CBSE Class 10 Science 2019 · Paper 31/5/1 · Question 23(a)
How is the presence of an acid tested with a strip of red litmus paper ?
Answer
Turn the red strip blue with a base, then dip it in the test solution: it turns red if an acid is present.
An acid does not change the colour of red litmus - the strip simply stays red, as it would in a neutral solution too - because an acid turns blue litmus red. So the red strip is first converted into blue litmus: dip it in a basic solution, such as dilute sodium hydroxide, and it turns blue. Then dip this blue strip in the solution to be tested: if it turns red, an acid is present; if it stays blue, the solution is not acidic.
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