In gases pressure is due to collision of molecules with the boundary, what is the reason of pressure in liquid

Answer 1

Although liquid molecules are in motion, it is the mass of fluid acting on an area that produces pressure.

Here's some extra, more easily understood, info for you: -

e.g. 1m³ of pure water weighs 1,000kg (1,000,000g). The base area of 1m³ = 100 x 100 = 10,000cm²
1,000,000g acting on the area of 10,000cm²
= 1,000,000 ÷ 10,000 = 100g/cm² pressure.
Therefore a 1m head (height) of pure water exerts 100g/cm²

In Imperial, the pressure exerted by pure water can be found by dividing the mass of 1ft³ by the area of its base.

= 62.4lbs ÷ 144inches² = 0.433 psi
1 ft³ has a height of 1 ft.
Therefore a 1ft head (height) of pure water exerts 0.433 psi.

Any of these pressure readings can be applied to find the pressure exerted by any liquid simply by multiplying the above pressure figures by the S.G. & head of the liquid.

e.g. Mercury has a S.G. of 13.6.
100 x 13.6 = 1,360g/cm²/m
0.433 x 13.6 = 5.89psi/foot

Gasoline has a S.G. of about 0.76.
100 x 0.76 = 76g/cm²/m
0.433 x 0.76 = 0.329 psi/ft

Answer 2

There is pressure in liquids for EXACTLY the same reason! There is still molecular agitation in a liquid, and it's the momentum of the molecules hitting the boundary that creates the pressure.

Note that, technically, you don't actually NEED a BOUNDARY to determine a pressure. 'Pressure' is also defined as the NET transfer of momentum per unit area per second through a surface IMAGINED to be drawn in a medium.

1. Imagine a surface drawn INSIDE some body of liquid perpendicular to the x-axis. In the momentum budget, NEGATIVE x-momentum carried to the LEFT contributes exactly as much as POSITIVE x-momentum carried to the RIGHT, thereby DOUBLING the x-momentum transfer envisaged solely from material arriving from the left. This is so even though the IDENTITY of the molecules has now actually changed. (The molecules originally arriving from the left and passing through the surface are distinct from those arriving fron the right.)

2. At an actual, physical boundary, you have the same x-momentum approaching it from the left, say. After those molecules have been perfectly reflected, the original perpendicularly arriving x-momentum of each individual molecule has itself been reversed. This took an impulse per unit area equal to TWICE the arriving x-momentum to reverse, giving one the SAME factor of TWO increment in the momentum transfer for what are now physically IDENTICAL molecules after the impact.

The presence of dissolved gas, mentioned by the first responder, will add to the net pressure, but it is NOT necessarily a contributor to the pressure of all liquids.

Live long and prosper.

POSTSCRIPT: Note that like many career engineers, the responder immediately below can apply cookbook recipes for the evaluation of pressure under certain prescribed conditions, but simply does not understand its fundamental atomic or molecular basis. That basis is covered in good upper division or graduate physics and astrophysics texts.

Yes, of course in a static situation the value the pressure HAS somewhere can be evaluated by considering what force per HORIZONTAL unit area is required to support the vertical head of fluid ABOVE it. But that "explanation" of the "reason" for the pressure existing hardly explains why the pressure on a VERTICAL wall has the same value at that depth! (After all, that vertical wall itself DOESN'T have to "support" a column of fluid.)

The pressure on the vertical wall can only be understood by appreciating that the internal atomic or molecular velocities are both randomly and isotropically distributed, so that the net transfer of momentum across ANY surface, whatever its orientation, is the same at a given depth.

Without such an appreciation, the pressure needed for VERTICAL support could be derived from internal molecular motions that were themseves largely vertical, with very little "sideways" momentum transfer and therefore almost no "pressure" on those vertical walls.

Clearly the responder below has never asked himself how a VERTICAL support requirement leads to a SIDEWAYS FORCE! (The anwer isn't simply : "Well, everyone knows that pressure is isotropic," because this demands the question "HOW or WHY do we know that pressure is isotropic?"

And once one inderstands THAT, one can go on to understand why VISCOSITY exists, why HEAT TRANSFER occurs, etc., etc., ALL very important in both the oil industry and in astrophysics, and ALL having a verifiable atomic or molecular basis.

Answer 3

Pretty much the same, small amounts of trace gases in the liquids like CO2 build up due to temperature and other outside factors, thus causing pressure.