Question

Which gives violet colour with borax?
A.Fe
B.Pb
C.Co
D.Mn

Answer 1

When microcosmic salt is heated till it is red, it leaves a transparent glass of sodium phosphate. The molecular formula of microcosmic salt is $N{a_2}HP{O_4}$ . This compound dissolves in metal oxides like borax and results in the formation of glasses that have peculiar hues of colours. This method is also used in the qualitative analysis of various other substances and is known as the borax bead test.

Complete Step-by-Step Answer:
This name has been given to this process because of the formation of beads of sodium phosphate upon heating. The molecular formula for borax is given as $N{a_2}[{B_4}{O_5}{(OH)_4}].8{H_2}O$ .
This test is widely used in qualitative inorganic analysis to test the presence of certain metals. Each metal results in the display of a certain colour when tested in this manner.
The experimental setup and procedure for this borax bead test can be explained as: first, a small is made using a platinum or nichrome wire and this loop is heated till it becomes red hot. Then it is dipped in powdered borax till the loop is completely covered in the powder. This is now again heated in the flame till a transparent glass – like bead is formed. This glass bead is then slightly dipped in the solution to be tested after being allowed to cool. The bead and adhering substance is then heated in the lower, reducing, part of the flame, allowed to cool, and the colour observed. It is then heated in the upper, oxidizing, part of the flame, allowed to cool, and the colour observed again.
Each metal produces its own distinguished colour profile in this test. Let us discuss the colours obtained by testing the given options in this method:
1.Fe: Fe exhibits yellow colour in oxidizing flame and bottle green colour in reducing flame.
2.Pb: Pb exhibits pale yellow colour
3.Co: Co exhibits deep blue colour in both oxidizing and reducing flames
4.Mn: Mn exhibits pink – purple colour in oxidizing flame.
Hence, Option D is the correct option

Note: Several metals may give the same colour. Some metals give only colourless or grey beads. Hence, in these cases, we must use alternate methods to identify the metals.