Specific heat capacity and heat of vaporization of water. Evaporative cooling. Why ice floats.
Log in miluutin 9 years agoPosted 9 years ago. Direct link to miluutin's post “In this paragraph of heat...” In this paragraph of heat of vaporization I got a bit confused by these numbers: "Water’s heat of vaporization depends on the temperature: it's around 540 cal/g at 100 °C (water's boiling point) and around 580 cal/g at 25 °C (room temperature)." • (49 votes) Immanuel Tolstoyevski 9 years agoPosted 9 years ago. Direct link to Immanuel Tolstoyevski's post “Yes, that part is not ver...” Yes, that part is not very clear. How come only 40 calories can increase the water temperature by 75 degrees to its boiling point, if the specific heat property tells us that 40 calories can only increase it by 40 degrees? The relationship is non-linear. Remember that when you apply energy to water, some of it will increase the avg kinetic energy of the molecules (related to the temperature) and some will be spent to break off all hydrogen bonding and send the molecules flying away (related to heat of vaporization at a GIVEN temperature). You don't need to wait until 100 degrees for vaporization to begin. It occurs more and more as you near towards it. And as more molecules fly off, less energy is needed to break off the remaining bonds. That's why the difference between heat of vaporization at 25C (energy required to break all H-bonds between 1 gram of initially slow moving molecules) and at 100C (energy to break all H-bonds of 1 gram of fast molecules) is LESS then the energy required to make all of those 1 gram of molecules faster. (103 votes) JLD 8 years agoPosted 8 years ago. Direct link to JLD's post “In the last paragraph it ...” In the last paragraph it says: "In lakes and ponds, a layer of ice forms on top of the liquid water, creating an insulating barrier..." How does ice provide an insulating barrier? • (24 votes) Ryan Hoyle 8 years agoPosted 8 years ago. Direct link to Ryan Hoyle's post “Awesome question. Part of...” Awesome question. Part of the answer is that less dense materials conduct less heat, and thus slow down heat transfer. If you think about using a metal vs wooden spoon in a hot pan of water, it's the metal one that will burn you, because it is more dense and a better conductor of heat. So the transfer of heat from water to air is slowed down by the layer of ice. Another part of the answer is the ice prevents evaporative cooling, the liquid water molecules become physically trapped and so the ones with the highest kinetic energy can't escape, which would reduce the overall average kinetic energy and thus temperature of the water (see Sal's video on evaporative cooling). Because this doesn't happen with the layer of ice in the way, water can stay warmer for longer. svetlana4ered 9 years agoPosted 9 years ago. Direct link to svetlana4ered's post “But why is the distance b...” But why is the distance between molecules in ice bigger than in water? They are still, but why not close enough to each other to make a dense body? • (21 votes) iShowRizz 3 years agoPosted 3 years ago. Direct link to iShowRizz's post “This is because when wate...” This is because when water goes lower than 4 degrees celsius it expands. Meaning the molecules are further apart. So when water reaches 0 degrees celsius it is frozen at an expanded state. And since it is frozen at an expanded state with molecules further apart, it is less dense than water which has it's molecules closer together. (8 votes) Maryam 5 years agoPosted 5 years ago. Direct link to Maryam's post “What if you drop a solid ...” What if you drop a solid into a liquid and they both have the same density. Will the solid float or sink? • (6 votes) Alex 5 years agoPosted 5 years ago. Direct link to Alex's post “The solid would be suspen...” The solid would be suspended in the middle of the liquid without moving. (20 votes) Hikari Akasaki 8 years agoPosted 8 years ago. Direct link to Hikari Akasaki's post “Why do the fastest-moving...” Why do the fastest-moving molecules leave the liquid? • (9 votes) ILoveToLearn 8 years agoPosted 8 years ago. Direct link to ILoveToLearn's post “The higher the speed, the...” The higher the speed, the greater the movement, the larger the likelihood that the particle will ricochet off the container or another particle in just the right way to escape. (12 votes) Ron Feldman 7 years agoPosted 7 years ago. Direct link to Ron Feldman's post “My question is related to...” My question is related to the first question listed. The answer seems to contradict what I thought I had learned 50 years ago in high school physics. My question relates to it taking such a small amount of additional energy to raise the temperature 75 degrees when compared to raising it 10 degrees.. I'm afraid I can't articulate my question above or my current understanding below very well.. But here goes.. What I recall learning is that it took a consistent amount of energy to raise the temperature of a volume of water 1 degree from room temperature up to 99 degrees. But that it took a great deal more energy to raise the temperature that last degree, from 99 to 100, because of the energy required to break the molecular bonds.. This is my first time using Khan, and I'm excited about having this resource. My thanks to whoever answers this question.. Ron • (11 votes) Wings of Fire Fan 5 months agoPosted 5 months ago. Direct link to Wings of Fire Fan's post “In my high school chemist...” In my high school chemistry class(recently), I think what I learned was that the reason a lot of energy is needed during the phase change is because of the energy required to break apart the bonds. I haven't taken physics yet, so I don't know what is being taught now, but it looks like you're pretty much right, at least as far as I know! (1 vote) hannadiop15 10 months agoPosted 10 months ago. Direct link to hannadiop15's post “why do some molecules of ...” why do some molecules of water escape from the surface of the water at lower temperatures and not others? • (4 votes) Richard 10 months agoPosted 10 months ago. Direct link to Richard's post “It’s important to underst...” It’s important to understand what temperature actually is for this question. It is the average kinetic energy of the matter, with the key being average. Not all particles of matter have the same kinetic energy. Some have greater than others, and temperature is simply the mean of those energies. This means that even if some liquid water isn’t at the boiling temperature, there exist some fraction of the water molecules which have sufficient energy to transition to the gas phase. So even though most of the water molecules remain liquid, the most energetic water molecules escape the liquid as water vapor. Hope that helps. (10 votes) Archana 5 years agoPosted 5 years ago. Direct link to Archana's post “At the surface of a liqui...” At the surface of a liquid, why do some molecules evaporate but others do not? • (6 votes) Ivana - Science trainee 5 years agoPosted 5 years ago. Direct link to Ivana - Science trainee's post “The layer which is most c...” The layer which is most closer to the air, interacts with air molecules. Rest of molecules cannot eadily evaporate because cohesion forces are stronger than cohesion forces. (5 votes) Marwan 8 years agoPosted 8 years ago. Direct link to Marwan's post “what is the difference be...” what is the difference between heat and temperature? • (5 votes) ironnerfer 8 years agoPosted 8 years ago. Direct link to ironnerfer's post “Heat is how fast the mole...” Heat is how fast the molecules are moving in a given object. Temperature is the way we measure heat. (6 votes) Quin Keza 4 years agoPosted 4 years ago. Direct link to Quin Keza's post “i don't understand really...” i don't understand really well how hydrogen bonds are oriented in ice • (5 votes) PAlex 4 years agoPosted 4 years ago. Direct link to PAlex's post “I'll try my best to answe...” I'll try my best to answer : (5 votes)Want to join the conversation?
So in room temperature it needs only (?) 40 calories more to heat up 1C as it takes in the boiling point? How come there is such a tiny difference, or is it actually a huge difference?
As the temperature lowers, water molecules aren't moving as quickly. It is more difficult for hydrogen bonds to form and break (as it occurs in liquid water), until the molecules are too slow to break the bonds, forming crystals (ice).
The negative pole of an H2O is still oriented towards the positive pole of another H2O, but there is a greater distance between them (because of the low temperature), which is why ice is less dense than liquid water.
Here's a comparison of water and ice (hydrogen bonds are seen as dotted lines), I hope this helped!
https://lh3.googleusercontent.com/-DJGwXk7Zv8U/VnFQy3rcKpI/AAAAAAAAAQs/C8tsSMRycVo/s800-Ic42/structure%252520of%252520molecules.PNG
