The light produced by very hot atoms in the gaseous state is a unique spectrum for each element. To observe the spectrum requires the use of a prism, diffraction grating, or spectroscope. Before complex instruments were invented to observe elemental spectra chemists sometimes identified metals in compounds by doing a flame test. Salts are a type of compound that include a metal and a non-metal. Sodium chloride ( NaCl ) is the most familiar example of a salt but others include calcium chloride( CaCl 2 ) and copper(II) chloride ( CuCl 2 ). In flame tests salts that are dissolved in water are evaporated using a hot flame. In the flame the metal atoms become excited and produce their characteristic spectrum of light. However, since the observer does not use a spectroscope only one color is observed. It turns out that many metals produce a unique single color under these conditions. Some metals do produce very similar colors but a practiced eye can often distinguish them. This requires experience and is based on a comparison between the color observed and the known colors produced by different metallic salts. It is a traditional art of the chemistry laboratory to use these colors to identify specimens of compounds that contain unknown metals.