Answer :
Blue litmus paper turns red. xH2O is heated it decomposed to produce HCl which is the cause for a change in color.
Blue litmus paper is mainly used for acid testing. It turns red when it comes in contact with acid. It remains blue in contact with basic or neutral substances. The dye in blue litmus reacts with the H+ ions and is chemically changed so that the bonds are tuned to reflect longer wavelength light, appearing red to our eyes.
A simple way to measure pH is to use litmus paper. When litmus paper is touched it changes color depending on whether the paper is acidic or basic. If the paper turns red it is acidic if it turns blue it is basic. There is no reaction when blue litmus paper is placed on the pad, so there is no change in the blue litmus paper.
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