Question

Lassaigne extract is acidified with acetic acid and then lead acetate solution is added. It forms black precipitate of :

• A. $PbS$
• B. $C{H}_{3}COONa$
• C. $PbS{O}_4$
• D. $PbO$

Answer 1

The correct option is A $PbS$

Lassaigne’s Test:

A small piece of a sodium metal is heated gently in a fusion tube till it melts to a shining globule. At this stage, a small amount of substance is added and the tube is heated strongly. The red hot tube is plunged into distilled water contained in a china dish. The contents are boiled for some time, cooled and then filtered. The filtrate is known as sodium extract or Lassaigne ‘s extract.

The extract contains sodium sulphide formed by the reaction between sulphur (present in the compound) and sodium:

$2Na+S⟶{Na}_{2}S$

The Lassaigne’s extract is divided into two parts and following tests are performed:

Lead acetate test: One part of the extract is acidified with acetic acid and then lead acetate solution is added. Formation of a black precipitate confirms the presence of sulphur in the organic compound.

${Na}_{2}S+Pb{\left(C{H}_{3}COO\right)}_2⟶PbS\downarrow +2C{H}_{3}COONa$

Sodium nitroprusside test. A few drops of sodium nitroprusside solution are added to another part of the Lassaigne’s extract. The appearance of purple colouration confirms the presence of sulphur:

${Na}_{2}S+{Na}_2\left[Fe{\left(CN\right)}_{5}NO\right]⟶{Na}_4[Fe{\left(CN\right)}_{5}NO.S]$