Question

the heat energy required to raise the temperature of a unit mass of water is 4.2 joule
is it correct?

Answer 1

We may define specific heat of water as the amount of heat energy required to raise the temperature of unit mass (say one gram) of water through unit degree.
As amount of heat energy required to raise the temperature of one gram of water through 1⁰C is one calorie (by definition), therefore specific heat of water is

s = 1 cal g^{-1 0}C^-1 = 1 cal g^-1 K^-1
= 4.2 J g^-1 K^-1 = 4200 J kg^-1 K^-1