**What is density?**

**Density** is the mass per unit volume of a substance. For example, the density of pure iron is 7.87 g/cm3, and this means that one cubic centimeter of iron has a mass of 7.87 grams. The density is also known physically by the symbol (ρ), which is pronounced: " Ru".

**Density units**

The units used to express the density of materials vary, for example, in the metric system, the units of g/cm3 and kg/m3 are the most popular units that express density. ; m3 , liter , or millilitre, or other.

**How to calculate the density**

Density can be calculated for any substance by knowing the mass and volume of the substance, and the law for calculating the density is as follows:

**Density = mass/volume**

It is intended to calculate the density of a sugar cube with a mass of 11.2 g, and the measurement of one side of a sugar cube is equal to 2 cm. The density is calculated by following the following steps:

**Determine mass:**mass = 11.2 g.**Finding the volume:**Using the volume laws, the volume of a cube = (side) 3 = 8 cm 3**Applying the Density Law:**Density of a sugar cube = mass/volume = 11.2/8 = 1.4 g/ cm3

**Material density difference**

There are many dense materials, such as iron, lead, platinum, and different types of rocks, and the feeling of heaviness and hardness of dense materials often increases if compared to the same size of less dense materials, such as glass, wood, and some light metals such as aluminum, Among the most famous materials whose densities are widely used in practical applications, are the following:

- Earth's density = 5.51 g/ cm3.

- The density of water = 1 g/ cm3.

- Air density = 1.2 kg/ m3.

**Density and buoyancy**

Materials can float on the surface of the water in general if the density of the material placed on the surface of the water is less than the density of the water, as the weight of the displaced water is equal to the weight of the material placed on the surface of the water, and this means that high-density materials such as solid metals will sink in the water.

An aluminum sheet can float on the surface of the water even though its density is higher than that of water. This is because its mass is distributed over a large surface area, as if this board is rolled into a ball, its mass will be concentrated in one place and it will sink, and this explains why ships did not sink even though they are made of materials denser than water; Its mass is distributed over a large surface area, and its body contains large amounts of air and other materials less dense than water; This makes the ship's hull as a whole less dense than a single piece of the ship's solid material.

The density difference between oil and water can be observed when trying to mix them. Oil will float on the surface of the water because it has a density less than the density of water, as the density of oil is approximately equal to 0.93 g/cm3, which is slightly less than the density of water, which is equal to 1 g/ cm3.