Understanding Alcohol Boiling Points: A Definitive Guide

The boiling point of a substance is the temperature at which it transitions from a liquid to a gas. Alcohols have different boiling points depending on their type, with ethanol, for example, boiling at around 78°C. The boiling point of an alcohol is influenced by factors such as atmospheric pressure, the number of carbon atoms, and the presence of other substances like salt or sugar. Distillation can be used to separate different types of alcohol based on their varying boiling points.

The role of atmospheric pressure

The boiling point of a liquid is influenced by atmospheric pressure, which is the pressure exerted by the weight of the air molecules above the liquid. Atmospheric pressure is also referred to as barometric pressure.

In the context of boiling points, vapour pressure is crucial. Vapour pressure is the pressure exerted by a vapour when it is in equilibrium with its liquid or solid form in a closed system. As a liquid is heated, its vapour pressure increases until it equals the pressure of the gas surrounding it, which is when boiling occurs. At this point, the vapour inside the bubbles exerts enough pressure to keep them from collapsing, and the bubbles rise to the surface and burst.

The boiling point of a liquid is the temperature at which the vapour pressure of the liquid equals the atmospheric pressure. Therefore, the boiling point of a liquid depends on the surrounding pressure. When the atmospheric pressure is lower, less energy is required for the liquid to transition into a gas, resulting in a lower boiling point. Conversely, when the atmospheric pressure is higher, more energy is needed for the liquid to boil, leading to a higher boiling point.

For example, ethyl alcohol (ethanol) has a boiling point of 78.4°C under standard atmospheric pressure (760 mm Hg or 1 atm). At this temperature, the vapour pressure of ethyl alcohol equals the atmospheric pressure, enabling its transition from a liquid to a gas.

Distillation

The boiling point of alcohol can be influenced by the presence of other substances, such as salt or sugar. For example, salt will make alcohol boil at a higher temperature, while sugar will cause it to boil at a lower temperature. This is because salt and sugar change the number of water molecules in the solution, affecting how easily they can escape as vapour. Additionally, the boiling point of alcohol increases with the number of carbon atoms in a molecule. This is because the hydroxyl group, or the "OH" group, allows alcohol molecules to engage in hydrogen bonding, which increases the boiling point.

Boiling point of ethanol

The boiling point of ethanol, also known as grain alcohol (C2H5OH), depends on the atmospheric pressure. At standard atmospheric pressure (14.7 psia, 1 bar absolute), the boiling point of ethanol is 173.1 °F (78.37 °C or 78 °C). The boiling point decreases as atmospheric pressure decreases, so it will be slightly lower than published values unless you are at sea level.

The boiling point of a substance is the temperature at which the vapour pressure of the liquid becomes equal to the atmospheric pressure acting on the liquid's surface. As a liquid is heated, its vapour pressure increases until it equals the pressure of the gas surrounding it. Vapour bubbles form within the liquid, rising to the surface where they burst and are released into the air. At the boiling point, the vapour inside the bubble exerts enough pressure to prevent the bubble from collapsing. The molecules of the liquid must be able to overcome the forces of attraction between them to create vapour.

The boiling point of ethanol is lower than that of water, which boils at 212 °F (100 °C). Distillation can be used to separate ethanol and water, as well as to separate ethanol from other liquids and organic compounds. However, distillation cannot completely separate ethanol from water because they form an azeotrope. The only way to fully separate ethanol from water is to boil it away or allow it to completely evaporate.

The boiling point of ethanol is also influenced by the presence of other substances. For example, when ethanol is added to a boiling liquid and then removed from the heat, about 85% of the ethanol remains. Flaming the liquid to burn off the ethanol results in 75% retention, while storing food containing ethanol overnight without heat results in 70% retention. Baking a recipe containing ethanol for one hour reduces ethanol content to 25%, while baking for 25 minutes without stirring results in 45% retention. To reduce ethanol content to 10% or lower, a recipe must be baked for two hours or longer.

Hydrogen bonding

The boiling point of a substance is the temperature at which the vapour pressure of the liquid becomes equal to the atmospheric pressure exerted on the liquid. At this point, the liquid transforms into a vapour without any increase in temperature. The boiling point of a liquid is dependent on the surrounding pressure. For instance, the boiling point of water is 212°F or 100°C.

Alcohol has a higher boiling point than alkanes with similar molecular weights. For example, ethanol has a molecular weight of 46 and a boiling point of 78°C (173°F), while propane, with the same number of electrons and a similar length, has a lower boiling point. The boiling point of alcohol also depends on the type of alcohol and the atmospheric pressure. The boiling point decreases as atmospheric pressure decreases.

The boiling points of alcohols increase as the number of carbon atoms increases, and the hydroxyl group is referred to as a hydrophilic ("water-loving") group because it forms hydrogen bonds with water and enhances the solubility of an alcohol in water. However, solubility decreases as the length of the hydrocarbon chain in the alcohol increases. Alcohols with higher molecular weights tend to be less water-soluble. The water solubility of lower-molecular-weight alcohols is pronounced, and as the hydroxyl group accounts for almost half of the weight of the molecule in methanol, it is completely soluble in water.

Primary, secondary, and tertiary alcohols

Alcohols are organic molecules containing the hydroxyl functional group, OH, directly bonded to carbon. The carbon attached to OH is known as the carbinol carbon. The number of carbon atoms attached to the carbinol carbon determines whether an alcohol is primary, secondary, or tertiary. If the carbinol carbon is attached to one carbon, the alcohol is primary; two, secondary; and three, tertiary.

The boiling point of alcohols increases with the number of carbon atoms as stronger van der Waals forces act between the molecules. For example, methanol, with a smaller number of carbon atoms, boils at around 60°C, whereas phenol, with a higher number of carbon atoms, boils at around 180°C.

The boiling points of primary, secondary, and tertiary alcohols differ due to the structure of their carbon chains. Branching in carbon chains significantly affects boiling points. More branching leads to a more compact structure, reducing the surface area available for intermolecular forces. Primary alcohols have the hydroxyl group attached to a carbon connected to one other carbon. Secondary alcohols have the hydroxyl group on a carbon connected to two other carbons. Tertiary alcohols have the hydroxyl group on a carbon connected to three other carbons. Due to the high degree of branching, tertiary alcohols have the lowest boiling points.

The exposure of the hydroxyl group also impacts the boiling point. A more exposed hydroxyl group results in stronger intermolecular H-bonding, leading to higher boiling points. This is because the hydroxyl group is more accessible for other molecules to hydrogen bond to, resulting in stronger intermolecular forces and higher boiling points.

Overall, the boiling point order of primary, secondary, and tertiary alcohols is primary > secondary > tertiary. For example, the primary alcohols 1-butanol and 2-methyl-1-propanol have higher boiling points than the secondary alcohol 2-butanol, which boils at a higher temperature than the tertiary alcohol t-butanol.

Frequently asked questions