How is the presence of an acid tested with a strip of red litmus paper?

No colour change on red litmus paper with acids as acid turns blue litmus to red, so red litmus is first to be changed to blue litmus. In order to do so,

  • Dip the red litmus paper into an alkaline solution. It will turn blue.
  • Then dip this blue litmus paper into the acid, it will turn red again, thus indicating the presence of an acid.

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