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As you know, 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 other. So, if you have the same substance, going thru the different phases (solid, liquid and gas) the relative densities make sense 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 to trends and the most common one here is water. Water's liquid phase is denser than it s solid phase (a reverse of most of there substance). The reason for this is the characteristic of the particles ( IMF & shape). 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 (a bent shape which you either know or will learn later). From this shape and IMF, water molecules arrange themselves in a hexagonal arrangement when it forms the solid phase (we call it ice). This hexagonal arrangement occupies more space/volume per molecule than in the liquid phase (where the molecules can move freely in 3-D space). So, this is why ice floats in water!<b
Procedures