When compressive forces are applied on a body, what causes increment in volume. According to me the volume should remain constant since the mass and density of body are constant.

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    $\begingroup$ Why do you assume that the density remains constant when a pressure is applied? Doesn't it strongly depend on the container of the object, if any? $\endgroup$ – AccidentalBismuthTransform May 8 at 7:46

Neither of them should remain constant. Have you seen any type of press ? There's plenty of them, take a look into a garbage collecting cars - they have standard garbage compressor integrated into machine. So this means that we can compress object volume into smaller one. Of course not all objects have same compressibility level. Ones (as water) are considered to be uncompressable. But this depends on amount of pressure you can give to object. Under extra-ordinary pressure conditions, such as inside of black-hole, water would be compressed too. Other objects compresses easily, such as most gasses at room temperature.

Compactifying volume means that we increase object's density, because density is inversely proportional to objects volume: $\rho = \frac mV$. So neither density stays constant.

If you have A LOT of compression power, then atoms of object can become so close to each other that electron capture reaction can begin : $$ p + e^- \to n + \nu_e $$ Proton captures electron and converts into neutron emitting neutrino in the process. If you do the math correctly you will see that $$ m_n - (m_p + m_e) = 0.7823 \,\,\left[\frac{\text{MeV}}{c^2}\right] $$ So in the end matter becomes more massive in the process of neutron accumulation. By rough analogies of mass-energy equivalence, $E=mc^2$, this means that you are accumulating/converting your compression energy into object mass, so object becomes heavier in the process. Thus mass too isn't constant under high compression. This process can be seen in neutron star formation. In Earth we don't have such huge pressure conditions, but astronomers can observe them in high-density star formation. Only in that case, pressure role is taken by gravity forces.

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