CK-12 Physical Science Concepts - For Middle School

http://www.ck12.org Chapter 3. Chemical Interactions

TABLE3.5:Melting and Boiling Points of Some Polar and Nonpolar Covalent Compounds

Name of Compound

(Chemical Formula)

Polar or Nonpolar? Melting Point(°C) Boiling Point (°C)

Methane (CH 4 ) nonpolar -182 -162

Ethylene (C 2 H 2 ) nonpolar -169 -104

Ammonia (NH 3 ) polar -78 -33

Water (H 2 O) polar 0 100

Q:Which compound in theTable3.5 do you think is more polar, ammonia or water?

A:Water is more polar than ammonia. Its strong polarity explains why its melting and boiling points are high even
for a polar covalent compound.

Summary

• Polar covalent compounds have molecules with a partial negative charge on one side and a partial positive
  charge on the other side. This occurs because the compounds contain polar bonds. In a polar bond, one atom
  attracts the shared electrons more strongly than the other atom does.


• In some polar molecules that contain hydrogen atoms, the partial positive charge of the hydrogen atoms of one
  molecule are attracted to the partial negative charge of an atom of a nearby molecule. This force of attraction
  is called a hydrogen bond.


• Hydrogen bonds are relatively weak, but they add to the energy needed for molecules to move apart from each
  other when matter changes state from a solid to a liquid or from a liquid to a gas. This explains why polar
  covalent compounds have relatively high melting and boiling points.

Vocabulary

• hydrogen bond: Weak bond that forms between a slightly positive hydrogen atom in one molecule and a
  slightly negative atom in another molecule.

Explore More

Watch the video about hydrogen bonds at the following URL, and then answer the questions below.

http://www.youtube.com/watch?v=ZWqC1uDNlJ0

1. What is electronegativity?


2. In water molecules, why do shared electrons spend more time orbiting the oxygen atom than the hydrogen
   atoms?


3. Hydrogen forms hydrogen bonds with oxygen. What is another element besides oxygen that may be involved
   in hydrogen bonds?


4. Water has the properties of cohesion and adhesion. Define these two properties, and explain why they occur
   in water.


5. Why are hydrogen bonds extremely important in biology?

Review

1. What are polar covalent compounds?


2. Define hydrogen bond.