Practical Chemistry in a Nut Shell - II
Flame Test
Some volatile salts impart characteristic colour to the non-lu
minous flame. The chlorides of the metals are more volatile in comparison to other salts. The metal chloride volatilises and its thermal ionisation takes place.
The cations impart a characteristic colour to the flame as these absorb energy from the flame and transmit the same as light as characteristic colour.
| Colour of flame | Inference |
| Golden Yellow | Sodium |
| Violet | Potassium |
| Brick Red | Calcium |
| Crimson Red | Strontium |
| Apple Green | Barium |
| Green with a blue centre | Copper |
Borax Bead Test
On heating borax the colourless glassy bead formed consist of sodium metaborate and boric anhydride.
On heating with a coloured salt, the glassy bead forms a coloured metaborate in oxidising flame.
The metaborates posses different characteristic colours. The shade of the colour gives a clue regarding the presence of the radical. However in reducing flame the colours may be different due to different reactions.
| Metal | Colour of Flame in | |||
| Oxidizing Flame | Reducing Flame | |||
| Hot | Cold | Hot | Cold | |
| Copper | Green | Blue | Colourless | Brown-Red |
| Iron | Brown-Yellow | Pale-Yellow | Bottle Green | Bottle Green |
| Chromium | Green | Green | Green | Green |
| Cobalt | Blue | Blue | Blue | Blue |
| Manganese | Violet | Amethyst Red | Grey | Grey |
| Nickel | Violet | Brown | Grey | Grey |
Charcoal Cavity Test
This test is carried out on a charcoal block in which a small cavity has been made by a knife. When a metallic salt is heated with Na2CO3, the metal carbonate is formed which decomposes into oxide. The carbon of the block reduces the oxide into metal.
| | Observation | Inference |
| 1 | Formation of metallic bead | Ag |
| | a)Lustruous white, malleable | Pb |
| | b)Greyish white, marks paper | Sn |
| | c)White, does not mark paper | Cu |
| | d)Red | |
| 2 | In crustation with metal | Sb |
| | a)White incrustation, brittle metal | Bi |
| | b)Yellow incrustation, brittle metal | Pb |
| | c)Yellow incrustation, malleable metal | |
| 3 | In crustation without metal | ZnO, SnO |
| | a)White and yellow when hot | BiO |
| | b)Yellow and orange when hot | CdO |
| | c)Brown | As2O3 |
| | d)White | As2O3 |
Post a Comment