What Is Relative Density?

Relative density refers to the ratio of the density of a substance to the density of a reference substance under the respective specified conditions. The symbol is d, a dimensionless quantity. The general reference substance is air or water: when air is used as the reference substance, the density of dry air under standard conditions (0 ° C and 101.325kPa) is 1.293kg / m3 (or 1.293g / L).

Relative density refers to the ratio of the density of a substance to the density of a reference substance under the respective specified conditions. The symbol is d, a dimensionless quantity. The general reference substance is air or water: when air is used as the reference substance, the density of dry air under standard conditions (0 ° C and 101.325kPa) is 1.293kg / m3 (or 1.293g / L).

Chinese name

Relative density

Foreign name

Relative density; Density ratio

Category

density

Meaning

The density ratio of the substance to the density of the reference substance

Relative density example

When water is used as the reference density, that is, 1 g / cm ^{3 is} used as the reference density (density of water at 4 ° C.), it has been called specific gravity in the past. The relative density is generally used as the density of water at 4 ° C, and the density of another substance is divided by it.

The relative density is just no unit, and the value is the same as the actual density.

For example: methane relative density: the relative density (air = 1) is 0.555. (At normal temperature and pressure)

It is very convenient to use and can also be used in calculations.

According to the relative density of certain substances, it is possible to infer certain fire protection characteristics and take corresponding fire protection measures. If a flammable and flammable liquid with a relative density <1 occurs in a fire, it should not be extinguished with water, because it will float on the water surface. Instead of being saved, it will disperse with the water and increase the loss. Therefore, foam and dry powder should be used to extinguish the fire. Another example is flammable gas and vapor with relative density <1, which is easy to diffuse and form an explosive mixture with air, and it is easy to flow along the ground and ditch over long distances. In case of open flame, flashback will occur. When determining the location of the vents in the workshop and warehouse, the air vents should be set above the space for the gas lighter than air; the air vents should be set below the space for the gas heavier than air.

Relative density reference

Density is the mass per unit volume of a substance, and it is measured by the ratio of the mass of a material to its volume.

That is, density = mass / volume, which means that it has units.

Now let's talk about relative density based on density, which is mainly used in gemology.

The ratio of the density of a substance to the density of a standard substance is called its relative density. In most cases, the standard substance is water, which can be understood as the ratio of the mass of the same volume of substance to the mass of water of the same volume.

The relative density can be measured by the still water weighing method, and the more specific method is the specific gravity liquid method in gem identification.

But be careful when using specific gravity fluid, because specific gravity fluid is toxic.

The ratio of the density of a substance to the density of water is called the relative density of the substance.

Relative density determination liquid

[Principle of Experiment]

The density of water is approximately 1X10 ³ kg / m ³ , and the density of an object relative to water is defined as:

Relative density of objects = mass of objects / mass of water of the same volume

experiment equipment

A bottle, a balance, liquid (such as methanol-containing alcohol), water, cloth, or absorbent paper.

[Experimental steps]

(1) Weigh the weight of the dry, empty bottle with a balance and record it as m ₁ (gram).

(2) Fill the bottle with the liquid of the relative density to be measured and stopper the bottle; wipe the surface of the bottle with a cloth or absorbent paper; weigh it again and record it as m ₂ (g).

(3) Pour out the liquid in the bottle and wash it with water.

(4) Fill the empty bottle with water, stopper the bottle and dry the surface of the bottle; weigh it and record it as m ₃ (g).

(5) Calculate using the following formula. The density of the measured liquid relative to water is

relative = (m ₂ -m ₁ ) / (m ₃ -m ₁ )

Compared with the results of balances or graduated cylinders, the liquid density values measured by the above method may be different. The main reason for this difference is that the density of water is approximately thought to be 1000 kg / m ³ .

Relative density determination of beans

[Principle of Experiment]

The density of water is about 1000kg / m ³ , and the density of an object relative to water is defined as:

Relative density of objects = mass of objects / mass of water of the same volume

experiment equipment

A bottle; a balance; several beans; water; cloth or absorbent paper.

[Experimental steps]

(1) Weigh the weight of the dry, empty bottle with a balance and record it as m ₁ (gram).

(2) Fill the bottle with beans to reach about 1/3 the height of the bottle; weigh it and record it as m ₂ (g).

(3) Fill the bottle with water and shake it gently to make the air bubbles in the bottle float out and stopper the bottle. Dry the surface of the bottle with a cloth or absorbent paper, weigh it again, and record it as m ₃ (g).

Note: In the above experimental steps, in order to avoid the beans swell due to water absorption, it is best to use cold water and shorten the operation time as much as possible.

(4) Pour the water and beans out of the bottle, clean the bottle, fill it with water, stopper the bottle and dry the surface of the bottle. Weigh its weight and record it as m ₄ (g).

(5) Calculate the density of the object relative to water according to the following formula

relative = (m ₂ -m ₁ ) / [(m ₄ -m ₁ )-(m ₃ -m ₂ )]

Through the above experiments, we already know that the mass of beans = (m ₂ -m ₁ ) (g), the mass of a bottle of water (excluding bottle) = (m ₄ -m ₁ ) (g), and about 2/3 bottles The mass of water (the rest of the bottle contains beans) = (m ₃ -m ₂ ) (g), so the mass of water with the same volume as the beans = (m ₄ -m ₁ )-(m ₃ -m ₂ ) ( G), so the density of beans relative to water is determined by the formula above.