I'm not a chemistry student. So, I humbly request you to limit yourself to the basics, please do not indulge in infighting. If you ignore the request, you would be investing your efforts at the wrong place and perhaps it would be considered a pretentious show of knowledge. Litmus is a water-soluble mixture of different dyes. It works between the pH range 4.5-8.3. To cover the full of spectrum of pH, 1-14, universal indicator is used. The dyes used in the litmus are extracted from lichens, especially Roccella tinctoria. Lichens are composite organisms consisting of a symbiotic association of a fungus with a photosynthetic partner, usually either a green alga or cyanobacterium. Here dye is certain chemical which can be single molecule or a group of different molecules and behaves in a particular way under different pH conditions. 1: As it is stated above that the litmus can only used when the range is between 4.5-8.3, what would happen to the litmus or dyes or their color patterns when pH is outside this bound? 2: How did someone come up with the idea that some weird composite organisms called lichens can be used to tell the acidity of the solution? It was not like that someone accidently dropped few grains of table salt or sugar into the solution and discovered that it could be used for acidity test. Wikipedia says: Chemical reactions other than acid-base reaction can also cause a color-change to litmus paper. For instance, chlorine gas turns blue litmus paper white – the litmus paper is bleached. This reaction is irreversible and therefore the litmus is not acting as an indicator in this situation. 3: What does it mean by 'irreversible' here? Perhaps, it means that chlorine somehow permanently damages the dyes and even if it is evaporated or removed fully, the dyes wouldn't retain their original property and couldn't be used for the test again.