Would flame test be useful for detecting metal ions present in a mixture of metal ions? ?
it is not a reliable method. atomic spectroscopy would be much more accurate and sensitive than the human eye.
to add to the information provided by bob pursely, which of course is correct, there are a limited number of ions that can be detected by blame tests. there are fewer than can be detected when in the presence of other ions. for example, na can be detected easily. so can k. however, na and k together, gives a problem because na masks the color of k. that can sometimes be avoided by using a cobalt glass to filter the yellow part of the na flame out. ba, sr, and ca are ok by themselves but in mixtures they give problems. copper, by itself, is usually seen easily but is often overlooked in mixtures. that is what mr. pursley means when he says the tests aren't reliable in mixtures.
does atoms of the same element have the same number of protons?