You might be wondering how that can happen when the temperature is low. It turns out that all liquids can evaporate at room temperature and normal air pressure. Evaporation happens when atoms or molecules escape from the liquid and turn into a vapor. Not all of the molecules in a liquid have the same energy. When you have a puddle of water H 2 O on a windy day, the wind can cause an increased rate of evaporation even when it is cold out.
Energy Transfer The energy you can measure with a thermometer is really the average energy of all the molecules in the system. There are always a few molecules with a lot of energy and some with barely any energy at all. There is a variety, because the molecules in a liquid can move around. The molecules can bump into each other, and when they hit A little bit of energy moves from one molecule to another. Put a lid on it for about 20 seconds, and then remove the lid and look at the inside of it.
Students wearing safety goggles can observe this close up. Evaporation, on the other hand, occurs only at the surface of the water. Recall Figure 2, showing the evaporating molecules leaving the surface of the water. Evaporation, however, uses the energy already in the liquid.
If you have a puddle of water, it has some heat energy, which usually just came from the environment. The heat in that water results in some molecules moving fast enough to escape into the air, that is, evaporate. No additional source of energy is required for evaporation, and the water does not need to reach the boiling point to evaporate. What you just read implies that evaporation, but not boiling, is a natural process.
Your puddle of water or the water on your hair that you just washed will evaporate without you doing anything special. Just wait, and it dries. But boiling does not usually happen naturally. We have to deliberately heat the liquid to get it to boil. Your eggs will cook just as fast either way. On the other hand, evaporation of water will cool the water—and any surface that the water is evaporating off of.
The evaporating water molecules carry away heat from your skin. This is also why you perspire on a hot, summer day. The additional moisture on your skin results in more evaporation, which cools your skin. Because at high altitudes, the air pressure is lower. Any cooks out there? I just picked up my old copy of Joy of Cooking, in which the authors include how cooking instructions must be modifi ed at high altitudes. Remind me to do an article on that. Then how it possible for water to evaporate at room temperature?
Think of temperature as average kinetic energy of the water molecules. While the average molecule doesn't have enough energy to break the inter-molecular bonds, a non-average molecule does.
Water is a liquid because the dipole attraction between polar water molecules makes them stick together. However, at the surface of the liquid, lone molecules may end up getting enough kinetic energy to break free due to the random nature of molecular motion at basically any temperature.
On the flip side, water molecules in the atmosphere may enter the liquid at the surface as well, which is measured by equilibrium vapour pressure. Imagine spinning a roulette wheel, but instead of dropping in one ball, you drop in They all rattle around at different speeds, like the molecules in water.
You can cool them down by spinning the wheel slower, so they bounce about less; heat them up by spinning faster so they bounce more; you can freeze them by stopping the wheel and waiting till they're all stationary; and you can boil them by spinning the wheel so fast that they all fly out of the top.
Now pick up all the balls and throw them back in with the wheel spinning at a moderate speed. If you watch for a while you'll see that although the average speed of the balls is below the "boiling point" where they all fly out the top, every now and again one ball will ricochet off another with enough force to send it flying out of the wheel. If you watch for long enough eventually all the balls will be gone.
Your balls just evaporated. Temperature is a measure for how much kinetic energy the molecules in a substance have. If the temperature is high, they are moving pretty fast, if the temperature is low, they are moving a lot slower. If molecules are moving slow, they bundle up and you get a solid.
Once you heat it up a bit, the substance starts to become liquid. When you heat it up even more, the molecules will start to move so fast they will spread out into the entire space gas. However, this is all averages. In a liquid all molecules are moving, some faster than others. If a molecule happens to break through the 'surface' of the water, it'll have escaped the inter-molecular forces holding the water together and it'll be evaporated.
This can also happen with solids, there it is called sublimation. If you're heating up water, you're adding energy so this process will start to go faster.
Then at boiling point, you'll reach the point where molecules will want to start moving so fast they start to form gas bubbles inside the liquid. The boiling temperature of a liquid is not the temperature at which it can enter the gaseous state. This is why, for example, water boils at lower temperatures at higher altitudes. Furthermore, water is always evaporating. It is also always condensing.
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