3.6 Cooling by Evaporation - Mr. Joey PDF

3.6 Cooling by evaporation Mr Joey Evaporation Evaporation: The change of state from liquid to gas occurring below the boiling point. Recall: Particles in liquids move at random / have different energy Particles in liquids are bound by moderately strong attractive forces The ones with t...

3.6 Cooling by evaporation Mr Joey Evaporation Evaporation: The change of state from liquid to gas occurring below the boiling point. Recall: Particles in liquids move at random / have different energy Particles in liquids are bound by moderately strong attractive forces The ones with the greatest energies can overcome the attractive forces Ultimately, they escape the liquid and become gaseous particles Evaporative cooling Evaporation causes the remaining liquid to cool down If the particles with the highest energies escape, the remaining particles have lower energies on average Therefore, the temperature of the remaining liquid reduces Analogy time Imagine a class of 25 students. The average height of the class is 1.6 m. The teacher asks the tallest 5 students to exit the class. As such, we expect the average height of the remaining students to be lower! This is why after some evaporation occurred, the liquid’s temperature will lessen. The most energetic particles escape, leaving the average energy (temperature) of the remaining liquid to be lower.  It is the evaporation of the sweat that cools us down, not Sweating just the presence of sweat! Our body automatically cools us down when it is hot by producing liquid on our skin But how? The liquid (sweat) is produced with the same temperature as our skin The sweat evaporates, cooling the remaining sweat Heat will then transfer from our skin (hotter) to the sweat (colder) As more sweat evaporates, it continues to absorb heat from our skin, making us feel cooler. Rate of evaporation There are some factors that causes evaporation to be faster: 1. Higher temperature 2. Lower humidity of surrounding air 3. Greater area of liquid surface 4. Evaporating liquid has lower boiling point 5. Windy conditions If our sweat evaporates faster, our body will be cooled down more rapidly Thus, our body temperature reduces faster and we feel colder Liquids with different boiling points Some liquids have lower boiling points than others The attractive forces between particles are weaker They evaporate more quickly For example, perfume evaporates quicker than water, which evaporates quicker than liquid soap Hence, when a liquid that evaporates quickly is placed on our skin, it removes heat from our skin faster, making us feel colder