Solutions

Litmus Paper and Litmus Test

What is the Litmus Test?

You must have heard of a litmus paper. It is quite common amongst chemistry practical. It is used to determine the pH of an aqueous solution. Thus, we refer to this determining as the litmus test.

litmus test

Litmus Paper

Litmus paper is basically a filter paper which has been treated with a particular indicator- a mix of 10-15 natural dyes. Furthermore, these dyes are acquired from lichens. It is so because they turn red when in contact with acidic conditions (pH 70).

Similarly, when the dye is purple it means the pH is neutral. However, one must always remember that all pH papers are not litmus paper.

The litmus paper is a kind of pH paper which we make by treating paper with natural dyes using lichens. Furthermore, the litmus paper is generally found in red or blue colour. While we use litmus paper commonly for testing the pH of liquids, we can also use it for testing gases.

Litmus Test

We perform the litmus test by dropping a little liquid sample on the small strip of paper. It can also be done by dipping that piece of litmus paper into the small specimen of the sample. However, it is preferable to not dip the litmus paper in the entire container of the chemical. This is so because the dye can soil a possibly valued sample.

This test is a very quick and easy way of finding whether the liquid or gaseous solution is either acidic or basic (alkaline). We can perform the litmus test by the use of litmus paper or an aqueous solution which contains litmus dye.

In the beginning, as we said, the litmus paper will either be red or blue. Further, when there is acidity ranging from pH 4.5 to 8.3, the blue paper will change to red. Similarly, if there is alkalinity, the red litmus paper will change to blue. Therefore, we can say that usually a pH of 4.5 and below means red litmus paper and a pH of 8.3 and above is blue litmus paper.

Furthermore, if the litmus paper turns into a purple colour, it will mean that pH is nearly neutral. When the red paper does not change colour, it means the sample is an acid. Similarly, if the blue one does not change, it is an indication of the sample being a base.

Always keep in mind that acids and bases only refer to the water-based solutions. Therefore, the pH paper will not change its colour in non-aqueous liquids like vegetable oil.

Limitations of Litmus Test

While we may think that the litmus test is quite simple and fast, it does have a few limitations. Firstly, we see it does not produce accurate results. In other words, the indication is not precise regarding the pH. As we do not have a specific numerical pH value, we cannot get precise results.

Moreover, the indication of whether the sample is acid or base is quite rough. In addition, there are other reasons as well as to why the paper is changing its colour other than the acid-base reaction.

Alternatives of Litmus Test

While the litmus paper is quite handy when we need a common acid-base indicator, we can get a better alternative that will produce more accurate results. For instance, one can use the red cabbage juice which changes its colour in reaction to pH from red through blue to greenish-yellow. Moreover, there are dyes orcein and azolitmin that produces results similar to the litmus paper only.

Solved Question for You

Question– What colour does litmus paper changes to when the sample is an acid?

  1. Blue
  2. Red
  3. Yellow
  4. Green

Answer– The correct answer is option B. Blue litmus paper turns into red colour under acidic conditions.

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