Question

A mixture of boric acid with ethyl alcohol burns with green edged flame due to the formation of:
(A) Ethyl borax
(B) Ethyl borate
(C)Methyl borax
(D) Methyl borate

Answer 1

When the mixture of boric acid with ethyl alcohol is heated and gives green edged flame. The green edged flame is obtained only when borate anion is present. This method is widely used in the inorganic quantitative analysis as a confirmatory test for presence of borate.

Complete step by step solution:
Firstly, we should write the chemical equation:
When borax is heated in sulphuric acid to yield boric acid. The mixture of boric acid and ethyl alcohol is heated to give triethyl borate.
$N{a_2}{B_4}{O_7} + {H_2}S{O_4} + 5{H_2}O\xrightarrow{{}}N{a_2}S{O_4} + {H_3}B{O_3}$……………reaction 1
${H_3}B{O_3} + {C_2}{H_5}OH\xrightarrow{{}}B{(O{C_2}{H_5})_3} + 3{H_2}O$…………………reaction 2
The presence of $B{(O{C_2}{H_5})_3}$ gives green edged flame.
-Option A is "Ethyl borax". This is not the correct answer because no compound actually exists as ethyl borax only borax ($N{a_2}{B_4}{O_7}$) as seen in reaction 1. Therefore, option A is the wrong answer.
-Option B is "Ethyl borate". The product obtained in reaction 2 is triethyl borate, but it is also known as ethyl borate. The reaction 1 and 2 is the reaction involved in the ethyl borate test which is a confirmatory test for borate. The ethyl borate obtained in reaction 2 imparts green edged flame indicating the presence of borate. Therefore, option B is the correct answer.
-Option C is "Methyl borax". Same as ethyl borax methyl borax too does not exist. Therefore, option C is wrong.
-Option D is "methyl borate". Methyl borate is the wrong answer according to our question. Methyl borate too imparts green flame, but here the reactant mentioned in the question is boric acid and ethyl alcohol. When reaction takes place between ethyl alcohol and boric acid the formation of methyl borate in this case is not possible. Therefore, option D is the wrong answer.

Thus, the correct answer is Option B.

Note: Methyl borate also imparts green flame. The intensity of light is more in methyl borate than ethyl borate. The ethyl borate test has been practiced from olden days till date. In addition, methanol and boric acid gives methyl borate which also imparts a green colour.