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As temperature rise the density become lower,When temperature goes down, density is higher but in higher temperature the body become bigger so why density become lower?

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read the definition of density. It is inversly proportional to volume. – anna v Oct 30 '12 at 7:16
@anna v,ok but is it right to think about density as the Measurement for the amount of particles inside the body and how close they are from each other ? – mar Oct 30 '12 at 7:25
In a general sense yes. For the same mass lower density means that the particles/molecules constituting the mass, are further apart. – anna v Oct 30 '12 at 8:25

You just said it! As the body is heated, it becomes bigger, so if it's mass doesn't change, it's density has decreased, since density is the mass divided by the volume.

Density is an intensive property. It is not dependent upon the amount of material inside the system (scale invariant).

A block of iron at a given temperature has the same density, regardless of how big or small the block is.

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For the density of something to reduce one of two things must happen.

  1. The amount of the material must be reduced,
  2. The volume occupies must increase.

Case 1 effectively isn't happening (you do get some evaporation but not enough to affect the density), so case 2 happens.

This is because for most materials as the temperature rises the atoms/molecules that make up the material move faster and further thus increasing the volume that the material takes up. There are exceptions to this - water being one in the range 0 to 3.8 °C - where the volume decreases as the temperature increases.

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The three answers I've seen are all correct. But one should note that the volume does not necessarily increase as the temperature goes up. Water in the range 0 to 3.8 C is but one example. Several metal alloys show similar behavior. – Paul J. Gans Jan 4 '13 at 20:13

density is inversely proportional to volume of the body from the eqation: density = m/v With increase in size of body with temperature, volume of body also increases hence, the density will reduce (as it is inversely related to volume) hope you understood the answer. regards

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