Density and Specific Weight

# Density of a Substance

Matter is everything that surrounds us. How we observe it depends on whether we observe it as a chemist, physicist, geologist, biologist, etc. A chemist is interested in the structure of matter and studies the laws according to which matter changes. Physicists are interested in its physical properties. Just like chemistry, physics is interested in the structure of matter, the size and mass of molecules and atoms, and the forces between them, as both the physical and chemical properties of matter depend on this.

## Physical properties of a substance

The physical properties of a substance are density, elasticity, compressibility, strength, thermal and electrical properties, etc. We will learn about density in this material, and all the others in separate materials.

Even when it is (chemically speaking) the same substance, its physical properties can be different. What do they depend on? Let's list some quantities that affect the physical properties of substances:

• Temperature

Example

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• State of a substance

The same substance can appear in three states. These are solid, liquid, and gas. Liquid and gas are also called fluid states, as the substance can flow and take the shape of the container in which it is located. A substance can change from one state to another under the influence of heat. The physical properties of the same substance change dramatically after these transitions.

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• Pressure

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The substance can be homogeneous or inhomogeneous:

• Homogeneous substance

In the case of a homogeneous substance, its physical properties are the same everywhere. They are independent of the place within the substance from where we take the sample for measurement.

Example

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• Inhomogeneous substance

In the case of an inhomogeneous substance, its physical properties depend on the location. Judging whether an object is homogeneous or not depends on the size of the particle within the substance being observed.

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## Density of a substance

A substance is denser if there is a greater mass of the substance in the same volume.

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The density of a substance is therefore the quotient of its mass and volume :

The unit for density is:

The density of a substance is given as:

or the mass is:

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The table of the density of some substances at temperature is as shown below:

 Substance Density Wood 0.6 to 0.8 Kerosene 0.84 Water 1 Aluminium 2.7 Iron 7.8 Copper 8.9 Silver 10.5 Lead 11.3 Mercury 13.6 Gold 19.3

## Specific weight

Similar to how density is the ratio of mass to volume, specific weight is the ratio of weight to volume . The symbol of specific weight is :

The unit for specific weight is:

How are density and specific weight related?

Let's write again the formula of the specific weight which is given as:

 We note that the weight of a substance is given as the product of its mass and volume : We note that from equation 1 above that:

The specific weight of a substance is therefore its density multiplied by the gravitational acceleration . It is not constant for a certain substance but rather depends on the magnitude of the gravitational acceleration, which varies slightly depending on the place of observation. Therefore, in practice, instead of specific weight , we prefer to use the product .

Let's make the subject of equation 2:

 We note from equation 3 that:

Specific weight of a substance is given as its weight divided by the volume :

Specific weight and density are related through free-fall acceleration, by the equation:

The weight of a substance can also be expressed in terms of density, as:

Example

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