Transcona Community Learning Centre
CHEM 30S
LESSON 5: BOILING AND CONDENSING
Lesson Outcomes
When you have completed this lesson, you will be able to:
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Operationally define boiling temperature in terms of observable and measurable
properties.
State the normal boiling temperature of water (i.e., boiling temperature at standard
pressure — also called boiling point).
Predict the ranking of normal boiling temperatures of a series of ionic or nonionic compounds from formula mass, and explain reasons for the ranking.
Compare the normal boiling temperatures of various liquids, and relate these
temperatures to intermolecular forces.
Describe the practical value of knowing the boiling temperatures of various
substances (e.g., distillation for purification or separation).
Defining Boiling and Condensing
We will begin our discussion by operationally defining boiling and condensing, that is,
we will describe them in terms of observable characteristics. In the next lesson we will
learn the true definition of boiling.
Boiling or vaporization is known as the change in state from liquid to gas. The
boiling point of a substance is the temperature at which the substance boils.
Boiling is recognized by vapour bubbles forming throughout the liquid, rising to
the top where the vapour is released.
The normal boiling point is the temperature at which a substance boils at
standard pressure. The normal boiling point of water is 100.0°C.
As a substance boils, the particles of the liquid absorb energy and begin to move
faster and further apart. The liquid gets progressively less dense as the particles
(move further apart) absorb more heat. In fact, water is at its most dense at 4°C.
Eventually, the particles have enough energy to overcome the forces of attraction
in the liquid state and form bubbles, or pockets of vapour, throughout the liquid.
Since these bubbles of vapour are less dense than the liquid, they rise to the
surface and break open.
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Condensation or liquefaction is the change in state from gas to liquid. As a gas is
cooled, the particles slow down and move closer together. The intermolecular
forces take hold and the particles move into the liquid phase. As they move
closer together, they no longer require as much energy and heat is released.
Heating Curve for a Liquid
Try this at home using the thermometer from the previous lesson. Heat a pot of water on
the stove and observe the temperature, ensuring the thermometer does not touch the sides
or bottom of the pot. Continue to observe the temperature until the water boils and
observe the temperature for 5 – 10 minutes more.
You should notice something similar to the melting of ice experiment. The
temperature remains constant during the change of state! If you continue
monitoring the temperature of the boiling water, the temperature will remain
constant at close to 100°C. If we were to graph the data, it would look like the
figure below.
Why does the temperature remain constant, even though we continue to heat the
water? We can explain this using the Kinetic Molecular Theory. Forces of
attraction hold the particles in a liquid together. As heat is absorbed, the kinetic
energy of the particles increases, so the temperature increases. Eventually,
when the boiling point is reached, energy is used to overcome the forces of
attraction between the particles in the liquid. The heat supplied by the stove, is
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used to overcome the intermolecular forces rather than increasing the
temperature. When all liquid particles have vaporized, the temperature of the gas
can begin to increase.
During boiling the temperature of the liquid water in the pot will not change, as
long as the pressure remains constant. The plateau or constant temperature
indicates the boiling point of the liquid.
Cooling Curve for Water Vapour
The cooling curve for water vapour would be similar to the one below.
The plateau occurs at the boiling point of the substance. The temperature
remains unchanged at the boiling point because as the particles condense,
energy is released. This energy prevents the water vapour-water mixture from
decreasing in temperature.
If the temperature were to remain constant at the boiling point, the liquid and gas
phases would remain in a dynamic equilibrium, as long as the vapour is not
allowed to escape. These particles would move back and forth between the liquid
and vapour phase indefinitely, as long as the container was closed and the
temperature remained constant.
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Boiling Point and Intermolecular Forces
As discussed with melting point, the stronger the forces of attraction between the
particles, the greater the energy needed to overcome these forces. This means that the
stronger the intermolecular forces, the higher the boiling point.
When comparing similar molecules, boiling point increases with increasing formula
mass. This suggests, with similar particles, larger particles have greater forces of
attraction.
Boiling Points of Noble Gases
Mass
(amu)
Boiling Point
(°C)
Helium
4.0
-269
Neon
20.1
-246
Argon
39.9
-186
Krypton
83.8
-153
Element
The intermolecular forces in ionic compounds such as NaCl (mass = 58.5
amu) are still much higher than those of molecular compounds, when
comparing mass. The boiling point of NaCl is 1413°C. The high boiling points
of ionic compounds is due to the strong electrostatic forces between the
positively and negatively charged ions.
Boiling point is a characteristic physical property. Many substances can be
distinguished by their boiling points. For example, the purification of
petroleum products involves a process called fractional distillation. The
products are separated by their differing boiling points. The crude oil is
heated to a very high temperature and the components of the oil will
condense at various temperatures. We will examine the fractional distillation
of crude oil further in the organic chemistry module.
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The Boiling Point of Water
As mentioned previously, there is a special intermolecular force of attraction holding
water molecules together. This intermolecular force is called hydrogen-bonding. If we
compare the boiling point of water to other hydrogen compounds of group 7 (VII)
elements, we would expect water to have a boiling point near –80°C.
Due to hydrogen-bonding, water's boiling point is considerably higher than what
would be expected based upon its size.
Lesson Summary
In this lesson, you have learned:
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A heating curve shows the graph of a substances temperature as it's heated, as a
function of time.
At the boiling point, both liquid and gas phases are present.
The boiling point of a substance is the temperature at which the heating curve of
its liquid plateaus.
As a substance is heated, the temperature of a pure substance remains unchanged
at its boiling point because energy is used to overcome intermolecular forces,
rather than increasing temperature.
As a substance is cooled, the temperature of the substance remains unchanged at
its boiling point because energy is released as the molecules become arranged into
a liquid, which requires the particles to have less energy. The released energy
maintains the temperature at the boiling point.
The normal boiling point is the temperature at which a substance changes from a
liquid to gas at standard pressure. Water's normal boiling point is 100°C.
The greater the intermolecular forces, the greater the boiling point.
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Exercise
Answer the following questions.
1. Why does the temperature remain constant at the boiling point until all the liquid
is gone, no matter how long the liquid is boiled?
2. Explain how intermolecular forces affect the boiling point of a substance.
3. HCl has a boiling point of -85C. HI has a boiling point of -35C. What could you
conclude regarding their intermolecular forces?
Answer Key
1. The temperature remains constant because the heat energy is used to overcome
forces of attraction, rather than increasing the temperature (motion) of the
particles.
2. Substances with strong imfs have a higher boiling temperature since more energy
is needed to overcome the stronger imfs. Substances which have lower imfs will
have lower boiling points.
3. HI has stronger imfs because its boiling point is higher. ( BP =  imfs)
Lab Assignment – do only if you have not done the one in class
Description: You will draw a labelled heating curve, given experimental data.
Method of Evaluation: This assignment will be marked out of a total of 13 marks.
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The graph will be marked out of 5 marks.
The graph should fill a page.
The graph should have a title and labelled axis.
The points should be visible and connected with a line.
The value of each question is shown below.
A student was given some solid Substance Y, placed it in a beaker and began
heating the substance. Temperature measurements were taken every minute for
35 minutes. The following data were collected and recorded in the following
table. Use spreadsheet software or graph paper to construct a heating curve of
the substance. Make sure to label axes and title the graph.
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Table 1. Heating of Substance Y.
Time
(minutes)
Temperature
(°C)
Time
(minutes)
Temperature
(°C)
0.0
15.2
19.0
92.5
1.0
19.8
20.0
98.7
2.0
24.7
21.0
107.0
3.0
29.2
22.0
109.1
4.0
31.3
23.0
109.4
5.0
32.2
24.0
109.3
6.0
32.0
25.0
109.3
7.0
32.1
26.0
109.3
8.0
32.1
27.0
109.4
9.0
32.1
28.0
109.3
10.0
32.2
29.0
109.4
11.0
35.4
30.0
110.5
12.0
45.0
31.0
111.8
13.0
54.6
32.0
119.6
14.0
61.2
33.0
127.1
15.0
66.8
34.0
133.4
16.0
75.0
35.0
139.7
17.0
83.7
18.0
88.1
Question(s):
1. What is the melting point of this substance? How do you know? (1.5
marks)
2. What is the boiling point of this substance? How do you know? (1.5
marks)
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3. What phase(s) is/are present at each of the following points in time?
a) 2 min b) 6 min c) 17 min d) 26 min e) 34 min (2.5
marks)
4. Explain why, at the intervals of 5-10 min and 23-29 min the substance
is being heated but the temperature is not increasing. (1.5 marks)
5. Explain how the length of the plateaus in the graph could be changed.
(Give at least two ways) (1 mark)
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