How does evaporation REALLY work

which dries clothes faster cold dry day or warm humid day and how does sweating cool us down and what do most people get wrong about evaporation keep watching to find out we all know evaporation when we see it if we leave cup of water out somewhere the water will steadily disappear as if by Magic and we know that the water is going out into the air and disappearing but is that all that's happening well let's take look at tiny drop of water in cloud Zoom down to the molecular level and find out what's going on if we really want to understand evaporation we need to understand little bit of thermodynamics but don't worry it's nothing too complicated so here are the molecules in our tiny cloud droplet they have enough energy to break some of the bonds that hold them together but not enough to break free of the droplet completely in fact that is what makes liquid liquid usually we imagine that all of the molecules that make up droplet of liquid have the same amount of kinetic energy but actually the energy is not shared equally between them all and they all exchange energy randomly between each other every time they bump into each other and that means that we get distribution most particles will be relatively low energy but some particles will have picked up more than the average share as they've been bumped around so let's imagine that the average amount of energy that these particles have is let's call it six quanta well most particles will have little less than that and some particles will have much much more now let's say that if particle can get 10 quanta it can break all of its bonds to the molecules around it but underneath the surface inside the droplet that doesn't really mean much because it will just have more collisions until it loses that energy again but at the surface that's big deal because as soon as it gets those 10 Quantum of energy it can go flying off now increasing the temperature means that we increase the average amount of kinetic energy so that means that the rate of evaporation will increase too because now it is much easier for molecules to get those 10 quanta that they need to fly away from the surface but let's look at what that evaporation has done to the rest of the liquid left behind if molecules with more than the average value of kinetic energy are leaving and taking that extra kinetic energy away with them then the average value of kinetic energy left in the droplet will steadily decrease and since temperature is essentially measure of average kinetic energy if the average kinetic energy of the liquid droplet is decreasing then so is the temperature and that is how evaporation calls liquids and we can actually see this happening in our really simple simulation here as the high energy particles leave from the surface taking their extra energy with them the particles that have been left behind start getting locked into this hexagonal pattern now for humans the evaporation of water which we call Sweat calls our bodies down enough to keep our temperature below about 37.5 degrees Centigrade but some liquids with very light molecules can evaporate so quickly that heat from the surroundings can't diffuse in fast enough to stop very dramatic temperature drop but there's something that most people don't realize or perhaps they just ignore and that's the evaporation is always going in reverse as well as those molecules fly off into the air they will still be having collisions and as they have those collisions they are going to lose their extra kinetic energy and then if they manage to find their way back to liquid again they'll be absorbed back into it in process called condensation so that means when we put lid on bottle of water it doesn't stop evaporating it's actually still evaporating at the same rate that it always was but now this empty headspace at the top is filling with those Vapor molecules and they start condensing back into the liquid over period of couple of seconds perhaps that rate of condensation will become the same as the rate of evaporation and at that point we've reached equilibrium where we can't physically see the evaporation happening anymore but at the molecular level it's all still going on now really important consequence of evaporation is something called vapor pressure if we have net evaporation inside container perhaps we've just put lid on or perhaps we're heating it up that extra evaporation will add Vapor molecules to whatever gas is already in there and that increases the total pressure because there is now the original pressure that was in there before the pressure of the vapor the vapor pressure and that pressure that total pressure and the vapor pressure will continue to increase until it reaches equilibrium where the vapor is now condensing back into the liquid at the same rate that it's evaporating now this pressure increase can be significant diethyl ether is liquid with very high vapor pressure and that means we need to be very careful when we are storing it with stopper on the top or if we are heating it in closed container for the same reason petrol caps on cars have valve that lets the petrol vapor escape on hot days and that brings me on to another very common mistake in gas the space between molecules is much larger than the size of the molecules themselves and that means that there is always space for liquid molecules to evaporate into and what that means is that the maximum vapor pressure of liquid depends only on temperature it does not depend on total pressure because there is always room for that liquid to evaporate so before we put the stopper in our container of diethyl ether the pressure will be one atmosphere inside the flask we put the stopper on and the diethyl ether will continue evaporating until it reaches its maximum vapor pressure for that temperature the point where the rate of evaporation is equal to the rate of condensation but it doesn't matter if we increase the total pressure inside that flask we could pump it full of 10 atmospheres of normal air and that ether will still evaporate and still add extra vapor pressure now I've seen scientists talking about things like drinking fizzy drink in submarine and they say that the carbon dioxide is not evaporating because the submarine is at two atmospheres of pressure well it is evaporating it's evaporating at the same rate that it would do on normal land at one atmosphere of pressure what it's not doing is bubbling or boiling but that's topic for another video so if you want to see that one make sure you click the like button and subscribe and you'll know when it comes out thank you so what else can we understand about our daily lives now that we understand the process of evaporation well how about the so-called Steam from hot cup of coffee the hot water of the coffee has got high vapor pressure so the water is evaporating very quickly into the air but very soon those water vapor molecules will hit the much cooler air and they'll lose their kinetic energy and then they'll start meeting each other and they will condense back to form liquid droplets so what you can see over the top of hot cup of coffee is not actually water vapor and it's certainly not Steam and it's definitely not smoke what it actually is is mist of liquid water droplets just like cloud in fact that is exactly what cloud is measuring the combination of heat and humidity is done by using something called the wet bulb temperature so we take thermometer and we wrap little bit of wet cloth around the thermometer bulb and the water will start evaporating from that wet cloth and cooling it down now on normal day the wet bulb temperature will be significantly cooler than the normal dry bulb temperature but on very humid day those temperatures will be very close and this brings us to very serious problem that's only going to get worse over the coming years humans and animals sweat and pant to cool down by evaporating water but at wet bulb temperature of 35 degrees Centigrade that's simply not enough to cool us down anymore sweating won't help you sitting in the shade won't help you drinking water won't help you and if you can't get to somewhere cooler you'll die in rich countries most people not all of them can get to somewhere with some air conditioning but in many parts of the world we have already seen wet bulb temperatures that are killing people because they simply have nowhere to escape to people die livestock die wild animals die this is problem that is happening now and it's only going to get worse so what can we do about this well the only way to fix Global heating is to force governments the rich and the powerful to take it seriously and that can leave us feeling helpless and angry but found an easy way to take action and that was to join 350.org who are not sponsoring this video by the way very simply they send you an email about once month giving you all kinds of small actions that you can take simple things like signing petition or sending an email to bank to pressure it to give up its investments in Coal you know public pressure is really important for these kinds of Corporations and if you want to you can also join direct action at protests from around the world why not check them out okay so now we can answer our question from the beginning of the video which will drive faster clothes on washing line on cold dry day or clothes on hot humid day well let's look at the situation on cold day the water in the clothes doesn't get as much energy for evaporating so it will still be evaporating and even ice evaporates very slowly but it won't be evaporating as quickly as clothes for example on day at 35 degrees Centigrade but on the other hand if we're talking about somewhere like Singapore where you can have 35 degrees centigrade and extremely high humidity our atmosphere is already full of water vapor so even though the water from our clothes is evaporating much faster we've got much more water vapor in the air that is going back and condensing much faster onto our clothes so the crucial question might be how windy is it because wind blows away the extra water vapor coming from our clothes now on cold dry day that water vapor will be blown away and it's not coming back but on hot humid day our wind is just carrying more water vapor it'll still dry the clothes but very slowly so which one is faster well it depends on how cold the cold dry day is and it depends on how humid the humid hot day is so how about you have you suffered through humid weather have you still got questions about evaporation and how it works or have you got some tips for us about how to stay cool let us know down there in the comments and I'll see you next time