1.48: Explain Why the Melting and Boiling Point of Substances with Simple Molecular Structures Increase, in General, with Increasing Relative Molecular Mass

SIMPLE MOLECULAR STRUCTURE: Molecules with atoms joined together by Strong Covalent Bonds

RELATIVE MOLECULAR MASS: Calculated by adding the Relative Atomic Mass for all the atoms shown in Chemical Formula

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MELTING AND BOILING POINT OF SIMPLE COMPOUNDS IN RELATION TO MOLECULAR MASS:

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• As Relative Molecular Mass of a substance increases, melting and boiling Point will increase in correlation (Positive Correlation)
• As the increase in Relative Molecular Mass of a substance means there are more covalent bonds in the structure, more intermolecular forces of attraction are needed to be broken down
• Hence, larger amounts of heat energy is needed to overcome these forces and break down the bonds

Example:

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SIMPLE MOLECULAR SUBSTANCE	RELATIVE MOLECULAR MASS	MELTING POINT	BOILING POINT
METHANE	16	-182°C	-161.5°C
AMMONIA	17	-77.73°C	-33.34°C
WATER	18	0°C	100°C