Is the square root of 12 a rational number or irrational number?

If the square root of a natural number is not an integer, then it is irrational. Another way to look at it: if a natural number N falls between two perfect squares, then the square root of N is irrational.

So in this case, 12 is between 9 (3 squared) and 16 (4 squared). So the square root of 12 is between 3 and 4, and also the square root of 12 is irrational.