Red And Blue Litmus Paper
Why use red litmus paper and blue paper when testing substances with litmus paper?
You should always test both, as using just one will not provide enough information about the substance.
As an example. Suppose you drink acid ... a cup of orange juice and want to know if it is sour. What if you dropped a sheet of red paper and only red paper? The color of the paper does not change! You should also use blue paper to check for acid.
What if you have a glass of water and for some reason you don't know why it's neutral? If you only use a sheet of blue litmus paper, you will find that it will not change color and say, ham, that means it's easy. This shortcoming is clearly wrong. You should also use red litmus paper to check that nothing has changed and that the substance is neutral.
What if someone asked about your blood pH? If you didn't know it was between 7:35 and 7:45 in the morning, you probably didn't realize it was easy. It will not work using just a sheet of blue litmus paper. You will then see that the substance must be basic or neutral, but it does not give you the correct picture. You can take Lok and put a piece of red paper in it to make it look easy ... but you don't always get Lok, so it's important to use both papers.
Test on red litmus paper