it’s a solid form that changes into a gaseous state, bypassing the liquid state, it’s called sublimation, such as in the case of dry ice and moth balls (naphthalene). Even water in the form of ice and snow can sublimate.

Therefore, the safe scientific answer to the question is this: there is no exact temperature at which water evaporates, but more of a range of temperatures. 

It’s below the boiling point of water, which is 100°C, but above the absolute zero temperature. As long as there is energy input, there is always the possibility that some molecules will be able to escape, transitioning from solid to liquid and then to gas.

Do All Liquids Evaporate At the Same Rate?

Different types of liquids have different evaporation rates. Under ideal conditions, the main factor that affects the evaporation rate of a substance is the intermolecular forces of the liquid. Substances that have stronger intermolecular forces have lower rates of evaporation. This is directly correlated with the vapour pressure of the substance.

Vapour pressure is the pressure of the vapour from a liquid in a closed container at certain temperatures. It’s directly proportional to temperature. Below are examples of liquids in their corresponding vapour pressures at different temperatures:

Table showing the vapour pressure of three different liquids at different temperatures

Evaporation rate is a comparative measurement. It’s simply the ratio of the time necessary to completely evaporate a given amount of a liquid compared to a comparison liquid. The test liquid must be subjected under the same conditions as the comparison/reference liquid.

How Does Temperature Affect the Rate of Evaporation Experiment?

The rate of evaporation is directly proportional to the liquid’s temperature. The higher the temperature, the faster the evaporation rate. At higher temperatures, molecules move more quickly. This facilitates the escape of some molecules by changing phase.

You can design a simple experiment to test the relationship between temperature and the rate of evaporation. Just make sure that all the parameters are the same, except for temperature. For instance, you can try the experiment shown in the video below:

This experiment is an inverse way of determining the relationships between the rate of evaporation and temperature. Temperature is the dependent variable here, while the rate of evaporation is the independent variable. We know that alcohol has a higher rate of evaporation than water. Therefore, we can predict that the temperature readings on the thermometer will be significantly lower when the alcohol-soaked cotton is placed on the thermometer bulb.


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