Question

Why do solids , liquids and gases differ in their physical state?

Answer 1

Whether a substance is gas, liquid or solid depends on the balance between strength of its chemical bonds within its molecules and the interactions between the molecules (inter-molecular interactions). Gas e.g. Cl2, O2,etc.and liquid molecules, e.g. H2O, have strong covalent bonds. The inter-molecular forces between O2 or Cl2 molecules are very weak. Thus, in gases we have either very weak or no order. In water the covalent bonds between atoms are strong but there are inter-molecular hydrogen bonds also. Thus, water molecules form some timely complexes and we say that there is “near order” in liquids. In solids the inter-molecular forces are comparable to the chemical bond forces, e.g. in NaCl the ionic chemical bonds are between Na and Cl atoms throughout of the salt crystal. The same is with the diamond, where the covalent bond forces are distributed throughout of the whole crystal. In sugar crystals the inter-molecular forces are quite strong. Thus we say that crystals have “remote order”. Additionally, there are solid liquids such as glass. They are formed by a balance of bond forces and inter-molecular forces but they were unable to form crystal structure. Here we have again “near order”.