Why does ice cube float on water?

Let us start with a simple experiment: Take a glass of water. Put some ice cubes. Observe. We see that ice cube, though solid, floats on water. Why?

Let us analyze this simple experiment scientifically. Ice cubes and water are different forms of the same substance. Ice cube is in the solid state and water is in the liquid state. Why does ice cube float on water?

This simple experiment is governed by certain scientific principles, concepts and theories. They are:

   1.    Buoyancy: When ice floats it experiences an up thrust or upward buoyant force exerted by water. This force is greater than the downward force of the weight of the ice cubes. This means that the density of ice cubes is lesser than the density of water. That means, the density of water is 1gm/cc. The density of ice is 0.92gm/cc.  Therefore, ice cubes float in water.

2.    Ice is formed when water freezes. When water freezes, the temperature of water steadily goes down. Water molecule is made up of two hydrogen atoms and one oxygen atom. Oxygen atom has a negative charge. When water freezes, the water molecules are brought together very closely. The negatively charged oxygen atoms try to repel each other or move away from the water molecules lowering the density of water.

3.    Anomalous Expansion of Water:

All substances expand when they are heated and their density decreases .They contract on cooling and their density increases. This is how substances generally react to heat. How does water react to heating and cooling?

If we cool water, it follows the general trend till 4^o C. Density of water gradually increases as we cool it. When we reach 4^oC, its density reaches a maximum. When we cool it further to 0^oC, water expands instead of contracting. As the temperature of water steadily decreases the density of water decreases, when we cool it from 4^oC to 0^oC. This behaviour is known as Anomalous Expansion of water.

The effect of this expansion of water is that the coolest water is always present on the surface. Water at 4^oC is having the greatest density, settles at the bottom of the rivers and lakes. The lightest and the coolest layer accumulates on the top of the river. So in the winter, the top of the water is always the first to freeze. Since ice and water are bad conductors of heat, the top layer of ice insulates the bottom layers of the water body. This nature’s mechanism protects the aquatic lives living in the rivers and lakes.

In general, almost all substances have a lower density when they are in a liquid phase than when they are in a solid phase. But water is an exception. Water reaches its maximum density at 4°C (40°F). As it cools further and freezes into ice, it actually becomes less dense. The reason for this is found in the molecular structure of water and because of its hydrogen bonding. 

 A water molecule is made of one oxygen atom and two hydrogen atoms. They are strongly joined to each other with covalent bonds. The sharing of the (negatively charged) electrons among the three atoms makes the hydrogen atoms slightly positive when compared to the charge of the oxygen atom which is negative. For this reason, water molecules are also attracted to each other by (weaker) hydrogen bonds between the positively charged hydrogen atoms and the negatively charged oxygen atoms of neighbouring water molecules. As water is cooled below 4°C, the hydrogen bonds adjust to hold the negatively charged oxygen atoms apart with space in between. This produces a crystal lattice, which is commonly known as 'ice'. 

Ice, like all solids, has a well-defined structure; each water molecule is surrounded by four neighbouring water molecules. Two of these are hydrogen-bonded to the oxygen atom on the central water molecule, and each of the two hydrogen atoms is similarly bonded to another neighbouring water molecule. Ice forms crystals having a hexagonal lattice structure, which in their full development would tend to form hexagonal prism like structure.

There is greater openness in the structure of ice. This is necessary to ensure the strongest degree of hydrogen bonding in a uniform, extended crystal lattice when amount of thermal energy available to the molecules is less. 

In the case of water, the molecules attract each other through the special type of dipole-dipole interaction known as hydrogen bonding. A hydrogen-bonded cluster in which four water molecules are located at the corners of an imaginary tetrahedron is an especially favourable (low-potential energy) configuration. The molecules undergo rapid thermal motions. In liquid water a more crowded and jumbled arrangement of water molecules is possible because of the greater amount of thermal energy available above the freezing point.

From the arrangement, water molecules in the structure of ice it is clear that hydrogen bonding is maximum in ice, has more open structure, and therefore has a lesser  density than liquid water. Therefore Ice cubes float on water.