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Boric acid reacts with methanol in the presence of sulfuric acid which acts as a catalyst, to produce an inorganic ester, trimethyl borate. These types of reactions are known as esterification reactions. The overall process can be described by the equation:
Trimethyl borate is a liquid with a low boiling point (68 - 69°C), it is highly flammable, and can be easily ignited with a Bunsen burner. An intense green flame is observed during the combustion. This green colored flame is specific for substances that contain boron atom. However, barium or copper compounds also color the flame in green, but there are small differences in the green colors. Out of this reason, we can check if sample contains boron atoms by introducing them into a flame. This is so called flame test and is used in qualitative chemical analysis to prove the presence of specific atoms. There is a lot of physical chemistry behind the process, but it is simplified in the paragraph bellow. Atoms and ions have two kinds of states: a ground state and an excited state. In the ground state all electrons in the specific atom are in their lowest energy levels possible. Atoms as well as ions can be excited by the heat of a flame, which means that electrons absorb energy. These excited atoms or ions are short living species, and have a great tendency to return to the ground state and release the excess of energy in form of a light. The color of the emitted light is specific for each element: yellow light is specific for sodium, red for lithium and strontium, violet for potassium and rubidium, blue for cesium, green for barium, copper and boron, etc.