石蕊试纸遇到酸就变红。
它的水溶液，或者做成石蕊纸后，是最古老和最常用的指示剂，可以指示一种物质是酸还是碱。
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.
这种变化虽然不同于碱滴定酸时石蕊由红到蓝的突然转变，但也相差不远了。
It was not quite the sudden shift from litmus red to blue of an acid being neutralised by an alkali, but it was not far off.
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