Test: turns blue litmus paper red.
In the form of a water solution or as litmus paper, it is the oldest and most-used indicator of whether a substance is an acid or a Base.
"Base: In chemistry, any substance that in water solution is slippery to the touch, tastes bitter, changes the colour of acid-base indicators (e.g., litmus paper), reacts with acids to form salts, and promotes certain chemical reactions (e.g., base catalysis)."
