Which of the following is a main characteristic of chemical reaction?
A. Formation of precipitate
B. Colour change
C. Evolution of gas
D. All of these

Hint: A chemical reaction is in which the bonds are broken of the reactant molecules, and then the new bonds are formed within the products in order to form a new molecule. The chemical reactions are present everywhere in our surroundings from burning a candle to boiling a glass of water and heating your food.

Complete Solution :
 A chemical reaction is in which the bonds are broken off theThere are characteristics of chemical reaction:
a) Change of colour: Most of the times you must have seen the change of colour in the substances due to a chemical change. For example, if you left an iron rod in the winter days outside, then you will see the formation of the rust, oxide of the iron that will turn the object into reddish brown.
b) Precipitate formation: When two salts are mixed and if one of them is solid and doesn’t mix in the solution well, then it gets settled down at the bottom as a precipitate. For example, when lead nitrate reacts with potassium iodide, it forms a yellow precipitate of Lead iodide and potassium nitrate.
c) Evolution of gas: When you eat a cake, you must feel that it is very fluffy because the gas which is formed is called Carbon Dioxide and it escapes through the tiny holes which are formed.
So, the correct answer is “Option D”.

Note: There is one more characteristic related to chemical reaction: Temperature Change- when a chemical reaction takes place, then either the heat is evolved or heat is used to either raise the temperature or drop the temperature of the reaction.