Question

State one relevant observation for the following:
Lead nitrate solution is mixed with dilute hydrochloric acid and heated.

Answer 1

The law of conservation of mass, mass can be neither be created nor destroyed in a chemical reaction. The total mass of the elements presents within the products of the chemical reaction has to be equal to the total mass of elements present within the reactants. The solubility decreases with cooling and precipitation occur. On increasing the temperature, solubility increases.

Complete step by step answer:
When dilute HCl is added to lead nitrate solution and heated, it forms a white precipitate of lead chloride and nitric acid. In this double displacement, a reaction takes place.
$Pb{(N{O_3})_2} + 2HCl\xrightarrow{\Delta }PbC{l_2} + 2HN{O_3}$
On heating, the precipitate dissolves slowly to give a clear solution because lead chloride is fairly soluble in hot water. But on cooling the precipitate re-appears as lead chloride is far less soluble in cold water as we know solubility decreases in cold water.
The substance that undergoes a chemical change in the above reaction, lead nitrate, and hydrochloric acid are the reactants. The substance lead chloride and nitric acid are the products.
In the above observation a double displacement reaction takes place. In this type of reaction, two compounds reacted by an exchange of ions to form two new compounds are called double displacement reactions. In double replacement reactions, the positive ions and the negative ions are exchanged. A double replacement reaction is represented by the general equation.
$AB + CD \to AD + BC$
In the double displacement reaction, atoms of different compounds switch places. The reactants are two compounds and the products are two different compounds. Double displacement reactions can also be called double replacement reactions.

Note:
A white precipitate of lead chloride is formed when dilute hydrochloric acid is added to lead solution at room temperature. On heating, the precipitate gradually dissolves to give a clear solution because lead chloride is fairly soluble in hot water.