Which formula represents an ionic compound?
(1) nacl (3) hcl
(2) n2o (4) h2o

Answer is: 1) NaCl.

NaCl is sodium chloride, ionic compound.

Ionic compounds are made of ions held together with ionic bonds.  

Ionic bond forms when a cation transfers its extra electron to an anion who needs it.  

Electronegativity (χ) is a property that describes the tendency of an atom to attract a shared pair of electrons.  

Δχ(Na-Cl) = 3.2 - 0.95 = 2.25; electronegativity difference between sodium and chlorine.

When the electronegativity difference is greater, the bond polarity is increasing.

Other compound have covalent polar bonds.

vanderwaal's forces of attraction are the electrostatic intermolecular forces that exist in a molecule due to the temporary or permanent dipoles that are developed in a molecule. in , the major intermolecular forces are dipole-dipole interactions as the molecule is polar (bent structure). so the vanderwaal's forces of interaction in between molecules are the permanent dipole-dipole interactions that develop due to the dipole that exists in between the polar h - s bond. the positively charged hydrogen atom is attracted towards the negative end of the dipole that is sulfur of the adjacent molecule. therefore, these forces of attraction are relatively stronger than that of london dispersion forces that exist in non polar molecules.

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.