Anomalous expansion of water

water has quite a few unusual properties one of them is famously called the anomalous expansion so we'll talk about what this is and what are its consequences all right let's begin let's start with the meaning of this anomalous what does that mean anomalous just means unusual unusual so we're talking about unusual expansion in water let's take an example imagine we have some water some amount of water and let's draw some water molecules and let's say the temperature right now is a room temperature let's call it as 27 degrees Celsius what we'll do now is we'll cool it down let's cool it will cool it down what's going to happen well as you cool it its temperature will drop and as the temperature drops the molecules of water start losing energy because of which they vibrate slower and they end up coming closer to each other and as a result the whole water ends up shrinking now we have spoken about this in a little bit more detail in some previous video and so if you require more clarity it would be a great idea to go watch that video and then come back over here but anyways water contracts on cooling all right so let's write that down as temperature decreases so we decrease the temperature what we will see is water ends up shrinking hmm so water contracts or shrinks let's write that shrinks now there's nothing unusual about this this is normal behavior when you heat things they expand and when you cool them they contract so again this is normal this is normal but this normal behavior only continues until you hit four degrees Celsius because once you go below four degrees Celsius something very unusual happens all right so let's write that down until we hit four degrees Celsius until it for resources we will see water contract like this water keeps contracting so the water would shrink now I have highly exaggerated over here for FX but water shrinks and this continues until four degree Celsius so until we have hit four degrees Celsius everything is normal so far but you cool it down even further cool it further and now you will see mysteriously water doesn't shrink anymore instead it expands and that is what we call as the anomalous expansion again let's write that down if you further decrease the temperature further decrease the temperature we would expect the water to continue shrinking because that's normal behavior but it doesn't instead you will see it expanding it expands and this is very unusual you don't expect this to happen right this is unusual so let's write that down again let's copy this copy and paste we would expect the world to shrink further but it doesn't instead it expands again I've exaggerated it for effects but you get the idea and you could cool all the way down to zero degrees Celsius and you will see this anomalous or unusual behavior but don't stop there cool it even further cool it even further what happens next well we have now reached the the freezing point of water so water will start crystallizing into ice and you will see that even when it is crystallizing even during this this particular process the unusual behavior continues so even during crystallization we will see a continued expansion taking place so it still expands over here again this is also unusual you don't expect that to happen all right so our ice at zero degrees Celsius temperature doesn't change remember only the phase changes at zero degrees Celsius but anyways during this time ice keeps expanding so from four degrees Celsius to all the way zero degrees Celsius ice during this region we are seeing an unusual expansion and the unusual behavior is not just when you cool it but even if you had to heat it up if you had to heat up ice it would melt and even if you were to further heat up and increase the temperature from 0 to 4 degrees Celsius you would see now instead of expanding it would contract alright so from 4 degrees Celsius to 0 degree Celsius ice in this phase you will see unusual behavior if you're cooling it it'll expand if you're hitting it it will contract that's the idea behind this now you may be curious to know what's the reason behind this anomalous behavior why does water act so weirdly between 4 to 0 degrees Celsius and why only between this temperature well we'll talk about the reasons behind this in another video but as of now let's look at the consequences of this I mean what's the big deal so yeah water expands when you cool it below 4 degrees Celsius but what's the big deal what happens because of that well let's talk about that let's let's make a little bit of room all right the first consequence can be thought of this way now imagine you have water now right at 4 degrees Celsius and suppose you cool it down well it expands what if you were to heat it up well it could still expand normal behavior so either ways notice water expands so if you look at the molecules of water either ways even if you cool it or you heat it from 4 degrees Celsius they will go farther away from each other that means the molecules are the closest at 4 degrees Celsius that makes sense in other words the molecules of water are most crowded at 4 degrees Celsius in physics we could say the density of the water is maximum at 4 degrees Celsius density is a measure of how crowded things are since water is the most crowded at 4 degrees Celsius it will have maximum density at 4 degree Celsius so that's one consequence we can write so consequences so the first one is it has maximum density at 4 degrees Celsius at four degrees Celsius and that density is thousand kilograms per meter cube which means if you take one meter cube of water at four degrees Celsius it would weigh a thousand kilograms at any other temperature it will have lower density so the density at any other temperature would be less than thousand kilograms per meter cube now this may not seem like a big deal but think of it this way the unusual behavior continues even when water is crystallizing into eyes even when ice is being formed molecules are going farther away and the density is decreasing because of this ice has lower density than water let's write that down ice is less dense less dense than water then water and if you think about that water is really special because most solids are denser than liquids you may already learnt about that that molecules are more tightly packed in solids high density but that's not the case with ice ice is less dense and then water and this is the reason why ice floats on water we take this for granted because we see this every day but if you think about this as really special ice floats on water because of this anomalous behavior but there's another very big consequence because of this anomalous behavior let's make a little bit more space for that all right so the third consequence is that aquatic life let me just draw on fish over here aquatic life can survive can survive in very cold conditions very cold conditions very cold conditions and here's how that works suppose this represents this represents a lake or a pond in a very cold place let's say the surrounding temperature is below zero degrees Celsius we would expect this entire water to freeze up right because the whole temperature is below zero degrees Celsius but it doesn't and it's all because of this anomalous behavior of water so what happens is that the top layer of the water which is in direct contact with the surrounding well that will end up freezing that'll end up becoming ice now imagine there was no anomalous expansion in water then ice would be more dense than water ice would sink and the rest of the water would come on top and again the process would continue and the whole water body would solidify just like we would predict but because of the anomalous behavior ice is less dense than water which is why the top layer of the ice doesn't sink but instead it floats and it doesn't allow the bottom water to freeze up it doesn't allow the bottom water to come in contact with the cold surrounding it insulates the bottom water and which is why the water below can stay warm and this is why oceans and lakes at very cold places don't freeze up only the top layer freezes and then it acts like an insulator and keeps the water below pretty warm making our aquatic friends very happy