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As you Density is an intensive physical property. Therefore, as long as you have the same substance in the same phase, the density is the same. There is an equation for determine density, D = m/V where m -mass of substance and V - volume of substance. From a particle view of matter (KMT), density is simply how close (in space) the base units (i.e. particles) are to each (assuming the same of the two substances are close to the same mass. So, if you have the same substance, going thru the phase (solid, liquid and gas) make sense using KMT knowing the gas can go anywhere (have obtained enough energy to overcome their Intermolecular Force) so they will be farthest apart (lowest Density), next closer is liquid (with a higher Density) and finally solids that are the closest (thereby having highest Density).
Of course, there are always exceptions and the most common on to the above trend is water. Water liquid phase is denser than it s solid ( reverse of most of there substance. The reason for this is the particles of water or we call them molecules of water. As you know or will know later in the year, water is a polar molecule which means it exerts a Coulombic force onto other water molecules (or anybody else). Also due to the shape of the molecule (either you it is bent shape or will learn it later) and its CF influence, it causes the water molecules to arrange in a hexagonal arrangement when it forms the solid phase (we call it ice). This hexagonal arrangement occupies more space/volume per molecule than if they could move more freely in 3-D space (a liquid particle) shape. So, that is why ice floats in water!
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