Acids, Bases and Salts

What Is Litmus Paper?

Litmus Paper

Litmus paper is a filter paper we treat with a natural-soluble dye that the lichens provide us. The piece of paper that gives the result that is referred to as litmus paper can come in use as a pH indicating tool.

Under the acidic conditions, the blue paper turns red where the pH level is below 4.5. While the red paper does not change its color under the alkaline conditions where the pH level is above 8.3.

Blue litmus doesn’t change its color under the alkaline conditions, while the red one doesn’t change its color under acidic conditions.

Neutral litmus paper is kind of purple in color. Neutral paper turns into red color under the acidic conditions and in blue color under alkaline conditions.

While it might come in use for determining whether an aqueous solution is an acid or a base, it is not good to use for estimating the pH value of the liquid.

History and Composition of Litmus Paper 

Spanish physician Arnaldus de Villa Nova used the litmus paper for the first time around 1300 AD. Originally, the litmus was a blue dye that any number gives of lichen species found in the area of Netherlands.

litmus paper

Today, Roccella Montagnei species from Mozambique and Dedographa Leucophoea from California help in preparing this paper. However, litmus might have a number of 10 to 15 different types of dyes.

Litmus Paper Colours

The Red litmus consists of a weak diprotic acid. Upon exposure to base, a reaction takes place between the hydrogen ions and the base. Moreover, it produces a change in color that turns into blue color.

On the other hand, the blue litmus paper already consists of the blue conjugate base. It reacts with an acid, as a result of which it changes to red color.

pH Scale Basics of Litmus Paper

pH is that value which helps in expressing the concentration of hydrogen into the solution. The range of the pH scale is from 0 to 14 and equates to the negative logarithm of the concentration of hydrogen ion.

The pure water is neutral and its pH level is 7. Solutions that have a level of pH less than 7 are acidic. However, the solutions that have a pH level of more than 7 are alkaline or basic. The lemon juice and stomach acid have a pH level of about 2.

Coffee has a pH level of about 5. The human blood is slightly alkaline, with a pH that stays near about to 7.4. Household cleaners like ammonia solution and bleach have a pH value of approximately 9 and 12, respectively.

Litmus Paper Experiment

The litmus paper gives a general indication of acidity or alkalinity to the user because it correlates to the shade of red or blue color that the paper turns to. For testing the pH of a substance, we have to drip a strip of the paper in the solution or use a dropper or a pipette for dropping a small amount of the solution over the paper.

Moreover, the blue litmus paper is capable of indicating an acid that has a pH level of around 4 to 5 or lower than that. T\Similarly, the red litmus paper can show a base that has a pH level greater than 8. Thus, if a solution is with a pH between 5 and 8, it shows little change in the color over the paper.

Solved Question for You

Ques. What happens when a blue litmus paper is dipped into an acid solution?

(A). It turns red.
(B). Litmus paper turns green.
(C). The color vanishes out.
(D). No change is present in the colour

Ans. (A). The paper turns red in colour.

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