Density and viscosity of water

Liquid water is a fluid, a deformable substance without its own form, which changes shape under the action of an external force applied to it. Its shape is preserved only if a solid body limits it. Liquids are generally considered to be non-compressible; they keep the same volume whatever their shape. The gases tend to occupy all the available space; they are compressible.

Density of water

It is the measure of the mass present in a certain quantity of fluid. It corresponds to the number of molecules contained in the volume. We confuse density and density r. If the temperature rises, the molecules of the fluid move apart and the density decreases. If the temperature drops, it's the other way around.

Viscosity of water

Measurement of the resistance of a fluid to change of shape: viscosity determines the speed of movement of the fluid (for example, the speed of movement of a spoon in a bowl: the more viscous the liquid, the slower the movement) . The addition of a small amount of suspended or dissolved substance can greatly increase the viscosity of the liquid. Molecular viscosity is denoted by µ; it is expressed in N.s.m-2. Liquids have a higher viscosity than gases: the molecules are closer to gas: the molecules are closer together, bonds are established between them which increase the cohesion of the whole. Viscosity varies inversely with temperature. The kinematic viscosity n is equal to the ratio of the molecular viscosity of the fluid to its density.

There are two types of viscosity:

Dynamic viscosity of water

the dynamic (or absolute) viscosity µ defined by the ratio of the tangential shear stress T (in Pa) to the speed gradient V (in m · s – 1) in a direction y (in m) perpendicular to the sliding plane :

T = µ (du / dy)

u: speed

µ: depth of flow

du / dy: speed gradient

T: tangential stress

kinematic viscosity of water

the kinematic viscosity v defined by the ratio of the dynamic viscosity µ to the density ρ of the fluid:

ν = µ / ρ

The viscosity of water decreases with increasing temperature . On the other hand, it increases with the content of dissolved salts; seawater is therefore more viscous than river water.

Pressure acts in a very special way on the absolute viscosity of water.

Unlike other liquids, moderate pressure makes water less viscous at low temperatures: it somehow crushes its molecular organization. As the pressure continues to increase, the water takes on a liquid structure devoid of internal stress and follows the general rule, that is, viscosity increases with pressure.

surface tension of water

It characterizes a property of interfaces (surfaces limiting two phases). It is defined as a tensile force which is exerted on the surface of the liquid while always tending to reduce the extent of this surface as much as possible.

It is such that it causes a capillary rise of 15 cm at 18 ° C in a tube 0.1 mm in diameter.

Surface tension decreases with increasing temperature.

The addition of dissolved salts generally increases the surface tension (= 74.6 mN · m – 1 for a 1 mol · L – 1 aqueous solution of NaCℓ at 18 ° C).

Physical Properties of Water (U.S. Customary Units)

Temp Specific weight Density Modulus elasticity Dynamic viscosity Kinermatic viscosity Surface tension Vapor pressure
ºF γ, lb/ft3 b ρ, slug/ft3 b E/103, lbf/in2 µ × 105, lb · s/ft2 υ × 105, ft2/s c σ, lb/ft pv, lbf/in2
32 62.42 1.940 287 3.746 1.931 0.00518 0.09
40 2.43 1.940 296 3.229 1.664 0.00614 0.12
50 62.41 1.940 305 2.735 1.410 0.00509 0.18
60 62.37 1.938 313 2.359 1.217 0.00504 0.26
70 62.30 1.936 319 2.050 1.059 <0.00498 0.36
80 62.22 1.934 324 1.799 0.930 0.00492 0.51
90 62.11 1.931 328 1.595 0.826 0.00486 0.70
100 62.00 1.927 331 1.424 0.739 0.00480 0.95
110 61.86 1.923 332 1.284 0.667 0.00473 1.27
120 61.71 1.918 332 1.168 0.609 0.00467 1.69
130 61.55 1.913 331 1.069 0.558 0.00460 2.22
140 61.38 1.908 330 0.981 0.514 0.00454 2.89
150 61.20 1.902 328 0.905 0.476 0.00447 3.72
160 61.00 1.896 326 0.838 0.442 0.00441 4.74
170 60.80 1.890 322 0.780 0.413 0.00434 5.99
180 60.58 1.883 318 0.726 0.385 0.00427 7.51
190 60.36 1.876 313 0.678 0.362 0.00420 9.34
200 60.12 1.868 308 0.637 0.341 0.00413 11.52
212 59.83 1.860 300 0.593 0.319 0.00404 14.70

 

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