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KS3

Classifying Materials

Question

# How would you describe a typical solid's density?

2 years ago

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Mark Murray

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Density is mass per volume. Water has a density of 1g/cm3. So any substance with a lower density will float on water. Substances with a density higher than 1g/cm3 will sink in water.

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Density is mass per unit volume. The unit of density is gram/cubic centimetre or mass units/volume units.

So, let's consider two solid cubes A and B. Let's assume each cube has the same volume of 1 cm by 1 cm by 1 cm (that is 1 cm^3 or cc). Now, for cube A the mass is 5 g and for cube B the mass is 10 g. So, density of cube A is 5 g/cc and density of cube B is 10 g/cc. Hence the cube B has a higher density than cube A.

However, please understand that density is not mass, it's mass per unit volume. So, suppose there are two solid cubes C and D of same mass of 5g. But, volume of cube C is 100 cc and volume of cube D is 10 cc, then density of cube C = 5/100 = 0.05 g/cc and density of cube D = 5/10 = 0.5 g/cc.

So, in this case, even though C and D have same mass, D has a higher density than C simply because the volume of cube D is lower. So, two objects with exactly the same mass can be more/less dense depending on their density.

When you generally put a solid of lower density on a liquid of higher density, the solid will float - that's why ice (density = 0.917 g/cc) floats on water (density = 1 g/cc)!

That's why an empty ship floats on water! The density of the ship (which is total mass of the hull and the mass of the enclosed air divided by the volume of the ship ) is less than the density of water!

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