Ishita says anything that tastes sour must be acidic, because sourness is exactly how you…

Why

Ishita has a genuinely useful clue — most common acids we eat, like lemon, tamarind and curd, really do taste sour, so sourness is a fair everyday hint. The trouble is turning a hint into a rule you can trust completely, and never testing acids you should never taste at all. Many acids used in science and industry are far too dangerous to put anywhere near your tongue — strong acids can burn skin badly — so tasting is never actually a safe or reliable general test. The reliable test scientists use instead is an indicator: a substance like litmus paper that changes colour on contact — acids turn blue litmus red, and this works whether or not something is safe, or even meant, to taste. Test: lemon juice turns blue litmus red just as expected, but so would substances no one should ever put in their mouth. Her friend is right: taste can be a rough clue for kitchen acids, but the sure test is a chemical indicator like litmus, not your tongue.