What is a ring test? Explain it?

Hint: So, basically to proceed with the question first we need to understand the meaning of the in test or we can say when the test is called the ring test. So the ring test is basically a reaction in which two species react with each other and at the position in the test tube , where the two reactants form the junction, is shown as the collared ring. Or at the junction of two reactants a market coloured ring is formed depicting that the reaction is positive or has taken place positively.

Complete answer: Generally in the field of chemistry the ring test is referred to as the ‘Brown ring test’ which is performed for the nitrate.
So let’s have a look at the chemical equation of the above reaction i.e. The Brown ring test:
\[2HN{O_3} + 3{H_2}S{O_4} + 6FeS{O_4} \to 3F{e_2}{(S{O_4})_3} + 2NO + 4{H_2}O\]
The overall reaction is the reduction of nitrate ion by iron (II) which is oxidised to iron (III) and formation of a nitrosonium complex.

Later on, the remaining: further reaction as shown below:
\[[Fe{({H_2}O)_6}]S{O_4} + NO \to [Fe{({H_2}O)_5}NO]S{O_4} + 4{H_2}O\]
So, the reaction is clear and so is the ring test.

Note:
In the above reaction or test for nitrate, the iron sulphate is added to the solution of nitrate and then slowly we will add sulphuric acid that will lead to the formation of the brown ring at the junction of the two layers. And the formation of this brown layer at the junction indicates the presence of the nitrate ions in the solution.