Question

With $C{{r}_{2}}{{O}_{3}}$, colour of the bead in sodium carbonate bead test:
(A)- Red
(B)- Blue
(C)- Yellow
(D)- Green

Answer 1

Hint: The metals are easily detected by the bead test whose salt have coloured cations within it, which further form another coloured salt on being heated and with the help of their unique colour it can be identified.

Complete step by step answer:
The Sodium carbonate bead test, is an analytical method through which the sodium carbonate reduces the oxides or sulphides of the metal to the metallic state. These oxides on being exposed to the flame produce characteristic colours. Hence, helps in identifying the metal present.
- The process involves the finely powdered metal that is to be identified, to amount only one-third of the total mixture, as it is mixed with sodium carbonate (which forms the bead) and charcoal.

- Then, the mixture is moistened a little and placed onto the hollow of the charcoal and heated over the flame. Thus, the colour of the flame is observed and matched with the corresponding bead type and colour data available.


The reaction of chromium oxide, $C{{r}_{2}}{{O}_{3}}$with the sodium carbonate produces sodium chromate. The chromium oxide has chromium in $(+3)$ oxidation state, which is green in colour and is reduced to $(+6)$oxidation state in sodium chromate, which shows a characteristic yellow colour.

\[2C{{r}_{2}}{{O}_{3}}+4N{{a}_{2}}C{{O}_{3}}+3{{O}_{2}}\to 4N{{a}_{2}}Cr{{O}_{4}}+4C{{O}_{2}}\]


Therefore, in the sodium carbonate bead test with $C{{r}_{2}}{{O}_{3}}$, the colour of the bead is option (C)- yellow.


Note: It is applicable to transition metals which have coloured cations present in its salts. Along, with chromium, the manganese also gives colour, as it forms sodium manganate. It is quite useful in the analysis of silicates as well.