Question

Hydrometer is used to measure
A. Pressure
B. Weight
C. Density
D. Mass

Answer 1

Hint: Recall Archimedes’s principle. The weight of the liquid displaced by the object is equal to the weight of the submerged part of the object in the liquid. In a hydrometer, the liquid to be tested is taken into the cylinder and the hydrometer is gently lowered into that liquid. Recall the property which resists sinking of the object.

Complete answer:
The hydrometer works on Archimedes’s principle which states that when an object is partially or completely submerged in the liquid, the weight of the liquid displaced by the object is equal to the net upward thrust known as buoyant force provided by the liquid. Also, the weight of the liquid displaced by the object is equal to the weight of the submerged part of the object. Therefore, we can express,
\[{m_{liquid}}g = {m_{object}}g\]
\[ \Rightarrow {m_{liquid}} = {m_{object}}\]
\[ \Rightarrow {\rho _l}{V_l} = {\rho _o}{V_o}\]
\[ \therefore\dfrac{{{V_l}}}{{{V_o}}} = \dfrac{{{\rho _o}}}{{{\rho _l}}}\]
Here, \[\rho \] is the density and V is the volume. Here, the volume V is the submerged part of the object.

Therefore, we can see that the right hand term of the above equation is the relative density of the object. Thus, we can say that the hydrometer is used to determine the specific density of liquids.

So, the correct answer is option C.

Additional information:
In a hydrometer, the vertical container is poured with the liquid to be taken for the determination of the density. The hydrometer is lowered into the liquid until the hydrometer floats freely in the liquid taken for the test. We know that the object floats on the surface of the water because of their relatively low density. The same happens in case of hydrometers. Lower the density of the test liquid, more the hydrometer sink in the liquid.

Note: Do not get confused between hydrometer and hygrometer. The hydrometer is used for measuring the density while the hygrometer is used for measuring the amount of vapor content in the air. Theoretically we can measure the relative density of the liquid but the hydrometer is calibrated such that it gives the density of the liquid and not the relative density.