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The Straight Liney = mx +b
where "m" is the slope, and "b" is the y-intercept.
In the case of the Charles Law lab, "y" is the column height of the entrapped air, and this is directly proportional to the volume, and "x" is the temperature.
Thus our equation is:
VOLUME = SLOPE x TEMPERATURE + VOLUME at 0°C
Since we are looking for the temperature when the volume falls to zero (the "x-intercept"), our equation becomes thus:
0 = SLOPE x TEMPERATURE + VOLUME at 0°C
Thus with doing a little surgery on this equation, we get:
TEMPERATURE = -(VOLUME at 0°C)/SLOPE
From this you should see that it is very important to get precise readings for the VOLUME at 0°C, and so that the slope has the greatest precision, you need good readings at the highest possible temperature you can reliably attain. You might think that 100°C is that number, but what if we dissolve a lot of salt in our water and bring it to a boil? It should be hotter, right? So consider trying that!