Boiling water is 100 degree in celsius, is that an obtuse angle?

No. An obtuse angle is any angle greater than 90 degree. The degree here is a unit of measure used to measure the magnitude of an angle, and it's a Geometric concept. Although boiling water is 100 degree, the degree here is a scale of temperature. The degrees refer to different things. Therefore, boiling water is not even an angle, and it's not an obtuse angle.