The alkali metals cesium (cs) and rubidium (rb) were discovered based on their characteristic flame colors. cesium is named after the sky and rubidium after the gem color. what colors of light do you think these metals give off when heated in a flame?

The color of the sky is blue and cesium been named after the sky shows that  it will most likely give off a blue light when heated.

The gem color ( ruby gem ) is known as red therefore naming rubidium after the gem color ( ruby gem ) based on the color of its light shows that when rubidium is heated it will most likely give off a red colored light/flame

Hence we can conclude that When cesium ( cs ) is heated it gives off a  Blue light while when rubidium ( rb ) is heated it gives off a Red light

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