Question

A ______ turns red litmus to blue.
A.Acid
B.Base
C.Salt
D.Water

Answer 1

: Hint: To answer this question, you should recall that litmus is the simplest way to establish the pH of a substance. Red litmus paper reacts to alkaline substances by turning blue, while blue litmus paper reacts to acidic substances by turning red.

Complete step by step answer:
An indicator is a chemical substance which when added to acid or base changes its colour up to a limit with variation in pH of the solution to which it is added. Indicators usually are either weak acids or weak bases with a special characteristic of exhibiting different colours in the ionized and unionized form e.g. phenolphthalein is a weak acid. Litmus paper is made from wood cellulose that is infused with an aqueous solution consisting primarily of lichens. During the production of red litmus paper, the lichens are left to ferment in potassium carbonate, ammonia and a small amount of sulfuric or hydrochloric acid. Bases turn red litmus blue and acids turn blue litmus red.

Thus, the correct answer to this question is option B.

Note:
The titration graphs help in determining the equivalence point and hence the suitable indicators for various acid-base reactions:
1. Strong acid vs strong base: Neither indicator changes colour at the equivalence point. If we use phenolphthalein, we would titrate until it just becomes colourless (at pH 8) because that point is the closest to the equivalence point. However, if we use methyl orange, the titration will be done until there is the very first trace of orange in the solution. If the solution becomes red, we are getting further from the equivalence point.
2. Strong acid vs weak base: Methyl orange starts to change from yellow towards orange very close to the equivalence point.
3. Weak acid vs strong base: Use of phenolphthalein is preferred as it changes colour exactly where you want it to.
4. Weak acid vs weak base: The curve is for a case where the acid and base are both equally weak. Neither indicator is of any use.