Chemistry · Some Basic Concepts Of Chemistry · NEET
Take the Celsius number and add 273.15. That is the whole rule: K = °C + 273.15. Example: 25 °C becomes 25 + 273.15 = 298.15 K. To go from Kelvin back to Celsius, do the opposite and subtract 273.15: 310 K = 310 − 273.15 = 36.85 °C. Nothing is multiplied or divided, so the maths is easy — you only shift the number up or down by 273.15.
The exact NCERT formula is K = °C + 273.15. In most calculation problems the extra 0.15 does not change the final answer to the given significant figures, so books often use the rounded value 273 for speed. Use 273.15 when the question gives precise data or asks for the exact value. If a NEET-style option list shows both, pick the one that matches the precision of the numbers in the question. When unsure, 273.15 is the correct textbook value.
Because the Celsius degree and the Kelvin unit are the same SIZE. A rise of 1 °C is exactly a rise of 1 K. The two scales only START at different points. Celsius starts (0°) at the freezing point of water. Kelvin starts (0 K) at absolute zero, the lowest possible temperature, which is 273.15 degrees colder than water's freezing point. So you only slide the scale by 273.15 — you never stretch it, which is why there is no multiplying.
0 °C (freezing point of water) = 0 + 273.15 = 273.15 K. 100 °C (boiling point of water) = 100 + 273.15 = 373.15 K. Notice the gap: 373.15 − 273.15 = 100 K, and on Celsius it was also 100 degrees. Same gap on both scales — that proves the units are equal in size.
Kelvin is an SI base unit, so it is written as just a number and the letter K, like 298 K, not 298 °K. The degree sign is only used for Celsius (°C) and Fahrenheit (°F). Writing "°K" is wrong and can cost a mark. Say "298 kelvin", not "298 degrees kelvin".
Any time you use a gas law or a formula with T in it — like PV = nRT, Charles's law, or Gay-Lussac's law — the temperature MUST be in Kelvin. Putting Celsius into these formulas gives wrong answers, and can even give a negative temperature which is impossible in a gas law. So convert to Kelvin first, then calculate. This is the number-one reason NEET expects you to know this conversion.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
K = °C + 273.15. Add 273.15 to the Celsius value to get Kelvin, and subtract 273.15 to go back to Celsius.
37 + 273.15 = 310.15 K, usually written as about 310 K.
273.15 is the exact NCERT value. Many problems round it to 273 for convenience; use 273.15 when the data are precise or the answer needs it.
No. Kelvin starts at absolute zero (0 K), the lowest possible temperature, so it can never be negative. Celsius can be negative, but Kelvin cannot.
Write 300 K. Kelvin is an SI unit and never uses the degree symbol. Only Celsius (°C) and Fahrenheit (°F) use °.
