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Module 4: Liquid Characteristics - Density and Specific Gravity
Density
Specific Gravity
Practice Problems
Objectives
- Calculate the density and specific gravity of liquids.
Density is an important fundamental property of liquids and gases. This lesson deals with the density of liquids (Module 2 discusses gas density).
Density is a useful means of displaying the mass in a liquid. The density of any substance is the mass of the material divided by the volume it occupies. This is summarized in Equation 1.
- Where:
Density has a direct effect on the power required for pumps. Liquids with low levels of suspended solids such as city water (low densities) have little effect on the pump power requirements. Increasing the amount of suspended solids (higher densities) will generally increase the horsepower required to pump the liquid.
The density of various liquids can be obtained from tables in numerous reference sources. The common units for fluid density are provided in Table 1.
Temperature and pressure do not significantly alter the volume of liquids for the range of interest in air pollution control. Therefore temperature and pressure are not taken into account in determining liquid density as they are for gas density. However, it is good practice to be consistent and include temperature in Equation 1.
The density of water at various temperatures is provided in Table 2. The mass of 1 cm3 of water at 4°C is taken as unity.
-
#1 - Using Table 2, find the percent increase in the density of water
for the following sets of temperatures:
- -5°C compared to 200°C
- 15°C compared to 75°C (Usual range used in air pollution
control)
- -5°C compared to 200°C
-
#2
- How does each of the following actions affect the density of a
liquid?
- Adding more dissolved solids into the liquid
- Adding more suspended solids to the liquid
- Changing the temperature
- Changing the pressure
- Adding more dissolved solids into the liquid
Specific gravity is often used as a measure of the change in density. The specific gravity is a comparison of the density of the liquid compared to the density of pure water at a specific temperature. This definition is illustrated in Equation 2.
- Where:
The reference temperature for the density of water is usually 4°C (39.2°F). In the case of city water, the density at the reference temperature is 1.00 gm/cm3 or 62.4 lbm/ft3 at standard pressure.
Specific gravity is often represented using a notation that identifies the temperature of both the sample and the reference material (water) as shown below.
For example, suppose a liquid at 28°C has a specific gravity of 1.02 measured with a standard hydrometer that is calibrated against the density of pure water at 4°C. Its specific gravity is expressed as follows:
The specific gravity of a liquid is measured using a hydrometer, which is similar to the various instruments used to measure the concentration of antifreeze in a car. The specific gravity provides a useful means of determining the concentration of constituents in the water. Liquids dissolved into the water (such as ethylene glycol in antifreeze), solids dissolved into the water (such as salt in ocean water), and solids suspended into water change the specific gravity.
Example Problem 1.
Converting Between Specific Gravity and Liquid Density
A wet scrubber uses a liquid with a specific gravity of 1.04(4°C/4°C Ref. T) to remove pollutants. What is the density of this liquid?
Solution:
Calculate the density 
of the liquid using the following equation:
Practice Problems
Density and Specific Gravity
- Instructions:
- Complete the Practice Problems before proceeding to the next lesson. Click on the button below.