Physics · Mechanical Properties Of Solids · NEET
No, they are opposite (reciprocal) quantities. Bulk modulus B measures how much a body RESISTS a volume change under pressure, while compressibility k measures how EASILY the volume changes. Their relation is k = 1/B. A material with a high bulk modulus (like steel) has a low compressibility; a material with a low bulk modulus (like air) has a high compressibility. Students lose marks by treating them as the same, so always remember: stiff = high B = low k.
Since k = 1/B and bulk modulus is measured in pascal (Pa or N/m squared), compressibility has the SI unit of per pascal, written Pa raised to power minus one (Pa^-1) or m squared per newton (m^2/N). In short, its unit is just the inverse of pressure. This is a common one-mark trap: the unit of B is Pa, but the unit of k is Pa^-1.
Gas molecules are far apart and very weakly bound to their neighbours, so a small pressure easily pushes them closer and shrinks the volume a lot. NCERT states gases are about a million times more compressible than solids. Solids have atoms tightly coupled together, so they resist volume change strongly and are the LEAST compressible. Order: gases most compressible > liquids > solids least compressible.
The formula is k = -(1/Δp)(ΔV/V). The negative sign is there because when pressure increases (Δp positive), the volume decreases (ΔV negative). The two negatives make k a positive number. So the minus sign only keeps k positive; the numerical value of compressibility is always positive for a stable material.
Compressibility is defined as the fractional change in volume per unit increase in pressure. If a pressure Δp is applied, then ΔV/V = -k·Δp = -(Δp)/B. So the bigger the compressibility, the bigger the fractional squeeze for the same pressure. This is exactly the idea NEET tests when they ask for fractional decrease in volume or radius.
Match List I with List II. List I: A. Young's Modulus, B. Compressibility, C. Bulk Modulus, D. Poisson's Ratio. List II: I. (Δd/d)/(ΔL/L), II. FL/(A·ΔL), III. -(1/V)(ΔV/P), IV. -V·P/ΔV. Choose the correct answer.
The bulk modulus of a spherical object is B. If it is subjected to uniform pressure p, the fractional decrease in radius is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Compressibility is the reciprocal of the bulk modulus, k = 1/B, and it measures the fractional change in volume produced per unit increase in pressure.
The relation is k = 1/B, equivalently k = -(1/Δp)(ΔV/V). A high bulk modulus always means a low compressibility.
Gases are the most compressible and solids are the least. NCERT notes gases are about a million times more compressible than solids because gas molecules are weakly bound.
Its SI unit is per pascal (Pa^-1) or m^2/N, which is simply the inverse of the unit of pressure and bulk modulus.
It links directly to bulk modulus and to numerical questions on fractional change in volume or radius under pressure, which appear regularly, including NEET 2017 and NEET 2026 matching questions.
