Question

Swimming coaches sometimes suggest that a drop of alcohol (ethanol) placed in an ear plugged with water "draws out the water." Explain this action from a molecular point of view.

Short answer

Ethanol mixes with water, reduces its surface tension, and aids in evaporation, helping remove water from the ear.

Step-by-step solution

Understanding Molecular Interactions

Water and ethanol are both polar molecules, meaning they have a positive and negative side (dipole). This polarity allows them to interact and mix at a molecular level due to hydrogen bonding, where the positive side of one molecule is attracted to the negative side of another.

Ethanol Increases Volatility

Ethanol is more volatile than water, meaning it has a higher tendency to vaporize. When ethanol is added to water in the ear, it begins to evaporate, pulling water molecules with it due to their mutual interactions.

Alcohol as a Solvent Effect

The presence of ethanol in a water-filled ear lowers the surface tension of water. This reduction in surface tension allows water to spread more readily, reducing the 'plug' effect of water in the ear.

Evaporation of Water-Ethanol Mixture

As both ethanol and water evaporate together due to the volatile nature of ethanol, the water trapped in the ear is gradually drawn out because the ethanol helps to extend the rate and ability of water to evaporate.

Key concepts

Molecular Interactions

Molecular interactions involve how molecules attract, repel, and fit together. In the case of ethanol and water, these interactions are crucial. Both molecules exhibit polarity, meaning they have a distinct positive and negative end due to the distribution of electrons. This creates a dipole-dipole attraction, allowing them to mix well.

• The positive side of one molecule is attracted to the negative side of another, a key part of hydrogen bonding.
• This attraction is why water and ethanol dissolve in each other easily.
• These strong interactions help explain why ethanol can mix with water already present in the ear.

Overall, understanding these interactions gives insights into why ethanol can affect water in our ears so effectively.

Polarity

Polarity refers to the distribution of electrical charge over the atoms joined by the bond. Both water and ethanol are polar molecules, meaning their molecules have parts that are slightly charged. For water, the oxygen is slightly negative, and the hydrogens are slightly positive.

• Polarity determines how molecules interact with one another, influencing solubility and cohesion.
• Polar molecules like water tend to bond with other polar substances through hydrogen bonds.
• This property allows ethanol, also polar, to effectively come into contact with and interact with water in the ear.

In essence, the polarity of ethanol allows it to interact closely with water, helping to draw water out from the ear canal.

Evaporation

Evaporation occurs when molecules at the surface of a liquid gain enough energy to enter the gaseous phase. Ethanol tends to evaporate more readily than water due to its chemical structure and lower boiling point.

Upon adding ethanol to a water-filled ear, ethanol's higher volatility causes it to evaporate quickly,

• This evaporation pulls along water molecules bonded to it, helping to remove water from the ear.
• As ethanol evaporates, it increases the surface area exposed to air, further facilitating the evaporation of water.

This combination of effects explains how ethanol assists in 'drawing out' water from the ear, effectively reducing the feeling of a plug.

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction between molecules. It largely involves hydrogen atoms bonded to electronegative atoms like oxygen. This is particularly relevant for water, which can form up to four hydrogen bonds due to its two hydrogen atoms and the capability of oxygen to accept two more.

• In the case of ethanol, it can form hydrogen bonds with the water in the ear, effectively linking the molecules.
• These hydrogen bonds are part of why ethanol can mix so readily with water.
• The ability to form these bonds aids ethanol in influencing water's propensity to evaporate, by creating a structured transition that favors evaporation.

Understanding hydrogen bonding is crucial to grasping how ethanol becomes intertwined with water, aiding in its evaporation from the ear.