Page 121 - Physics

Sinking and fl oating

relative density is 0.8 and sinks less in water Example 5.9
whose relative density is 1. This means that The hydrometre shown in Figure 5.16 is
water is denser than methanol.
used to measure the densities of liquids
Determination of relative density of a between 1 g/cm and 0.8 g/cm . Assume
3
3
liquid using hydrometer that the density of water = 1 g/cm , the
3
cross-section area of the stem is 0.5
Acti vity 5.8 cm and the height of the stem above
2
Aim: To determine the relative the liquid level is 16 cm. Determine
density of a liquid. the volume of the hydrometre below

Materials: Liquid, hydrometer, sand
or lead ballast the 1.0 mark.

Procedure
1. Take a thin-walled test tube of
cross-section area A and put a strip 16 cm
of graduated graph paper inside it.
Ensure that the strip is waterproofed.
2. Take a glass jar containing water.
3. Try to fl oat the test tube in the vertical
position and partially submerged in
water. Let the density of water be ρ 1 .
4. Put some mass of sand or lead ballast Bulb (B)
in the test tube to make the bottom of
the test tube slightly heavy and read
the level of water on the graph paper. Lead ballast
Record the reading as L .
1
5. Replace water in the jar with the Figure 5.16
liquid whose density is ρ 2 . Solution
6. Again, record the level of the liquid Let V cm be the volume of the
3
on the graph paper as L . 1
2 hydrometre below 1.0 mark.
Since, the weight of the test tube with
the mass is the same in both cases, the The volume between 0.8 and 1.0 marks
Upthrust of water on the test tube = be V cm 3
upthrust of the liquid on the test tube. 2
The volume below the 0.8 mark be V cm 3
That means, 3
AL ρ g = AL ρ g Then, V = Area × height
2
1 1
2
2
2
L ρ = 0.5 cm ×16 cm
1 = 2 .
L ρ = 8 cm 3
2 1
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Physics Form 1 Final.indd 115 16/10/2024 20:57