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Specific heat of water | Water, acids, and bases | Biology | Khan Academy

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- [Voiceover] In chemistry, there's a very valuable concept called Specific Heat, and specific heat is particular to a given substance. Every substance has a different specific heat. It's defined as the amount of heat, the amount of heat energy needed to raise one gram of a substance one degree celsius. Sometimes the definition might say to raise a given mass of a substance one degree celsius, but I'm just being a little bit more particular. One gram of a substance, one degree celsius. So this is telling us, well how much energy do I have to put into something to heat it up? So, for example. For example, water, water. If I wanna raise that one degree celsius, so if I wanna raise that one degree celsius, I would have to put in, I would have to put in a certain amount of, I would have to put in a certain amount of heat, which would be different than, say if I had sand. Let's say this is, let's say this is sand. This is sand. I have trouble picking colors. So this is sand right over here. I have to do a better job of drawing sand. So this is sand right over here. If I wanna raise that one degree celsius, I would need a different amount. It actually turns out that I need less heat, I need less heat to raise the sand one degree celsius than I need to raise the temperature of the water or one gram of the water one degree celsius. So let's say this is a gram of water and this is a gram of sand. I'm going to need more heat here to raise this one degree celsius than to raise that. That's because water has a higher specific heat. So higher, higher, relatively higher specific heat. Specific heat. And sand, or at least relative to water, has a lower specific heat. Lower specific heat. So two ways you could think about it. Let me write this. Lower, lower specific, specific heat. Two ways to think about it. If you wanna raise one gram of each of them one degree celsius, you're gonna have to put more energy into the water than you're going to have to put into the sand. Or the other way around, if you put the same amount of energy into both, you're gonna raise the temperature of the sand a lot more than you would raise the temperature of the water. And water, actually, its specific heat has a special name and this is a name that you have seen before. The specific heat of water is called the calorie. Specific, specific heat of water is called the calorie. And you have seen this word before. When you've wanted to cut calories, when you've looked at the back of nutritional labels on food. There's one clarification. The calorie that people talk about when they're talking about nutritional labels or how many calories are actually in food, that's actually kilocalories. So if you see, if someone hands you a, let's say a bowl of ice cream. Let me draw a bowl of ice cream here. So if someone hands you a bowl of ice cream right over here and they tell you that this is 500 calories. Let me write that down. This is 500 calories. If we're thinking of it in terms of specific heat, it's actually 500 kilocalories. 500 kilocalories. So there's a couple of ways that you could think about 500 kilocalories. You could think about it as, this is, this ice cream has enough energy to raise, to raise 500 kilograms of water one degree celsius. You could also view it as the amount, well actually if you wanna think of it in more human terms, most humans are roughly grown people are between 50 kilograms or 60, 70 kilograms roughly over there. You could say 50 kilograms of water. Actually a grown male might be composed of about 50 kilograms of water and then there's obviously other things that make up their weight, I'm just approximating. 50 kilograms of water and then we're raising it one degree celsius in the top case, but in this case you would raise it 10. You would raise it 10 degrees celsius. And that's actually happening in our body. Your body heat is actually caused, partially, the energy from food, some of it is to process your movement and the different functions of your muscles and the brain and all the things you body does, and some of it is just producing heat. Sometimes as a byproduct of that movement and sometimes, frankly, just for the sake of producing heat. So, 500 calories you see on a food label, that's really 500 kilocalories, and that's enough energy to raise 500,000 grams of water, remember 500 kilograms, 500,000 grams of water, one degree celsius, or 50,000 grams of water 10 degrees celsius. But anyway, water has a special name, it's calorie. It's neat to be able to connect it to, well, what we're eating and to think about what a calorie actually means, or what kilocalorie actually means. But this notion that water has a higher relative specific heat to, say, things like land, actually has huge impacts on our climate. It's actually one of the reasons why it's often nice to live near the coast. Because, let me draw, let me draw a coastline. Right over here. This is, actually I'll draw it from, I'll draw it from above. So that's the coastline. This is land. Let's just say it's made up of sand, for the sake of argument, and other things. This is water. Let's think about it, first let's think about it in the summer. Let's think about in the summer, when it is hot. Think about just a sunny day. So in the summer, when things are hot, so you have the sun, you have the sun right over here and it's radiating energy and obviously the area also has, is also warming up the things that it comes in touch with. But you're having, at least at the coastline, the air is roughly the same temperature over both and, although there will be some variation, and they're getting the same amount of sunlight. But since water has a higher specific heat, that heat is going to warm up water less. So this is going to get less hot. Let me write it over here. This is going to get less hot. Less hot. And the land is going to get more hot. More hot. More hot. And that's why, when you're on the coastline, the air is also affected by what's in touch with it, it's gonna be in tough with this less hot water, and so as the air, especially if it's coming from the ocean, if the air is going in this direction, then if you're at the coast, you're gonna get probably a cooler breeze than if you're more inland over here. If you're more inland over here, the air is going to be hotter. So this is going to be hotter air if you're inland. And it's less hot air if you're at the coastline because the air is being cooled down to some degree, it's not being warmed up as much by the water. Now you have the opposite effect if you think about the coldest times of the year. If you think about the middle of the night in the winter. So let me draw that. If you think about nighttime in the winter, when everything is cooling down, when everything is cooling down once you have your coastline. Well now, both the land over here and the water, they're going to be radiating heat and the water is going to be radiating heat, there's no other sources of heat, there's radiating heat into the air. But if they radiate roughly the same amount of heat, the land is going to drop in temperature much much more. So it's going to get colder here. So the land is going to get colder and the water is going to get less cold. Because it works in both directions. More energy goes in the water, the temperature changes less, and it can also radiate the same amount of heat, but also lose less, it would also lose less temperature. So in the winter, in really cold times of the year, well the air above the water, when it comes in touch with the water, is going to be less cold than the air above the land because the water is less cold than the land. And so it's nice to live near the water. It's going to be less cold in the coastal areas than it would be if you're further inland. And this phenomenon, this is frankly anywhere where you might see water, but it's very pronounced where I live. I live in the San Fransisco Bay area. And if you live, you literally have the Pacific Ocean. Let me draw that. You have the Pacific Ocean here. You have the ocean right over here. And you have the San Fransisco Bay that looks something like that. You have your Golden Gate Bridge right over here, for anyone who is curious. This is San Fransisco right up here. I actually live right about there. And you see it, if you hang out in the summer, in the middle of the day in the coastline, it could be maybe 10 degrees, even 20 degrees cooler than if you are well inland over here. So it has a very profound impact on weather patterns. So the last thing you might be wondering is, well why does water have such a high specific heat? Well that goes back to our good old friend, the hydrogen bond. Because, if we're talking about a solid substance, if we're talking about a solid substance, and I'll just draw a generic solid substance. So solid substance right over here. I don't know what this is, let's say it's some type of structure that looks something like this. As it gets energy, as it gets heat energy, the heat energy just increases the kinetic energy of the actual particles. It actually just makes them vibrate more. Makes them vibrate in place more. And that's actually what temperature is. It's really average kinetic energy. When you touch something and go ooh, that's hot, it's really because those things are just vibrating super super super super fast. So it's gonna make these things vibrate more. Almost in place, especially if we're thinking about something that is a solid. While in the case of water, sure, when you get more energy, it's gonna make the water molecules move around faster, vibrate more, have more kinetic energy. But in order to have more kinetic energy, they're gonna have to overcome this kind of creation and breaking up of the hydrogen bonds. You can almost imagine the friction of a hydrogen bond. You can imagine if this water molecule wants to move in this direction, it has to break this hydrogen bond, it's gonna form another hydrogen bond, it's gotta break that. So it needs energy to keep breaking those hydrogen bonds, overcoming the friction of those hydrogen bonds and I'll use the friction in quotes 'cause it's, well actually all friction is actually some type of atomic force that's occurring at a microscopic level. But it's having to overcome that. And so some of this heat energy is actually used to overcome that as opposed to increasing the actual kinetic energy of the individual molecules, or making them vibrate or whatever to actually increase their temperature. And this is why water is able to store more heat and why it has a higher specific heat.

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Duration: 10 minutes and 50 seconds
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Language: English
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Posted by: jake.goldwasser on Nov 19, 2015

Specific heat of water. A calorie as the specific heat of water. How water moderates temperature.
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