Question

During chemistry, Dan mixes two clear, colorless solution and mixing them produces a yellow precipitate in a clear, colorless final solution as shown by the chemical equation.
Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)

What physical evidence indicates that there has been a chemical reaction?

Answer 1

Answer : Formation of yellow precipitate is the physical evidence that there has been a chemical reaction.

Explanation :

A chemical reaction happens when 2 or more substances chemically react with each other.

There are 4 evidences indicating that a chemical reaction has occurred which are mentioned below.

1. Change in color : On mixing, the reaction mixture can change color indicating that a chemical change may have occurred.

2. Formation of precipitate : A reaction in which a solid is formed on mixing 2 aqueous solutions is called as precipitation reaction. Here, the formation of solid indicates a chemical change.

ex. $AgNO_{3} (aq) + NaCl (aq) \rightarrow AgCl (s) + NaNO_{3} (aq)$

In above reaction, on mixing the solutions, a white precipitate of AgCl is formed which indicates a chemical change.

3. Evolution of gas : A chemical change also occurs when a gas is evolved on mixing 2 substances.

ex. $2HCl (aq) + Na_{2}CO_{3} (aq) \rightarrow 2NaCl (aq) + H_{2}O (l) + CO_{2} (g)$

In above reaction, CO2 gas is evolved and bubbles can be seen in the form of effervescence.

4. Change in temperature : Certain chemical reactions absorb or evolve heat. When a chemical reaction evolves heat, the temperature of the final reaction mixture increases and when it absorbs heat, the temperature decreases. This increase and decrease in temperature indicates a chemical change.

ex.  $HCl (aq) + NaOH (aq) \rightarrow NaCl(aq) + H_{2}O (l)$

In above reaction, the final mixture becomes hot as the reaction releases heat. ( Exothermic)

Change in odor also indicates a chemical change.

For the given reaction, a yellow precipitate of PbI2 is getting formed.

Therefore formation of this precipitate is the physical evidence that there has been a chemical reaction.