Which of the following instruments is used for measuring the relative density of liquids?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Hydrometer
A hydrometer is a weighted glass float with a graduated neck. It works on Archimedes’ principle: it sinks deeper in a lighter liquid and less deep in a denser one, so the mark at the liquid surface gives the relative density (specific gravity) of the liquid directly.
A lactometer is a hydrometer made for milk only: it shows whether water has been added to milk, so it is not the general instrument for liquids.