polar

1) what is the meaning of polar and non-polar?

Polar means a covalent molecule possessing some ionic characters due to the difference between electronegativity of the constituent atoms.

Say for example, a water molecule consists of H and O atoms. Since O is more electronegative than H (i.e.having higher affinity to electron then H), the bond pair electrons between a O - H bond is unevenly distributed with more biased to the O atoms. As a result, O atom will share more negative charges then H atoms and hence a water molecule exhibits an ionic character with O being more negative.

2) what is the difference between a polar substance and a non-polar substance?

Polar substance: exhibits ionic characters and the intermolecular forces between molecules are higher than expected, say for water, its melting and boiling points are adnormally high due to the existence of hydrogen bond between the molecules.

Non-polar substance: does not exhibit ionic characters and hence their melting and boiling points are generally lower than those of polar substances, say tetrachloromethane (CCl4)