Question

Assertion: “A golden yellow precipitate of lead iodide is formed when colourless lead nitrate solution is mixed with colourless solution and potassium iodide.” So, it is a chemical change.
Reason: Formation of new products in the reaction with the precipitate indicates a chemical change.
Read the above assertion and reason and choose the correct option from the following.
A.Both assertion and reason are correct and reason is correct explanation of assertion
B.Both assertion and reason are correct and reason is not the correct explanation of assertion.
C.Assertion is correct but reason is not.
D.Assertion is incorrect but reason is correct.

Answer 1

For this question we must know how a chemical change occurs. If the new product forms have different physical and chemical properties then the chemical change occurs otherwise no chemical change occurs.

Complete step by step solution:
Chemical change occurs when reactants react with each other to form a product with different physical and chemical properties than that of reactants. In a more precise sense this is called chemical synthesis. Chemical decomposition occurs when one reactant decomposes into two different compounds.
Let us look at the assertion statement once,
The reaction of lead nitrate and potassium iodide is as follow:
${\text{Pb}}({\text{N}}{{\text{O}}_3}) + {\text{KI}} \to {\text{KN}}{{\text{O}}_3} + {\text{Pb}}{{\text{I}}_2}$
As we can see, precipitate of potassium iodide forms which have a different molecular formula and nature than that of the reactants. Also the colour of reactants and product is different as given in the question. Hence a chemical change is occurring over here. So the given assertion is correct.
Now let us look at the reason statement. It is obvious that the new compound form confirms the chemical change occurring and the precipitate formed is the proof of chemical change because both the reactants are in the aqueous phase and precipitate is in solid phase. This means that a new product which is insoluble in water unlike reactants has formed. So the reason statement is correct as well.
Hence both the statements are correct and the reason is the correct explanation of the assertion.
So the correct option is option A.

Note: chemical changes can be exothermic that is heat is released during the reaction as well as endothermic that is heat is required or consumed by the reactants. Chemical changes are mostly irreversible especially the formation of precipitate like in the above question.