# Density and Specific Gravity

### DENSITY:

Density has got a great deal to do with the mass/weight of substances. This is so because equal volumes of different substance have got different masses and weight; for instance; 10 gram of iron will be heavier (have a higher density) when compared to 10 gram of wood (which has a lower density). Similarly, 100ml of water will have a higher density than 100ml of oil; and for this reason, oil floats on water. Please read on Separation of mixtures (water and oil) here.

Note: What makes the difference in the density of different materials is as a result of the different atoms that make them up; for instance; water (H2O), which is made up of (Hydrogen and Oxygen in the ratio of 2:1) is different the constituents making up oil.

By Definition, Density is the mass of a unit volume of a material or substance.

You can read more on mass here.

Note: The principle of density was determined by a Greek scientist named Archimedes.

The formula for density is:
ρ = M/V
ρ = density
M = mass
V = volume
Note: Density can also be expressed by the Greek letters "rho"

The Standard International (SI) Unit for Density is Kilogram per Cubic Metre (Kg/m3). Below are some substances with their varying densities:

 SUBSTANCE DENSITY (Kg/m3) Helium 0.179 Air (1atm and 20oC) 1.20 Bamboo Wood 400 Ethanol 810 Methylated Spirit 800 Benzene 900 Paraffin Wax 900 Ice 920 Water (Fresh Water) 1000 Sea Water (Saline/Salt Water) 1030 1600 Glass 2600 Aluminium  (Al) 2700 Zinc (Zn) 7100 Iron (Fe) 7900 Copper (Cu) 8900 Lead (Pb) 11300 Mercury (Hg) 13600 Gold (Au). Please read interesting facts on Gold 19300 Platinum (Pt) 21500

Notice from the table that helium has a lower density than air and that's one reason why it's used in filling balloons.

Having an understanding of density offers a convenient means of deriving the mass of a body from its volume. On the flip side, when we know the mass of a body, we can derive it's density.

Below are the derivation steps

Mass =  volume x density (M = Vd)

Volume = mass divided by the density (V = M/d).

Note: To get the weight of a body, we will need to multiply its mass by the acceleration of gravity.

A common application of density is seen in the way at which different materials interact with each other. For instance, a wood may float on water while an iron may sink in it. Also, balloons will float in air because it's filled with helium gas which is much lighter (has a lower density) than air.

### Relative Density / Specific Gravity

Relative density involves the density of one substance in relation to that of water (at 4oC). it's also called specific gravity.

The Relative Density (R.D) of a material is the ratio of it's density (the material) to that of water at 4oC.

By principle, an object with a specific gravity/relative density of less than 1 will float on water, while that with value greater than 1 will sink in water.

Below are the formulas for calculating relative density.

Relative Density; R.D = Mass of Substance/Mass of Water at 4oC

R.D = Weight of Substance/Weight of Water

R.D = Density of Substance/Density of Water at 4oC

R.D = ρsubstance / ρreference (water at 4oC)

Where ρ = Density

Note: Relative Density has no Standard International (SI) unit.

The table below shows the relative density of some substances:

 SUBSTANCE RELATIVE DENSITY Balsa Wood 0.2 Oak Wood 0.75 Ethanol (C2H5OH) 0.78 Liquid Paraffin 0.8 Olive Oil 0.91 Coconut Oil 0.925 Petroleum 0.98 Water (Fresh Water) 1 Salt Water (Reef Aquarium) 1.025 Ironwood 1.5 Graphite 1.9 – 2.3 Common  Salt (NaCl) 2.17 Aluminium (Al) 2.7 Cement 3.15 Iron  (Fe) 7.87 Copper (Cu) 8.96 Lead (Pb) 11.35 Gold (Au) 19.3 Osmium (Os) 22.59 Alfred Ajibola is a Medical Biochemist, a passionate Academician with over 7 years of experience, a Versatile Writer, a Web Developer, a Cisco Certified Network Associate and a Cisco CyberOps Associate.

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