Alcohols such as ethanol are more volatile than water because which physical property of alcohol is lower than that of water?

Introduction / Context:
Volatility refers to how readily a liquid evaporates at a given temperature. More volatile liquids evaporate more quickly and generally have lower boiling points. Alcohols such as ethanol are often described as more volatile than water, which is why the smell of spirit is easily detected and why alcohol based liquids dry quickly. This question asks which physical property of alcohol is lower than that of water that explains this higher volatility.

Given Data / Assumptions:
- The comparison is between an alcohol, such as ethanol, and water at similar conditions.
- Options include boiling point, density, viscosity, surface tension and refractive index.
- Volatility is linked to how easily molecules escape from the liquid phase into the vapour phase.
- We assume standard atmospheric pressure when discussing boiling points.

Concept / Approach:
The boiling point of a liquid is the temperature at which its vapour pressure equals the external pressure. Liquids with lower boiling points reach high vapour pressure at lower temperatures and therefore evaporate more readily, making them more volatile. Ethanol has a significantly lower boiling point than water. While density, viscosity and surface tension also differ between alcohol and water, they are not the primary factors directly defining volatility in basic physical chemistry discussions. Thus, the lower boiling point of alcohol compared with water is the key property explaining its higher volatility.

Step-by-Step Solution:
Step 1: Recall that volatility is associated with how easily a liquid changes into vapour at a given temperature. Step 2: Understand that a liquid with a lower boiling point typically has a higher vapour pressure at room temperature and therefore evaporates more readily. Step 3: Note that ethanol, a common alcohol, boils at a temperature much lower than the boiling point of water. Step 4: Recognise that although alcohol may also differ in density, viscosity and surface tension, these properties are not the main reasons given in school level explanations for its volatility. Step 5: Conclude that the property responsible for higher volatility is the lower boiling point of alcohol compared with water.

Verification / Alternative check:
An everyday observation supports this idea. Spirits or alcohol based sanitiser on the skin evaporate quickly, giving a cooling sensation, whereas pure water takes longer to evaporate under the same conditions. Data tables list the boiling point of ethanol well below one hundred degrees Celsius, while water boils at one hundred degrees Celsius at one atmosphere pressure. This numerical difference directly reflects the fact that ethanol reaches the necessary vapour pressure for boiling at a lower temperature, confirming that lower boiling point is the key physical property linked to higher volatility.

Why Other Options Are Wrong:
Density refers to mass per unit volume and does not directly determine how fast a liquid evaporates, though it may influence convection currents. Viscosity measures internal friction or resistance to flow and is more closely related to how a liquid pours than to its tendency to evaporate. Surface tension affects the shape of drops and capillary action, not primarily the volatility. Refractive index concerns how light bends when entering the liquid and has no direct link to evaporative behaviour. Therefore these properties are not the main explanation for alcohol being more volatile than water.

Common Pitfalls:
Students sometimes associate any property that sounds like it influences movement, such as viscosity or surface tension, with volatility without carefully considering definitions. Another pitfall is to treat volatility as a vague concept rather than connecting it to vapour pressure and boiling point. By focusing on the clear relationship between lower boiling point and easier evaporation, learners can understand why ethanol and similar alcohols are more volatile than water.

Final Answer:
Alcohol is more volatile than water because its boiling point is lower than that of water.