Everything Science Grade 11

CHAPTER 2. INTERMOLECULAR FORCES 2.3

discussed more in Chapter ??. You should also notethat the strength of theintermolecular forces

increases with an increase in formula mass. This can be seen by the increasing melting and boiling

points of substances as formula mass increases.

Exercise 2 - 2

Refer to the data in Table 2.2 and then use yourknowledge of different types of intermolecular forces

to explain the followingstatements:

1. The boiling point ofF 2 is much lower than theboiling point of NH 3


2. At room temperature, many elements exist naturally as gases


3. The boiling point ofHF is higher than the boiling point of Cl 2


4. The boiling point ofwater is much higher than HF, even though they both contain hydrogen
   bonds

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2.3 Intermolecular forces in liquids ESBZ

Intermolecular forces affect a number of properties in liquids. We will use the example of water to

explain this.

• Surface tension
  You may have noticedhow some insects areable to walk across a water surface, and how
  leaves float in water. This is because of surface tension. In water, each molecule is held to
  the surrounding molecules by strong hydrogenbonds. Molecules in the centre of the liquid
  are completely surrounded by other molecules, so these forces are exerted in all directions.
  However, molecules atthe surface do not haveany water molecules above them to pull them
  upwards. Because theyare only pulled sideways and downwards, the water molecules at the
  surface are held more closely together. This is called surface tension.

For molecules in the

centre of the liquid, the

intermolecular forces

act in all directions.

For molecules at the

surface there are no

upward forces, so the

molecules are closer

together.

Figure 2.5: Surface tension in a liquid

• Capillarity
  Half fill a beaker with water and hold a hollowglass tube in the centreas shown below. Mark
  the level of the water inthe glass tube, and lookcarefully at the shape ofthe air-water interface
  in the tube. What do you notice?