Question

If we dissolve sugar in water the sugar molecules break and get into the spaces of water molecule but when a piece of iron is placed into water it donot dissolve , why sugar breaks into molecules and iron donot.

Answer 1

For a liquid to dissolve a solid, the molecules of the liquid and solid must attract one another.The bond between the oxygen and hydrogen atoms (O–H bond) in sugar (sucrose) gives the oxygen a slight negative charge and the hydrogen a slight positive charge. Sucrose is a polar molecule.
The polar water molecules attract the negative and positive areas on the polar sucrose molecules which makes sucrose dissolve in water.

In order for something to dissolve in water it needs to form ions that can become hydrogen bonded to the water molecules. As elemental iron is locked in solid form and is not ionised it is therefore unable to dissolve in water.