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Arrange the following substances in order of increasing melting point: Si, KCl, CH3OH (methanol), C2H6 (ethane).

1. Si < KCl < CH3OH < C2H6
2. CH3OH < C2H6 < Si < KCl
3. KCl < Si < C2H6 < CH3OH
4. C2H6 < CH3OH < KCl < Si

Correct answer: C2H6 < CH3OH < KCl < Si

Solution

C2H6 is a nonpolar molecular solid held by weak van der Waals forces (MP about -183 deg C). CH3OH is a polar molecular solid with hydrogen bonding (MP about -98 deg C). KCl is an ionic solid with strong electrostatic interactions (MP about 770 deg C). Si is a covalent network solid with very strong Si-Si covalent bonds throughout the lattice (MP about 1414 deg C). Increasing MP: C2H6 < CH3OH < KCl < Si.

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