Specific Gravity of Water

Specific Gravity of Water Points : specific gravity of water, surface tension of water, viscosity of water, compressibility of water Specific Gravity of Water The specific gravity (briefly written as sp. gr.) of a liquid may be defined as the ratio of its specific weight to that of a standard substance at a standard temperature. For liquids, pure water is taken as a standard substance and at 4°C. Mathematically specific gravity

Specific weight of liquid / Specific weight of pure water = liquid/water

Notes
1. The specific gravity of water, in the calculation of Hydraulics. Fluid Mechanics and Hydraulic Machines is taken as unity.
2. If density of a liquid is in tone per nrl (t/m³), it is numerically equal to specific gravity of the liquid.

Example
Find the specific gravity of an oil whose specific weight is 7.85 kN/m³.

Solution:
Given:
Specific weight of oil = 7.85 kN/m3.
We know that specific gravity of oil
Specific Weight of liquid/Specific weight of pure water =7.85/ 9.81 Surface tension of water The surface tension of a liquid is its property, which enables it to resist tensile stress. It is due to the cohesion between the molecules at the surface of a liquid. The effect of surface tension may be easily seen in the case of tubes of smaller diameters, open to the atmosphere. For example1 when a glass tube of small diameter is dipped in water, the water rises up iii the tube with an upward concave surface as shown.

(a) But when the same tube is dipped in mercury, the mercury depresses down in the tube with an upward convex surface as shown.

For the purposes of all calculations, relating to Hydraulics, Fluid Mechanics and Hydraulic Machines, the effect of surface tension is generally neglected. But the surface tension has got its own importance in a variety of ways. As a result of surface tension, the liquid surface has a tendency to reduce its surface as small as possible. That is why the falling drops of rain water become sphere. The property of surface tension is utilized in the manufacturing of lead shots. The molten lead is made to pass through a sieve from a high tower, and allowed to fall into water. The molten lead particles, while descending, assume a spherical shape and solidify in this form before falling into the water.

The following table gives the values of specific weight and surface tension of important liquids at 20°C (Le, normal temperature).

Viscosity of water We see that the flow of thin liquids, such as alcohol or water, is much easier than thick liquids, such as syrup or heavy oil. It is thus obvious, that each liquid possesses some property, which controls its rate of flow. This property is termed as viscosity and is an essential property of the liquid. We shall study the viscosity in detail in a separate chapter. Compressibility of water The compressibility of a liquid may be defined as the variation in its volume, with the variation of pressure. The variation in the volume of water, with the Variation of pressure, is so small that for all practical purposes it is neglected. Thus, the water is considered to be an incompressible liquid.