When two substances (reactants) are mixed in solution form, a chemical reaction occurs, resulting in the formation of new substances (products).
Do you like a coffee? What is the process involved in making coffee?
When milk is combined with coffee decoction, the colour of the milk and decoction changes due to a chemical reaction. After that, you can add just the right amount of sugar to make it delicious.
Note: If the milk powder and coffee powder are mixed in their solid-state, there will be no reaction that takes place. So it proves that some chemical changes take place when the reactants are in their liquid state.
Let us move on with the second example:

Case 1: In a test tube, combine a small volume of solid silver nitrate and sodium chloride.
Observation: There will be no change that takes place.
Result: It is because in solid-state, these reactants have no reactions.

Case 2: Take one test tubes with a small volume of solid silver nitrate and another with sodium chloride. Now dissolve these reactants in water separately in their test tubes respectively.
Observation: Let us now mix both the solutions. We can observe that silver nitrate solution reacts with sodium chloride solution to form a white precipitate.
Result: Chemical reaction takes place with the formation of a new substances.
From the above reaction we can conclude that certain chemical reactions take place only in solution and not in solid form.