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 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!
Procedures