# Properties of Fluids- Density or Mass density, Specific weight, Specific Volume, Specific Gravity, Surface Tension

**1..** **Density or Mass density( ρ) :** The density of a fluid is defined as the ratio of the mass of the fluid to its volume of fluid. Density is a crucial property that defines how much mass is packed into a given volume of a substance. It is expressed as the ratio of mass to volume. For gases, density varies with pressure and temperature, while for liquids, it remains relatively constant. Water, for instance, has a density of 1000 kg/m³, whereas air has a density of 1.225 kg/m³.

The unit of density in S.I. unit is kg/m3. The value of density for water is 1000kg/m. With the increase in temperature volume of fluid increases and hence mass density decreases in case of fluids as the pressure increases volume decreases and hence mass density increases.

**2. Specific weight or weight density (γ):** Specific weight or weight density of a fluid is the ratio between the weight of a fluid to its volume. The weight per unit volume of a fluid is called weight density.

The unit of specific weight in S.I. units is N/m3. The value of specific weight or weight density of water is 9810N/m3. With increase in temperature volume increases and hence specific weight decreases. With increases in pressure volume decreases and hence specific weight increases.

**3. Specific Volume (**) **:** The specific volume of a fluid is defined as the volume of the fluid per unit mass. It is denoted by the symbol ν (nu) and is measured in units of cubic meters per kilogram (m3/kg) or cubic centimeters per gram (cm3/g). Specific volume represents the volume occupied by a unit mass of fluid and is inversely related to density.

**4.** **Specific Gravity of Fluid** **(S):** The specific gravity of a fluid is defined as the ratio of the density of the fluid to the density of water at 4°C. It is a dimensionless quantity that helps in comparing the density of a substance to that of water. Water at 4°C is often used as a reference point for specific gravity calculations.

To calculate the specific gravity of a fluid, you can use the formula:

Specific Gravity=Density of Fluid/Density of Water at 4°C

Given that water has a density of 1 g/cm³ at 4°C, the specific gravity of water at this temperature is 1.

When determining the specific gravity of other fluids, you need to know their densities. The specific gravity values for common liquids like alcohol, oils, benzene, and water can be found in tables or databases. These values are crucial in various industries such as manufacturing, engineering, and chemistry.

**5. Surface Tension** (S): Surface tension plays a crucial role in allowing small objects to float on the surface of a liquid. It is the property of the surface of a liquid that enables it to resist external forces due to the cohesive nature of its molecules. In the case of water, the cohesive forces between water molecules are responsible for this phenomenon.

Measurement and Units of Surface Tension: Surface tension is typically measured in force per unit length, with units such as newton per meter (N/m) or dyne per centimeter (dyn/cm). It can also be expressed as energy per unit area, known as surface energy. The dimensional formula for surface tension is MT^-2.

**6. Viscosity of a fluid (η)**:

The viscosity of a fluid is a measure of its resistance to deformation at a given rate. It quantifies the internal frictional force between adjacent layers of fluid that are in relative motion. Viscosity depends on various factors such as temperature, pressure, and rate of deformation. The SI unit for viscosity is newton-seconds per square meter or pascal-seconds.

In general, liquids with higher viscosity exhibit more resistance to flow compared to those with lower viscosity. For example, syrup has a higher viscosity than water. Viscosity plays a crucial role in determining the forces required to overcome internal friction within fluids during processes like lubrication, transportation in pipelines, spraying, injection molding, and surface coating.

**Numerical on fluid properties:**

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