litmus paper

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Litmus paper turns red on contact with an acid.
石蕊试纸来试验这液体是不是酸性的。
Strong bases that turn litmus paper blue, they react with acids to yield salts, are caustic, and in concentrated form corrode tissues.
它们与酸作用成为中性盐；它们是苛性的，其浓缩物可腐蚀有机组织。
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。

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