Lesson 15: Properties of Liquids and Solids

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Intermolecular forces rule when it comes to liquids and solids.

Textbook Reading: Chapter 12, pages 411-437.

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Let’s start out with an introduction to liquids and solids by Bozeman Science.


 

Intermolecular forces

Remember the forces that hold atoms together to form molecules that we learned about in chapter 10: covalent bonds, polar covalent bonds and ionic bonds? Now we are going to find out about forces between molecules or intermolecular forces, which is what solids and liquids are all about.

The three types of intermolecular forces are dispersion forces (also called London forces), dipole-dipole forces and hydrogen bonds.

Dispersion forces were first recognized by Fritz London, which is why they are often called London forces. They are found between all molecules (both polar and nonpolar) and a formed due to temporary unequal sharing of electrons. In larger atoms or molecules, the valence electrons are, on average, farther from the nuclei than in smaller ones. The electrons are thus held less tightly and can more easily form temporary dipoles. Because of this, larger and heavier molecules exhibit stronger dispersion forces than smaller ones.

This quick video shows how the electrons move around an atom, but the forces work in a similar way around molecules as well.

Dipole-dipole forces occur between molecules that are permanently dipolar. You can figure out whether a molecule is dipolar by examining the differences in electronegativity between the atoms and also by examining the molecule’s shape.

dipole-force-graphic

Hydrogen bonds are the strongest type of intermolecular force. They form in the special case of hydrogen bonded to fluorine, nitrogen or oxygen. Although very specific, hydrogen bonds are not uncommon and in fact form important links between molecules in our DNA.

Base_pair_GC.svg

Hydrogen bonding between guanine and cytosine in DNA

(Illustration public domain from Wikimedia)

Intermolecular forces determine the physical properties of liquids and solids, such as surface tension, viscosity, melting point, boiling point, volatility, etc. We will examine some of these properties in lab.

This final video goes in depth about physical properties and intermolecular forces. Stick with it and you will find out why Kevlar is so strong!

Please feel free to contact me if you have any questions about this week’s lesson.

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