<P> Chemical reactions other than acid - base can also cause a color change to litmus paper . For instance, chlorine gas turns blue litmus paper white--the litmus dye is bleached, because of presence of hypochlorite ions . This reaction is irreversible, so the litmus is not acting as an indicator in this situation . </P> <P> The litmus mixture has the CAS number 1393 - 92 - 6 and contains 10 to 15 different dyes . All of the chemical components of litmus are likely to be the same as those of the related mixture known as orcein, but in different proportions . In contrast with orcein, the principal constituent of litmus has an average molecular mass of 3300 . Acid - base indicators on litmus owe their properties to a 7 - hydroxyphenoxazone chromophore . Some fractions of litmus were given specific names including erythrolitmin (or erythrolein), azolitmin, spaniolitmin, leucoorcein, and leucazolitmin . Azolitmin shows nearly the same effect as litmus . </P> <P> Red litmus contains a weak diprotic acid . When it is exposed to a basic compound, the hydrogen ions react with the added base . The conjugated base, formed from the litmus acid, has a blue color, so the wet red litmus paper turns blue in alkaline solution . </P>

You can test the acidity of a gas by putting litmus paper in a gas