why does water have a higher melting and boiling points than other molecules with similar molecular weights?

Answer 1

It's because of the H-bonding of water molecules (18g/mol). H-bonding is stronger than Van der Waals forces - for example in methane CH4 (16g/mol), which is gas at room temperature. Besides, water is unique because its oxygen atom has two lone pairs and two hydrogen atoms, meaning that the total number of H-bonds of a water molecule is up to four. Unlike in ammonia NH3 (17g/mol), there's only one H-bond since nitrogen has only one lone pair. Consequently, its melting and boiling points are lower than water (ammonia is also gas at room temp).

Answer 2

It's because water is a strongly polar molecule. As a result, it is cohesive, meaning it is attracted to itself, and that gives it a high poiling point since the attraction tries to keep the molecules sticking together as a liquid. The same polarity allows the molecules to easily fall into a crystalline solid structure, making it a solid at a relatively high temperature.

Answer 3

Hydrogen bonding. If you ever get a question to explain an anomaly in boiling points / melting points the answer is often hydrogen bonding! Its a very strong effect (normally the strongest of the intermolecular forces)

Answer 4

hydrogen bonding !!!