Litmus paper turns red in the presence of an acid.

石蕊试纸遇到酸就变红。

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。

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。