Complex sentence: Chemists use litmus paper to check acidity, using litmus, is a water-soluble dye extracted from certain lichens and absorbed on to filter paper; this is one of the oldest pH indicators.
Split into simpler sentences: Chemists use litmus paper to check acidity, using litmus, is a water-soluble dye extracted from certain lichens. Litmus is absorbed on to filter paper to make litmus papers; this is one of the oldest pH indicators.