Why is ice slippery?

Ice is solid form of water. Ice is made from water molecules. When water freezes ice is formed. The structure of water molecules in ice is the same as the structure of water molecules in steam.

Water molecule is made up of two hydrogen atoms and one oxygen atom. The electric field around oxygen atom is stronger. The electric field around hydrogen atom is weaker. Electrons from hydrogen atoms are pulled towards the oxygen atom. So, hydrogen atoms are electrically positive and oxygen atoms are electrically negative.

The region between hydrogen and oxygen atoms is linked with covalent bond. The electrons near the oxygen atom arrange in such a way that they stay away from one another. The lone pair of electrons in the oxygen atom arrange like a tetrahedron with respect to hydrogen atom.

Hydrogen bond is formed in liquid water as the hydrogen atoms of one water molecule are attracted towards the oxygen atom of a neighbouring water molecule. The hydrogen atoms are covalently attached to their oxygen atoms and also attracted towards other nearby oxygen atoms. This attraction is the basis of the 'hydrogen' bonds.  Hydrogen bonds are weak bonds. Several properties of water and ice are controlled by hydrogen bonds.

In liquid water, all water molecules have at least one hydrogen bond to neighbouring water molecules with effectively no free water molecules. 

The solid ice has the following properties:

1. Oxygen atoms are linked to each other by the combination of covalent bond and hydrogen bond.

2. Each water molecule can form two hydrogen bonds involving their hydrogen atoms and two further hydrogen bonds utilizing the hydrogen atoms attached to the neighbouring water molecules.

3. These four hydrogen bonds optimally arrange themselves as tetrahedron around each water molecule.

4. Each oxygen atom is linked by a combination of covalent bond and hydrogen bond to four other oxygen atoms.

5. Ice possesses crystalline structure because in ice the atoms are arranged in regular periodic manner.

6. Ice may have impurities such as particles of soil, bubbles of air etc. So it may appear transparent, opaque or bluish white.

7. Ice appears blue like water because it absorbs light at the red end of the spectrum and oxygen–hydrogen (O–H) bond are stretched. 

8. We have to cool water to get ice. When we cool water at room temperature we find that it goes on contracting but as the temperature falls below 4 degree C. On reaching 4 degree C it begins to expand instead of contracting. Conversely if water is heated from 0 degree centigrade to 4 degree C instead of expanding it contracts. This peculiar behaviour of water is known as Anomalous Behaviour of Water. The anomalous behaviour of water plays an important role in the survival of aquatic animals & plants in cold countries.

9. During winter season as the temperature decreases water at the surface of lakes, rivers, sea etc start contracting. As the water contracts its density increases & it sinks to the bottom. This process continues until the temperature of all the water in a lake or river falls to 4 degree C.

10. As the water at the surface cools below 4 degree C it expands instead of contracting & consequently it’s density decreases, as a result it remains on the surface of lake or river.

11. The temperature of water in the surface goes on decreasing to 0 degree C & finally is converted into hexagonal crystals of ice, while the water below the ice continues to remain at 4 degree C. As water remains at 4 degree C, fish & other aquatic animals remain alive.

12. This is due to hydrogen bond dominating the intermolecular forces, which results in less compact packing of the molecules in the solid.

13. When ice melts, the temperature remains constant at 0 Degree C. While melting, any energy added absorbed by ice breaks the hydrogen bonds between ice molecules. When all the hydrogen bonds are broken, ice will be converted into liquid water. The amount of energy consumed in breaking hydrogen bonds in the transformation from ice to water is known as the heat of fusion.

14. From the above properties of ice, we can conclude that ice cannot be slippery. When we walk on ice we may feel slippery.  But ice is not the reason for slipperiness.  Slipperiness is caused by the thin layer of liquid water present below the surface of ice. Viscosity of water is low. The inter-molecular bonds are not strong in liquid water. They are weak because the molecules move about freely, slide with each other to fill the holes and cracks they come across. The liquid layer of ice that is water has no friction. Therefore iced surface is slippery.