If I put some rubbing alcohol on my skin, it feels colder than if I put water on my skin. Am I correct in thinking this is because alcohol evaporates faster than water? And why do different liquids have different evaporation speeds? And why do faster liquids feel colder than slower ones?
Yes, it is because it evaporates faster, carrying the heat away from your skin at a faster rate. Without doing the math, I’d guess that one cc of water on the back of your hand and one cc of alcohol would transfer the same amount of heat away from your hand, but the alcohol would feel cooler because it was taking that heat away more quickly, and your body can’t replace that heat instantly.
Liquids evaporate faster if they are more volatile. This means, at a given temperature, they have a higher vapor pressure (which can be measured by putting an arbitrary amount of the liquid in a closed container, and measuring the pressure of the gas when the liquid-gas two phase mixture reaches equilibrium).
While we’re at it, boiling occurs when the vapor pressure exceeds the atmospheric pressure. That’s why water can be made to boil simply by putting it in a partial vacuum.
Not wanting to get bogged down in the chemistry of hydrogen bonding, heat of vaporisation and so forth there are two major factors.
- The molecules in some liquids stick to each other more than others. If they stick to each other they find it much harder to fly off into space because the molecules that haven’t yet vaporised will tend to pull them back into the pool.
2)Molecular weight. Just as it takes more energy to heave a bowling ball into the air than to heave a tennis ball, so it takes more energy to propel a large molecule into the air than a small molecule. The more energy a liquid needs to vaporise the longer it needs to sit next to the heat source to absorb that energy, and thus the slower it vaporises.
Those two factors interact in some odd ways though. For example based on weight water should evaporate far faster than alcohol. Indeed it should be a gas at room temperature. However water molecules are very sticky and so water remains a liquid long after much heavier molecules.
Water has a higher heat capacity and a higher heat of vaporisation, and will thus carry away more heat/volume. However it does so much more slowly for precisely the same reasons
Just wanted to clarify, in case it’s not completely clear, that it’s not just that heat is carried away - the heat is used to turn the liquid into vapour - latent heat of evaporation