The Celsius scale has negative temperatures. As a result, it is not a ratio scale. When zero means "none" as it does for distance, it is possible to make ratios. A distance of 6 m is twice as big as 3 m. A temperature of 40°C is not twice as hot as 20°C, however. In the 1880s, Lord Kelvin used the behavior of gases to project how low temperature could go. This Powerpoint presentation describes the problem with the Celsius scale, and explains how Kelvin developed a ratio scale with an absolute zero. It provides the formulas for temperature conversions and some practice problems.
Free worksheets for this topic are available on my website at http://www.evanschemistrycorner.com.