Question

How can you use a flame test to distinguish between hard and soft water

Answer 1

Hard water contains Ca$^{2+}$ and $/$ or $Mg^{2+}$

A clean wire loop made out of platinum or nickel-chromium wire is dipped in water to be tested and then placed in the hottest portion of Bunsen burner flame.

Calcium gives a Brick red coloration to the bunsen burner flame indicating hard water.