If mass remains the same while the volume of a substance the density of the substance will

If mass remains the same while the volume of a substance increases, the density of the substance will decrease. The increase in the volume is called expansion, while the decrease in the volume is called contraction. 
The density measures how much matter (mass) there is in a given amount of space (volume).  So, if the mass stays the same and the volume increases then there is the same amount of mass ad before but in bigger volume so the density is decreased. 

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