Since the SI unit for temperature is Kelvin, why is the Celsius scale most often used in the science labs?

I don't know that it IS the most often used. For example, almost all of the calculations require changing from C to K (gas law problems, and almost all others EXCEPT those where a difference of T is used). For hands on lab work, however, you are correct that it is the most often used scale. Although I don't know this, I suspect that it dates back to the "old" days when the centigrade scale was the ordinary European system way (for science and for households) of making lab measurement of T and it became more or less a standard out of convenience. Others may know of some specific reason.

The Celsius scale is most often used in science labs for practical reasons and its historical significance. Here's how you can understand why:

1. Practicality: The Celsius scale is often used because it is convenient for scientific calculations. Many scientific equations and formulas, such as the gas laws, require temperature values to be in Kelvin (K) rather than Celsius (°C). The Celsius scale and the Kelvin scale have the same size of temperature increments (1 degree Celsius is equal to 1 Kelvin), making it easy to convert between the two scales by simply adding or subtracting 273.15.

2. Historical Significance: The Celsius scale has a long history and widespread use in everyday life and science. It was named after the Swedish astronomer Anders Celsius, who proposed the scale in 1742. The Celsius scale gained popularity and became widely adopted, particularly in scientific research and laboratory work, especially in Europe.

3. International System of Units (SI): While the SI unit for temperature is Kelvin, the Celsius scale still remains widely used within the scientific community. This is partly due to convention and the familiarity of scientists with the Celsius scale. Additionally, many experimental data and scientific literature are often reported in Celsius, further reinforcing its usage in scientific labs.

In conclusion, while the SI unit for temperature is Kelvin, the Celsius scale is most often used in science labs due to its practicality for calculations and its historical significance in scientific research.