Question

What are the indications that a chemical reaction has occurred?

Answer 1

Hint: As we know that a chemical reaction is a process in which the bonds are broken within the reactant molecules and new bonds are formed in order to yield a new substance which is called a product. Here we are asked about the indications that show us whether the reaction has occurred or not.

Complete answer:
The indications which tells us whether the reaction has occurred or not are as follows:-
-Change in temperature: When a reaction occurs, the mixture or solution cools down or heats up.
-Change of colour: when a colored solution changes its colour or colourless solution shows some colouration, then we can say that reaction has occurred. For example while making dibenzylideneacetone upon mixing colorless substrates we get a bright yellowish product.
-Emission of gas: This means that when a reaction occurs, the gas or fumes are formed in the reaction which gets liberated or evolved. For example upon reacting sodium with alcohol, hydrogen gas gets generated.
-Formation of precipitates: If we mix two liquids, and formation of precipitate occurs at the bottom of the beaker or test tube, this means we had a reaction. For example when sodium chloride solution is mixed with silver nitrate, we get a white precipitate.
-Change in the pH: When two solutions of different pH are mixed together and the resultant solution has completely different pH, this means that the reaction has taken place.

Note:
-Change in temperature is not enough to confirm that a reaction has occurred because some substances cause a similar effect.
-Few chemical reactions when it takes place, they produce different kinds of smell, which is also a sign of the occurrence of a chemical reaction.