Which is heavier – the wood in the trunk of a tree or the metal in a coin? Your first answer might be to say the trunk of the tree, but it can float on water while a coin would quickly sink to the bottom,
To be a fair comparison we need to find the masses of equal volumes, if we found the mass of a piece of wood the size of a coin we would see it was lighter.
In the school laboratory, when small amounts of materials are used, the density of a substance is often calculated using masses measured in grams and volumes in centimeters cubed, giving a density value in g/cm3, The density value in units of g/cm3 can be converted to a value in kg/m3 by multiplying it by 1000. For example, ice was found to have a density of 0.920 g/cm3. This can also be expressed as 0.920 x 1000 = 920 kg/m3
Density is the mass of unit volume
In SI density measured in kg/m3
But can be measured also in g/cm3
Density (d) = mass (m) / volume (V)
At constant mass
The density is inversely proportional with volume of the substance.
d inversely proportional to V
At constant volume
The density is directly proportional with mass of the substance
d is directly proportional to m
|Material||Density / kg/m3|
|Pure water (at 40C)||1000|
If equal volumes are compared, the one of the greatest mass will have the greatest density.
If equal masses are compared, the one with smallest volume will have the greatest density.
1. Arrange the materials in this table in order of density, starting with the least dense material.
2. Which is heavier, 1 m3 of steel or 1 m3 of aluminum?
3. Which is heavier, 1 kg of steel or 1 kg of cork?
i. Density of water at 4 0C is 1000 kg/m3 = 1 g/cm3.
ii. Density doesn’t depend on the volume (how big) of the body only; it depends on both its volume and its mass.
iii. Density changes with temperature but the density of water is a very exceptional with heat change.
iv. Densities of gases depend on their pressure too.
v. The less dense float over the surface of the denser.
vi. The denser will sink or goes down.
A- To determine the density of a liquid
Measuring cylinder, electronic balance
1. Find the mass of the dry measuring cylinder using an electronic balance m1.
2. Add a specific volume of the liquid into the measuring cylinder, record the volume V.
3. Measure the mass of the cylinder with the liquid m2.
The mass of the liquid m = m2 – m1
The density of the liquid can be calculated from the formula ρ = m/v
When reading from the burette, make sure that the eye is placed at the same level as the meniscus of the liquid. (To avoid ……… error)
B- To determine the density of a regular object
Ruler, Vernier calipers or micrometer screw gauge
Electronic or digital balance
1. Measure the mass of the regular object using an electronic balance m.
2. Find the volume (v) of the regular object by measuring its dimensions then the volume can be calculated using the respective formulae.
Density can be calculate using the formula ρ = m/v
Density can be used to indicate the degree of purity.