(i) What property does Magnesium Oxide have that makes it useful to create heat-resistant bricks to line furnaces? (ii) Explain why H2S exists as a gas and H2O exists as a liquid (at r.t.p).

 (i)  Magnesium oxide is an ionic compound meaning the elements reside in a lattice held together by very strong electrostatic forces of attraction between their oppositely charged ions. Since bonds are broken by the input of energy (an endothermic process) MgO has a very high melting point making it an appropriate material to use. (ii)  The reason H₂O exists as a liquid is due to Oxygen being more electronegative than Sulphur which allows it to polarise the oxygen-hydrogen bond. The partially positive Hydrogens are then attracted to the ‘negative’ Oxygens of other water molecules resulting in an additional set of intermolecular attractive forces known as hydrogen bonding. Since Sulphur cannot polarise Hydrogen very much there is no Hydrogen bonding. You can think of it as water molecules not having enough energy to break off from each other to become a gas whereas H₂S molecules don’t have the same type of ‘hold’ on each other. This ‘hold’ between molecules is determined by intermolecular forces. The more you have, the tighter the hold.