Explain the following briefly:
(a) Sodium chloride dissolves in water but carbon tetrachloride is insoluble in water.
(b) Helium does not form He2 molecule.
(c) Pure water does not conduct electricity, but on adding sodium chloride to it, it starts conducting electricity.
(d) Cl2 is a non-polar molecule, while HCl is a polar molecule.
(e) Metals are electropositive.

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(a) Sodium chloride is an ionic compound. Ions present in sodium chloride can interact with polar water molecules and dissolve in it. But carbon tetrachloride is a non-polar covalent compound in nature and cannot interact with polar water molecules. Hence, it does not dissolve in water.

(b) Helium has completely filled K shell. Hence, it has no tendency to share, lose or gain electrons from another helium atom. Therefore, it does not form He2 molecule.

(c) Pure water does not conduct electricity due to absence of charge carriers or free ions in it. Addition of sodium chloride leads to the release of sodium and chloride ions. These ions in the solution can move freely and conduct electricity.

(d) Cl2 molecule is a non-polar molecule because two chlorine atoms have same electronegativity. Hence, the shared pair of electrons lies exactly between the two atoms without the development of any partial charge on either chlorine atom. While in HCl, there is a large electronegativity difference between chlorine and hydrogen atoms. Due to the high electronegativity of a chlorine atom, the shared pair of electrons shifts more towards chlorine atom and develops a partial negative charge on it. Meanwhile, an equivalent partial positive charge is developed on hydrogen atom, resulting in the formation of a polar molecule.

(e) Metals have low ionisation energy and can easily lose their valence electrons to attain noble gas electronic configuration. Hence, they are electropositive in nature.

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