Difference Between Density and Specific Gravity

Density and specific gravity are two properties of matter that are closely related to each other; However, they are not the same. Both are intensive properties related in one way or another to the mass and volume of substances, and both can be used to predict the buoyancy of different bodies in different fluids, but they also have differences that make the use of one more practical than the other. the other in certain situations.

Next, we will see what density and specific gravity are and we will see the most relevant characteristics that distinguish one from the other.

What is density?

The density of a material is its mass per unit volume. It is an intensive property characteristic of each material. In many fields of knowledge, density is represented by the letter d ; however, in some natural sciences such as physics and chemistry, as well as in most engineering, it is represented by the symbol ρ (the lowercase Greek letter rho ).

The density is calculated by means of the following formula:

where ρ is the density, m is the mass, and V is the volume of the material or object.

Density units

The units of the density are [m]/[V] or, what is the same, [m]/[L] ³ . Some examples of units of density in different systems of units are:

scale type

Density is a property that is measured on an absolute scale. That is to say that its value goes from 0 onwards, regardless of the units that are used, and its value only depends on the material in question, not on any other material or reference system.

Dependence of density with temperature

The mass of a body is independent of temperature, but its volume is not. Most materials expand with an increase in temperature. When this happens, the density, which is divided by the volume, decreases.

There are, however, examples of substances that contract with temperature. Such is the case with water. In general, the density of water decreases with increasing temperature and increases with decreasing temperature. However, by cooling the water, just before the freezing point, the density decreases instead of increasing. This explains why ice, which floats on top of water, is less dense than water.

Measure instrument

The density of liquids is determined using an instrument called a pycnometer and with an analytical balance. The pycnometer allows volumes to be measured with a very high degree of accuracy, while the difference between the full and empty mass determined with the use of an analytical balance allows mass to be measured with equal precision and accuracy.

Density Uses

Density is used for different types of calculations. On the one hand, it allows determining volumes or masses of any substance, given that we know the mass or volume respectively.

It is useful for identifying or differentiating different materials. Being a characteristic property of matter, each material has a particular density at a given temperature.

It is of great importance in fluid mechanics since the difference between the densities of an object and that of a fluid determines the buoyancy of the former in the latter.

What is specific gravity?

Specific gravity , also called relative density, is the ratio between the density of a substance or material and the density of another reference material under the same experimental conditions of temperature and pressure . It is often represented by the symbol SG (specific gravity ) and, like density, is a characteristic quantity of a material at a given temperature.

The relative density or specific gravity is calculated differently depending on whether it is condensed matter (solids or liquids) or gases. In either case, a specific gravity less than 1 implies that the substance will float above the reference substance, while a specific gravity greater than one indicates that it will sink.

Specific gravity of solids or liquids

When the material in question is a solid or a liquid, the density of liquid water is taken as a reference, generally at the temperature at which its density is maximum, which corresponds to 4°C. At this temperature, the density of water is 1,000 kg/m ³ . In this case, the specific gravity is given by the following expression:

Gas Specific Gravity

On the other hand, when the material whose relative density is being measured or determined is a gas, the reference material is not water but air at the same given conditions of temperature and pressure. In other words, in this case, the specific gravity is given by:

Specific Gravity Units

The most important feature of specific gravity compared to density is the fact that, being a quantity obtained by dividing two densities, specific gravity has no units. In other words, it is a dimensionless quantity. This means that it is a pure number whose value will always be the same for a given substance at a given temperature and pressure, no matter what system of units was used to express the original densities.

In other words, relative density provides a scale of measurement for density that is independent of the system of units in which all other calculations are being worked. This makes it particularly useful for communicating between teams of engineers who tend to use non-SI systems of units, with scientists or specialists in other fields who tend to use either the metric or SI system.

scale type

As it is the ratio between the density of a substance and the density of a reference substance, specific gravity corresponds to a relative quantity and not an absolute one. In other words, by saying that the specific gravity of mercury, for example, is 13.59, we are really saying that its density is 13.59 times greater than the density of water. Note that it is a density relative to the density of water, so without knowing the latter, we cannot know the real density of mercury.

Reference density values

As can be seen, the specific gravity calculation depends on the density of the reference material, and this, in turn, depends on the temperature and pressure conditions at which the specific gravity is being measured or calculated. In the case of solids and liquids, whenever a specific temperature is not indicated, it is assumed that said SG is established based on the density of water at 4°C. In the case of gases, if the temperature and pressure conditions are not specified, the density is assumed under normal conditions of temperature and pressure, or NTP, corresponding to a temperature of 20 °C and a pressure of 1 atm, in which case In this case, air (dry) has a density of 1.204 kg/m ³ .

The following table presents these reference values ​​in different units:

Dependence of specific gravity with temperature

Being a function of two densities, which vary with temperature, the relative density or specific gravity also varies as a function of said property.

However, in general the variation is smaller than the variation of the absolute density. This is because, as mentioned above, the density of most substances decreases with increasing temperature, and this includes water for most temperatures that are not between 0 and 8 °C. Then, when the temperature increases, both the density of the material in question and that of the reference material decrease. This means that the change in the numerator partially offsets the change in the denominator, lessening the effect of temperature on specific gravity.

Measure instrument

Specific gravity is measured experimentally using an instrument called a hydrometer. This consists of a heavy bulb provided with a stem which has a scale calibrated according to the reference substance, in most cases water. When the bulb is placed in a liquid, it will sink until the buoyant force counteracts the weight of the hydrometer. The reading is made on the scale at the point that protrudes from the surface of the liquid.

Specific Gravity Uses

An immediate utility of the specific gravity is that its value immediately indicates whether or not the material will float in water or in the air, depending on whether it is solid and liquid, or gases, respectively. In either case, if the relative density is less than unity, the material will be less dense and will float, and vice versa.

Another very common application of SG is its relationship with the concentration of solutions. Depending on the interactions between solute and solvent, the density of a solution can be different from that of pure water, and, in general, this density varies as a function of concentration. Thus, the measurement of SG using a hydrometer makes it possible to determine the concentration of different solutions.

Some examples of the use of the SG for this purpose are:

• Fuel evaluation.
• The determination of the alcohol content during the fermentation of musts for the production of beers, wines and other alcoholic beverages.
• The evaluation of the concentration of sulfuric acid in lead/sulfuric acid batteries or accumulators commonly used in gasoline-powered automobiles, etc.

How to Determine Density from Specific Gravity

Specific gravity can easily be converted to absolute density by simply multiplying the former by the density of the reference substance in the required units:

Or, in the case of gases:

In either case, very accurate density tables are usually available at a wide variety of pressure and temperature values.

Summary of Differences Between Density and Specific Gravity

The following table summarizes the most important differences between density and specific gravity:

References

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Giner, S. (2020, August 18). Hydrometer or hydrometer . 2D2Dfoam. https://www.2d2dspuma.com/blog/que-es/hidrometro/

Libretexts. (2020, August 13). 1.14: Density and Specific Gravity . Chemistry LibreTexts. https://chem.libretexts.org/Courses/Saint_Francis_University/CHEM_113%3A_Human_Chemistry_I_(Muino)/01%3A_Matter_and_Measurements/1.14%3A_Density_and_Specific_Gravity

National Physical Laboratory. (2021). What is the difference between density and specific gravity? NPL Website. https://www.npl.co.uk/resources/qa/density-specific-gravity-differences