Noah Vinter
Alcohol is often added to water to speed up evaporation because it has a lower boiling point and heat of evaporation. This means that for the same amount of liquid, more alcohol evaporates than water, and at a faster rate. The exact mechanism behind this is that alcohol and water are highly soluble in each other, and when mixed, the solution evaporates faster than water alone due to the lower boiling point of ethanol (78.3°C) compared to water (100°C).
| Characteristics | Values |
|---|---|
| Boiling point of ethanol | 78.3°C |
| Boiling point of water | 100°C |
| Ethanol evaporates faster than water | True |
| Water molecules | Attract and stick together |
| Alcohol molecules | Less attraction and stickiness |
| Heat of evaporation | Lower in alcohol than water |
| Evaporation rate | Faster for alcohol than water |
| Cooling effect | Greater for alcohol than water |
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What You'll Learn
Ethanol's lower boiling point
Ethanol has a lower boiling point than water, with ethanol's boiling point being 78.3°C or 82°C, and water's being 100°C. This is due to the high degree of hydrogen bonding in liquid ethanol, which results in a higher degree of order and lower configuration entropy. The lower boiling point of ethanol means that it evaporates more quickly at the same temperature as water, and it takes some water vapour along with it.
The fact that ethanol evaporates faster than water is utilised in various applications. For instance, ethanol is used to remove water residue from cleaned lab equipment. In addition, placing a small amount of water and ethanol in two separate cups and observing their evaporation rates over time can be an interesting experiment to conduct.
Furthermore, ethanol's faster evaporation rate contributes to its cooling effect when applied to the skin. As ethanol evaporates faster, it carries away more heat from the skin, resulting in a greater cooling sensation compared to water.
In summary, ethanol's lower boiling point compared to water is a result of its molecular structure and hydrogen bonding. This property of ethanol has practical applications in various contexts, including laboratory procedures, scientific experiments, and cooling applications.
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Water and ethanol's azeotrope
An azeotrope is derived from the Greek words "ζέειν" (boil) and "τρόπος" (turning), with the prefix "α-" (no) to give the overall meaning, "no change on boiling". The term was first used in 1911 by English chemists John Wade and Richard William Merriman. Azeotropes are also called constant boiling point mixtures because their composition remains unchanged by distillation.
Water and ethanol form an azeotropic mixture when ethanol is present at a molecular percentage of ~91% (or 95.63% by mass). This mixture boils at 78.2 °C, which is lower than the boiling points of either pure ethanol (78.4 °C) or water (100 °C). At this composition, the liquid and vapour have the same composition, and no further separation occurs.
The addition of ethanol to water can speed up evaporation because ethanol has a lower boiling point than water. This means that ethanol evaporates more quickly at the same temperature and carries water vapour with it. The two substances are highly soluble in each other due to their shared hydroxyl groups (-OH). However, it is important to note that while ethanol evaporates faster, water transfers more heat during evaporation due to its higher heat of evaporation.
Distillation cannot separate the constituents of an azeotrope. Instead, repeated distillations of an ethanol-water mixture will produce mixtures that are progressively closer to the azeotropic ratio of 95.5/4.5%. No number of distillations will ever result in a distillate that exceeds the azeotropic ratio. This presents a challenge in alcohol distillation, as it is difficult to separate ethanol and water completely through distillation. However, distillation can be used to reduce the water content in ethanol.
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Alcohol's lower heat of evaporation
Alcohols have a lower heat of evaporation than water. This means that for the same amount of liquid, more heat transfer occurs during water's evaporation compared to alcohol. The degree of cooling is dependent on the evaporation rate and heat of evaporation.
The molecules held together inside a liquid need to break free to turn into a gas. This means that the hydrogen bonds holding the molecules together need to be broken. Molecules that can form lots of hydrogen bonds have a higher heat of evaporation. This also affects the boiling temperature of a liquid. Molecules that attract each other very strongly start to boil at higher temperatures compared to those with weak attractions. A lower boiling point generally means a liquid will evaporate more quickly. Water can form two hydrogen bonds per molecule. Its heat of evaporation is 2,260 joules per gram, or 541 calories per gram, and it starts boiling at 100 degrees Celsius.
Ethanol, a type of alcohol, has a lower boiling point of 78.3 degrees Celsius, which is significantly lower than the boiling point of water. However, these materials cannot be separated completely by distillation. While you can reduce the water content in ethanol by distillation, it takes many distillations to increase the ethanol content to greater than 50%. Given 10 or 20 distillations, you can get "absolute ethanol".
The rate of evaporation depends on temperature, humidity, and storage time. Even under ideal conditions, the loss of alcohol content in an unopened bottle is very gradual. Typically, less than 1% per year would be lost, an amount so small that it rarely affects the taste or potency of the liquor.
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Alcohol's higher volatility
Alcohols have a higher volatility than water due to their lower boiling point, which causes them to evaporate more quickly at the same temperature. The hydroxyl groups (-OH) present in both ethanol and water make them highly soluble in each other, but ethanol's lower boiling point of 78.3°C compared to water's 100°C means it vaporizes more rapidly. This is why alcohol is used to remove water traces from lab equipment, as it evaporates faster, taking some water vapour with it.
The rate of evaporation is influenced by factors such as temperature, airflow, and surface area, but the type of liquid is also significant. Water molecules have a higher attraction to each other than alcohol molecules, causing them to stick together more. This difference in intermolecular forces contributes to the faster evaporation rate of alcohol.
Experiments comparing the evaporation rates of water and alcohol support these findings. For example, placing equal amounts of water and alcohol in the sun and observing their evaporation rates demonstrates that alcohol evaporates much faster. Additionally, blowing on hands coated with water and alcohol shows that the alcohol evaporates more rapidly, resulting in a cooler sensation due to increased heat transfer during evaporation.
The volatility of alcohol is also utilized in practical applications such as cleaning pipes. Methanol and isopropyl alcohol (IPA) are used as organic solvents to remove organic deposits from pipes. Their high volatility ensures that they do not leave residue behind and effectively flush out any remaining water.
While the addition of alcohol does not significantly affect the boiling point of water, it still plays a role in enhancing the evaporation process. The presence of alcohol in a water mixture increases the overall volatility, leading to a faster rate of evaporation compared to pure water.
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Water molecules' higher attraction
Water molecules have a higher attraction to each other than alcohol molecules. This is because water molecules are more polar than alcohol molecules, meaning they have a greater ability to form hydrogen bonds with each other. As a result, water molecules tend to stick together more than alcohol molecules, which have a lower boiling point and evaporate more quickly.
The hydroxyl groups (-OH) in both water and alcohol molecules contribute to their mutual high solubility. However, the primary reason for the faster evaporation of alcohol is its lower boiling point. For example, ethanol has a boiling point of 78.3°C, while water boils at 100°C. This difference in boiling points means that at the same temperature, ethanol evaporates more rapidly and carries some water vapour along.
The rate of evaporation is influenced by factors such as surface area, temperature, and airflow. When comparing the evaporation rates of water and alcohol, it is evident that alcohol evaporates much faster. This is because alcohol has a lower heat of evaporation, which means that for the same quantity of liquid, more heat is transferred during water's evaporation than alcohol's.
The higher attraction between water molecules contributes to their slower evaporation rate compared to alcohol. Water molecules need more energy to break their intermolecular bonds and transition from a liquid to a gas phase during evaporation. In contrast, alcohol molecules experience weaker intermolecular forces, allowing them to evaporate more readily at lower temperatures.
The experiment of placing a drop of water and alcohol on separate pieces of paper or a penny demonstrates this concept. Water molecules tend to stick together, allowing multiple drops to accumulate on the surface before falling off. In contrast, the weaker attraction between alcohol molecules means fewer drops can be added before the alcohol spreads out or evaporates.
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Frequently asked questions
Alcohol is added to water as it evaporates faster than water due to its lower boiling point.
The boiling point of ethanol, a type of alcohol, is 78.3°C, while the boiling point of water is 100°C.
Alcohol has a lower heat of evaporation than water, which means that for the same amount of liquid, more heat transfer occurs during the evaporation of water compared to alcohol.
Yes, in addition to the type of liquid, the evaporation rate is also influenced by factors such as surface area, temperature, and airflow.
