(a) Red litmus paper turns blue in soap solution and no change occurs on blue litmus paper because soap is basic in nature.
(b) Denatured alcohol is ethyl alcohol which has been made unfit for drinking purposes by adding small amounts of poisonous substances like methanol, pyridine, copper sulphate etc. This is done to prevent the misuse of industrial alcohol for drinking purposes or black marketing (as it is supplied duty free for industrial purposes by the government).
