Question

Name an instrument based on this principle. State two uses of the instrument that you describe.

Answer 1

A hydrometer is based on the principle of floatation.

A hydrometer is a device used for measuring the relative densi ty of a liquid directly.

Hydrometer

It usually consists of a glass float with a long thin stem which is graduated. The glass float is a large hollow bulb which increases with buoyancy. The narrow stem increases the sensitivity of hydrometer. The bottom of the hydrometer is made heavier by loading it with lead shots so that it floats vertically.

The hydrometer works on the principle of floatation. Consider a thin-walled and flat bottomed test tube. Add some lead shots in the test tube and place it in a jar containing water. Adjust the number of lead shots such that it floats vertically with some of its portion outside the surface of the water in a jar.

If l is the length of the test tube inside water, a is an area of cross-section of the test tube, d is the density of water in which it floats, then the weight of water displaced = aldg and it is equal to the weight of loaded test tube.

Now allow the test tube to float in the other jar filled with a liquid of density d₁ and note the level l₁ at which it floats in that liquid.

Weight of the liquid displaced = al₁d₁g

Using the law of floatation
aldg = al₁d₁g

Id = l₁d₁

l/l₁ = d₁/d

or l is inversely proportional to the density.

It sinks more in a lighter liquid so as to displace more volume of the lighter liquid whose weight is equal to the weight of Hydrometer. Hence, it will sink less in a denser liquid so that it has displaced less volume of the denser liquid whose weight will be equal to the hydrometer.

In this way, this measure relative density of a liquid Barometer can be used as a lactometer which can be used to check the purity of milk.

The barometer can be used as an acid battery hydrometer which can be used to check the concentration of sulphuric acid in an acid battery.