Video: Knowing Key Facts about Temperature Scales

What is the value of the freezing point of water in Kelvin? Give your answer to 3 significant figures.

03:10

Video Transcript

What is the value of the freezing point of water in kelvin? Give your answer to three significant figures.

Okay, so in this question, we’re being asked to find the freezing point of water in the Kelvin temperature scale. Now, some of us may remember that the temperature at which water turns into solid ice, which is exactly what the freezing point of water is in the Celsius scale, happens to be zero degree Celsius. And some of us may remember it in the Fahrenheit scale, which happens to be 32 degrees Fahrenheit. Now, both of these temperatures are equivalent. Zero degrees Celsius is the same thing as 32 degrees Fahrenheit. They’re just expressed in different temperature scales, but they both represent the freezing point of water. However, in this question, we’ve been asked to find that freezing point in kelvin.

So starting with our temperature of zero degrees Celsius, we can recall that the temperature in kelvin that we’re trying to find is equal to the temperature in degrees Celsius plus 273. Which means that if we want to find the freezing point of water in kelvin, we can say that the temperature in kelvin of the freezing point of water is equal to zero, which is the temperature in Celsius of the freezing point of water, plus 273. And that just ends up being 273. Therefore, we can say that the temperature at which water freezes is 273 kelvin. Remember, we don’t use degrees kelvin for the Kelvin temperature scale, even though we do use degrees for the Celsius and Fahrenheit scales.

Anyway, so let’s see the procedure we would have to undergo if we started with our Fahrenheit temperature. We know that 32 degrees Fahrenheit is the freezing point of water. And we want to convert this to kelvin. Now, the easiest way to convert from Fahrenheit to kelvin is to first convert our temperature into degrees Celsius because there’s a very simple relationship between the temperature in Celsius and in kelvin. We simply add 273 to the temperature in Celsius to give us the temperature in kelvin. So as we’ve mentioned already, we first want to convert our temperature in Fahrenheit to a temperature in Celsius. To do this, we can recall that a temperature in degrees Celsius is equal to five-ninths multiplied by the equivalent temperature in Fahrenheit minus 32.

And so specifically for the freezing point of water, we can say that the temperature in degrees Celsius is equal to five-ninths multiplied by the temperature in degrees Fahrenheit, which is 32 minus 32. And 32 minus 32 inside this parenthesis gives us zero. And so on the right-hand side, we have five-ninths multiplied by zero, which means that the temperature in degrees Celsius of the freezing point of water is zero, exactly as we saw earlier in the video. Except in this instance, we started with the assumed knowledge of zero degrees Celsius being the freezing point of water.

If we can only remember 32 degrees Fahrenheit as being the freezing point of water, then we first need to find this temperature in degrees Celsius. And then once we’ve done this, once we know that zero degrees Celsius is the freezing point of water in degrees Celsius, we can then follow the same steps as earlier in this video to find the temperature in kelvin of the freezing point of water. And so both calculations lead us to the exact same answer, which is that 273 kelvin is the freezing point of water in kelvin. And because this quantity already has been written to three significant figures — one, two, three — we can say that 273 kelvin is the final answer to our question.

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