NCERT Solutions
Class 11 Physics
Mechanical Properties of Solids

Q. 8.13
What is the density of water at a depth where pressure is 80.0 atm, given that its density at the surface is 1.03 × 103 kg m–3?
Let the given depth be h.
Pressure at the given depth, p = 80.0 atm = 80 × 1.01 × 105 Pa
Density of water at the surface, ρ1 = 1.03 × 103 kg m–3
Let ρ2 be the density of water at the depth h.
Let V1 be the volume of water of mass m at the surface.
Let V2 be the volume of water of mass m at the depth h.
Let ΔV be the change in volume.
ΔV = (V1 - V2)
=m ((1/ ρ1) – (1/ ρ2))
Therefore, Volumetric strain = (ΔV/ V1)
=m ((1/ ρ1) – (1/ ρ2)) x (ρ1/m)
Therefore, ΔV/ V1 = (1- ρ1/ ρ2) … (i)
Bulk modulus, B = (p V1/ ΔV)
(ΔV/ V1) = (p/B)
Compressibility of water = (1/B) =45.8x10-11 Pa-1
Therefore, (ΔV/ V1) = (80x1.013x105x45.8x10-11) = 3.71x10-3 … (ii)
For equations (i) and (ii), we get:
(1- ρ1 / ρ2) = 3.71 x10-3
ρ2 = 1.03x103/(1-(3.71x10-3)
=1.034x103kgm-3
Therefore, the density of water at the given depth (h) is 1.034 × 103 kg m–3.