temperature scales and conversions all right another fundamental quality in science is temperature and we haven't talked much about temperature so far it's basically a measure of the average amount of energy of motion or kinetic energy in a system and temperatures are expressed using scales that use units called degrees I'm sure you've heard of this so degrees Fahrenheit for instance there are several temperature scales in use science uses two of them and the Fahrenheit scale is commonly used in the United States and so we discuss degrees Fahrenheit when we talk about temperature in this country but one thing to keep in mind on this scale the freezing point of liquid water is 32 degrees Fahrenheit and the boiling point of water and that's the temperature at which the liquid water turns to steam is 212 degrees Fahrenheit so it is useful to memorize these quantities if you haven't already and then we'll talk about similar points for the other two temperature scales as we go along okay so as I mentioned other temperature scales exist and these are the ones that science uses and also much of the rest of the world the first is the Celsius scale and that is symbolized by degree C and we say degree Celsius and a lot of times you'll hear degree C and basically on this temperature scale zero degrees is the freezing point of water and a hundred degrees is the boiling point of water and that Celsius scale is divided into a hundred divisions between these two landmarks and then it's extended higher and lower than that so there are two conversions between the Fahrenheit and Celsius scales and as you can see they're not the the simplest easiest thing to use but they're not bad once you get used to it once you get a little bit of practice to go to degrees C from Fahrenheit you want to take your Fahrenheit temperature subtract off 32 degrees and then multiply it by the fraction 5/9 okay if you want to go to Fahrenheit from Celsius you're going to take your degree Celsius multiply it by 9/5 so the other way around and then you're going to add 32 degrees all right so let's do a little conversion here a little practice so 98.6 that's the normal human body temperature and we want to convert that to degrees Celsius and right now it's in Fahrenheit so we're going to use the conversion factor that gives us degrees Celsius so we're going to take our degrees Fahrenheit right here we're going to plug that in we're going to subtract off 32 we're gonna multiply by 5 divided by 9 so 98.6 divided by 32 is going to give us 66.6 going to multiply it by 5 divided by 9 or five ninths and we're going to end up with 37 point 0 degrees C and again three sig figs three sig figs and in the conversions those are considered exact numbers so we're not going to count those for four significant figures all right so let's go the other way around we have 25 degrees C and what is that in degrees Fahrenheit so we're going to use the other equations we're gonna plug in degrees C here and we're going to multiply it by 9/5 we're gonna add on 32 so we're gonna get 45 plus 32 and that's going to give us seventy seven point zero degrees Fahrenheit all right another temperature scale very commonly used in chemistry and lots of other sciences is the Kelvin and and one thing to keep in mind about Kelvin just for the future I'm just putting this in your head is when we talk about gas laws the temperature that we use is always going to be in Kelvin so it has to be four gasless that's just my plug for later on in the course but this is the SI unit for temperature now the Kelvin temperature scale uses degrees that are the exact same unit size as the Celsius degree but the numerical scale has been shifted up by 273.15 units so let's take a look at these conversions and then it'll give you you know a more clear example of what we mean so if we want it if we want to go from degree C to Kelvin then basically well all we're going to do is take our degree C we're going to add 273 point one five to it okay if we want to go the other way around and get degrees C we're gonna put in our kelvins and we're going to subtract off 273.15 now most of the time is acceptable to just use 273 instead of 273.15 especially in this class but it's gonna depend on you know on the accuracy that you need for your problem okay another little interesting thing is that the Kelvin scale doesn't use the word degrees so we don't say we say 295 K or 295 Kelvin but we don't say 295 degrees Kelvin okay so there's no degrees in Kelvin and one of the reasons why well the reason why the Kelvin scale is defined the way it is is that there is a minimum possible temperature called absolute zero and so basically at Absolute Zero that is the bottom of the Kelvin scale so zero Kelvin is absolute zero and then the temperature has counted up from there so instead of using the freezing point of water as a baseline it's going to use absolute zero and count up from there and as I mentioned science uses the Celsius and Kelvin temperature scales exclusively not Fahrenheit at all all right so here's just a few pictures of thermometers that are just comparing so you can see 32 degrees C that's the freezing point of water zero degrees C for Celsius 273 K for Kelvin and absolute zero is zero Kelvin which we already discussed it's negative 273 degrees Celsius negative for 59 degrees Fahrenheit boiling point of water so 212 Fahrenheit 100 degrees Celsius 373 Kelvin okay so so that just gives you kind of a pictorial way to relate those different quantities all right so let's put it all together now so we have a normal room temperature and it's seventy two point zero degrees Fahrenheit we want to know the temperature of that room in degrees Celsius and also in Kelvin so first we're going to use the formula to determine the temperature in degrees Celsius so let's pick out that formula we're going to plug in our degrees Fahrenheit we're going to subtract off 32 multiply it by 5 divided by 9 and when we're done with that we're going to end up with twenty two point two degrees C so degrees Celsius now we can take that temperature in Celsius and plug it in to the Kelvin conversion fat equation to get kelvins from that so we have twenty two point two degrees C we're going to add on to seventy-three point one five we'll get to ninety five point four Kelvin and and so we have three significant figures up here four right here that's not correct we need to cut it down four is less than five so we're going to round down so we're going to end up at 295 Kelvin all right so just in summary chemistry uses Celsius and Kelvin scales to express temperatures the temperature on the Kelvin scale is the Celsius temperature plus 273.15 and the minimum possible temperature is absolute zero and that's you're okay on the Kelvin scale so you really want to make sure you can practice converting temperatures between all three temperature scales but especially you need to be able to easily convert from Celsius to Kelvin and vice versa just consistently so make sure you can do that very very easily