MODULE 3
4
WORKSHEET
WORKSHEET
EXPLAINING SOME PROPERTIES
OF WATER USING MODELS
Syllabus reference 8.4.1
Water is a common substance with apparent molecular simplicity, but it is also one of the most
important and unusual substances on Earth. It is the only substance to exist naturally in all three states
and it has the largest known number of possible crystal structures.
The formula for water is H2O and the shape of the water molecule is bent.
O
H
H
104.5°
The basic geometry in a normal ice crystal is shown below.
H
O
H
H
H
O
H
weak hydrogen bonds
between hydrogen
and oxygen atoms
O
H
H
O
H
H
O
H
(a) Five water molecules form
a tetrahedral cluster in ice
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O
O
O
O
O
O
O
O
O
O
open-cage
structure
O
O
O
O
O
O
O
O
O
O
O
(b) Ice crystal structure
Each oxygen atom is surrounded by four hydrogen atoms arranged in a tetrahedron. Two of the
hydrogen atoms are close to the oxygen atom and these are the ones that are covalently bonded to that
oxygen. The other two are slightly further away and are part of two other water molecules but are
attracted to an adjacent oxygen by hydrogen bonding.
When ice melts the water molecules gain energy. The crystalline structure present in ice begins to
disappear as molecular motion increases and positioning of the molecules becomes more random.
Using the above information complete the following.
1
In each of the boxes below draw a labelled diagram to represent the different arrangement of
molecules in ice (solid water) and liquid water. (Represent the water molecule by a circle.)
SOLID WATER
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LIQUID
LIQUID WATER
WATER
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2
Describe in your own words the arrangement of molecules in:
a ice (solid water)
An ice crystal has five water molecules in a tetrahedral cluster with hydrogen bonding between
water molecules.
b
liquid water
Liquid water has a random arrangement between molecules with no fixed positions as in ice.
There are hydrogen bonds between water molecules but these are continually being broken and
reformed.
3
Using the above models account for the differing densities of ice and liquid water.
Ice has a more open structure with the molecules in fixed positions further apart. This means
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there is more space between molecules in ice than in water so ice is less dense.
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MODULE 3 WS 4