How can I explain with illustrations the cohesive forces of water molecules?

1 Answer
Jun 10, 2014

When I teach about the intermolecular forces in water I get the students to bet on how many water droplets they can place on top of a penny without any falling off. They usually bet about 10, and then find out when they do the experiment that with care about 40 drops can be added.

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We can then go on to talk about how hydrogen bonds form between the lone pairs on the oxygen atoms in water molecules and the #delta+# hydrogen atoms in other water molecules, and how since each water molecule has two lone pairs and two such hydrogens, a network of hydrogen bonds can be built up, leading to surface tension. We'd also draw the hydrogen bonds:

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Finally we'd look at other anomalous properties of water such as relatively high melting and boiling point (for a Group 6 hydride), and the fact that ice floats as it has a lower density than liquid water.