### Density

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• #### Message 1.

Posted by BitesizePhysicsTeacher (U15165682) on Sunday, 10th February 2013

Density is defined as 'the mass per unit volume for a material' and is calculated using the formula -

ρ = m / v

where -

ρ (rho) - density
m - mass
v - volume

Density is usually measured in kg / m³, but can be expressed in g / cm³.

1 g / cm³ = 1000 kg / m³

In solids and liquid, where the particles are closely packed, the mass is more concentrated per unit volume, so densities for solids and liquids are large.

In gases the particles are spaced around 10 times farther apart than in solids and liquids, so the volume for the same quantity of matter will be around 10x10x10 = 1000 times greater, making the density of the gas around 1000 times smaller.

In Higher you are expected to be able to describe an experiment to find the density of air. this can be done in one of two ways -

Method 1 -
Removing the air from a rigid container (round bottomed flask)

+ find mass of flask and air, m₁
+ use vacuum pump to evacuate air from inside flask, then find mass of flask without air, m₂
+ find the mass of air by subtracting, m = m₁ - m₂
+ draw water in to evacuated flask, which will occupy the volume of the evacuated air
+ measure the volume of the water, v
+ calculate the density of air, ρ = m / v

Method 2 -

+ find mass of flask and air, m₁
+ use pump to add more air inside flask, then find mass of flask with extra air, m₂
+ find the mass of added air by subtracting, m = m₂ - m₁
+ release extra air from flask and collect over water by downward displacement
+ measure the volume of the air collected, v
+ calculate the density of air, ρ = m / v

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