-vi-Density

Density

starting with a question…..

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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,

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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.

===Explain=================

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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
Ice 920
Cork 250
Wood 650
Pure water (at 40C) 1000
Steel 7900
Aluminum 2700
Copper 8940
Lead 11350
Gold 19320
Polythene 920
Perspex 1200
Expanded polystyrene 15

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.

 

Questions

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?

Notes:

     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.

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Density Experiments

A- To determine the density of a liquid

Apparatus

Measuring cylinder, electronic balance

Procedures

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.

Calculation

The mass of the liquid m = m2 – m1

The density of the liquid can be calculated from the formula ρ = m/v 

Precaution

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

Apparatus

Ruler, Vernier calipers or micrometer screw gauge

Electronic or digital balance

Procedures

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.

Calculation

Density can be calculate using the formula ρ = m/v

Note:

Density can be used to indicate the degree of purity.

Quiz 1:

http://www.edinformatics.com/math_science/mvd_quiz.htm

Quiz 2:

http://www.sciencequiz.net/jcscience/jcphysics/density/density1a.htm

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