Litmus paper turns red in the presence of an acid.

石蕊试纸遇到酸就变红。

《牛津词典》

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.

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。

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We have no litmus - paper, but, see, here they come with the syrup of violets.

我们没有蓝色试纸，但是，听！他们拿堇菜汁来了。

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"It's very much the same process that's involved in litmus paper, " lead researcher Kenneth Suslick told CNN Radio.

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When the government waves its litmus paper in their face, the Lib Dems retort that if the prime minister is serious about his project, he needs to understand that coalition politics cannot work unless on some issues, at some times the smaller party is allowed to win.

ECONOMIST: The Scottish play