Question

Before giving you an injection, a physician swabs your arm with isopropyl alcohol at room temperature. Why does this make your arm feel cold? (Hint:The reason is notthe fear of the injection! The boiling point of isopropyl alcohol is 82.4° C.)

Short answer

The alcohol will evaporate and will make the skin that was in contact colder.

Step-by-step solution

Given data

A physician is swabbing the arm with isopropyl alcohol at room temperature.

 Step 2: Understanding the evaporation of alcohol   

Alcohols, in general, have the tendency to evaporate at room temperature; when the physician swabs the arm with isopropyl alcohol, it will interact with the skin and evaporate.

Reason for feeling cold in the arm

The reason is evaporation. When he puts the alcohol on our arm, the alcohol layer now available on the skin starts to evaporate, and in this change of physical form, it will turn from liquid form to gas form.

For this transformation to occur, it will require a certain amount of heat energy. Alcohol will take the heat from the body and surroundings and increase its temperature and change into vapor form. The change in temperature of the skin makes it colder.

Thus we feel cold in the arm.