Introduction:
When blue litmus paper is placed in an acidic solution, a reaction occurs between the acid and the blue litmus paper. To determine whether this reaction is a chemical change or a physical change, we need to understand the characteristics of each type of change.

Physical Change:
A physical change refers to a change in the appearance or physical properties of a substance without any change in its chemical composition. In other words, the substance retains its identity even though its form may change. Examples of physical changes include melting, freezing, boiling, and dissolving.

Chemical Change:
A chemical change, on the other hand, involves the formation of new substances with different chemical compositions. During a chemical change, there is a rearrangement of atoms and bonds break and form. Examples of chemical changes include combustion, rusting, and digestion.

Reaction between Blue Litmus Paper and Acid:
In the case of blue litmus paper and acid, a chemical change occurs. When blue litmus paper is exposed to an acidic solution, the blue litmus paper turns red. This change in color indicates a chemical reaction has taken place. The blue litmus paper contains a dye that is sensitive to changes in pH. In an acidic solution, the dye in the blue litmus paper reacts with the acid, causing a chemical change in the dye molecules. As a result, the blue dye is converted into a red dye.

Evidence of Chemical Change:
There are several pieces of evidence that support the conclusion that a chemical change has occurred when blue litmus paper is placed in acid:

1. Change in Color: The blue litmus paper turns red when exposed to acid, indicating a chemical reaction has taken place.
2. Irreversibility: The change in color from blue to red is not easily reversible. Even if the litmus paper is removed from the acid, it will remain red, indicating a permanent change has occurred.
3. Formation of New Substances: The conversion of the blue dye in the litmus paper into a red dye demonstrates that new substances have been formed during the reaction.

Conclusion:
In summary, the reaction between blue litmus paper and acid is a chemical change. The change in color from blue to red, the irreversibility of the change, and the formation of new substances all support this conclusion.

If we put blue paper into acid it will turn into read and it will be a chemical change