Explain why a solid mothball disappears without leaving any liquid & why our skin feels cold when a little alcohol is rubbed on it?
Mothballs, like dry ice (solid CO2), undergo a physical change called sublimation. In this change the solid goes directly into the gas state without first melting.
Your skin feels cool when alcohol is rubbed on it because the alcohol absorbs some of the heat from your skin and as a result it evaporates. The transfer of heat from your skin to the alcohol gives the cool feeling you experience.
Some substances change from solid to the vapour state without passing through the intermediate liquid state. This process of materials changing from solid to directly vapour state is called sublimation, and the material that exhibit the property of sublimation are called sublime material. Some example such material are iodine, arsenic, camphor, and dry ice. There are a few substances, such as iodine, arsenic, camphor, and dry ice. Mothballs are deliberately made of sublime substances so as liquids substances can soil the material to be protected against mothballs.
Our skin feels cold when alcohol is rubbed on it because of the latent heat of evaporation absorbed by the alcohol when it evaporates. Alcohol has very low boiling point and evaporates very fast even at room temperature. Because of this, during evaporation it absorbs heat from the skin faster than other liquids like water, making the skin feel cooler.