Why we determined (K-Na) by flamphot method?this qausion is in chamestry.?

Answer 1

I assume by quasion you mean question...

K and Na can be determined qualitatively by a large variety of methods. One of them is inserting their salts in flame and examining the resulting flame color. The color diference is due to diferent energy for atomic excitation. This phenomenon belongs to chemistry but also to spectrometry and physics.
I hope this answers your questions.

Answer 2

Their flames are quite distinctive (violet for K and golden yellow for Na).