1.4 Density

We must have all noticed that the weight of objects can vary greatly. A plastic teaspoon weighs less than a metal one, and a gold ring weighs twice as much as a silver one, even if the objects are exactly the same size.

The density of a material is a measure of how ‘squashed up’ it is, and a dense object contains more mass than a light object of the same size.

Density  of a substance is defined as its mass per unit volume.

d=M/V

where d– Density, M-mass, and V-volume of the substance.

It is a scalar quantity.

Note that the density of water is , and the rule is that an object of greater density will sink in a liquid of lower density. So, perhaps not surprisingly, the brick will sink in water. But will it sink in mercury? See the table right.

Density of some substances

Metal or alloy kg/cu.m
aluminium – melted 2560 – 2640
aluminium bronze (3-10% Al) 7700 – 8700
aluminium foil 2700 -2750
antifriction metal 9130 -10600
beryllium 1840
beryllium copper 8100 – 8250
brass – casting 8400 – 8700
brass – rolled and drawn 8430 – 8730
bronze – lead 7700 – 8700
bronze – phosphorous 8780 – 8920
bronze (8-14% Sn) 7400 – 8900
cast iron 6800 – 7800
cobolt 8746
copper 8930
delta metal 8600
electrum 8400 – 8900
gold 19320
iron 7850
lead 11340
light alloy based on Al 2560 – 2800
light alloy based on Mg 1760 – 1870
magnesium 1738
mercury 13593
molybdenum 10188
monel 8360 – 8840
nickel 8800
nickel silver 8400 – 8900
platinum 21400
plutonium 19800
silver 10490
steel – rolled 7850
steel – stainless 7480 – 8000
tin 7280
titanium 4500
tungsten 19600
uranium 18900
vanadium 5494
white metal 7100
zinc 7135

Determination of Density of a regular shaped object

1. First weigh the given solid in a physical balance and find its mass m.

2. Then measure the dimensions of the solid with the help of a meter rule (or vernier or micrometer) and calculate its volume V and use the formula

d=M/V

Determination of Density of a irregular shaped solid

1. First weigh the given solid in a physical balance and find its mass

2. Now take a suitable liquid (which does not react with the solid and in which the solid sinks) in a clean measuring cylinder. Note the level of the liquid in the cylinder. Then immerse the given solid completely into the liquid and note the reading again. The difference in the readings give the volume of the solid. Now use the above formula to find its density.

Determination of Density of an irregular shaped solid(Try it now)