What happens to the temperature of a block of
ice when you put a Bunsen burner underneath it? You might think that the
temperature goes up smoothly, but that's not what happens. The graph of
temperature against time is called a heating curve. Let's look at the
heating curve for water.
Notice that, in general, the temperature goes
up the longer the heating continues. However, there are two horizontal
flat parts to the graph. These happen when there is a change of state.
The plateaus are
also called phase changes.
The first change
of state is melting (changing from a
solid to a liquid). The temperature stays the same
while a substance melts. For water, this temperature is 0°C because
the melting point
for water is 0°C.
The second
change of state is boiling (changing from a liquid to a gas). The temperature stays the same
while a substance boils. For water, this temperature is 100°C
because the boiling
point for water is
100°C.
Different
substances have different melting points and boiling points, but the
shapes of their heating curves are very similar. For example, this is the
heating curve for iron, a metal that melts at 1538°C and boils at
2861°C.
Cooling Curves
Heating curves show how the temperature
changes as a substance is heated up. Cooling curves are the opposite. They
show how the temperature changes as a substance is cooled down. Just like
heating curves, cooling curves have horizontal flat parts where the state
changes from gas to liquid, or from liquid to solid.
You are likely
to have used salol or stearic acid in a school practical lesson to make
your own cooling curve. Salol has a melting point of about 45°C and
stearic acid has a melting point of about 69°C. They are easily melted in
a boiling tube placed in a beaker of hot water. The temperature can be
followed using a thermometer or temperature probe connected to a data
logger. The liquid may be cooled by putting the boiling tube in a beaker
of cold water or just leaving it in the air.
***Note- The melting and freezing
occur at the same temperature.
During freezing, energy is
removed and during melting, energy is absorbed.
1 The solid and liquid phases of water
can exist in a state of equilibrium at 1 atmosphere of pressure and
a temperature of
(1) 0oC (2)
100oC (3)
273oC (4)
373oC
ANSWER--->
The melting point of water
is OoC or 273K
2 Given the equation:
H2O(s) <--> H2O(l)
At which temperature will equilibrium exits when the atmospheric
pressure is 1 atm?
(1) 0 K (2) 100 K (3) 273
K (4) 373
K
ANSWER--->
The melting point of water
is OoC or 273K
3 The table below shows the normal
boiling point of four compounds.
Which compound has the strongest
intermolecular forces? (1)
HF(l) (2)
CH3Cl(l) (3)
CH3F(l)
(4) HCl(l)
ANSWER--->
HF The higher the boiling
Point the stronger the force of attraction between the
molecules
A sample of water is heated from
a liquid at 40oC to a gas at 110oC.
a)On the
heating curve diagram provided above, label each of the
following regions:
Liquid, only ;
Gas, only; Phase change
b) For section
QR of the graph, state what is happening to the water molecules as
heat is added.
c) For section
RS of the graph, state what is happening to the water molecules as
heat is added.
ANSWER
a--->
QR-liquid
only RS-Phase Change
ST Gas only
ANSWER
b--->
QR-Kinetic Energy
increases
ANSWER
c--->
RS- Potential Energy
Increases
3 The graph
below represents the heating curve of a substance that starts as a
solid below its freezing point.
What is the
melting point of this substance? (1) 30oC (2)
55oC (3) 90oC (4)
120oC
ANSWER--->
55 degrees the lower
plateu is the melting/freezing
point
8/03
Base your answers to questions 59 through 62
on the information below. Given the heating curve where substance
X starts as a solid below its melting point and is heated
uniformly:
59 Identify the process that takes place
during line segment DE of the heating
curve.
ANSWER--->
Vaporization, boiling or
phase change. NOT evaporation (that occurs below the boiling
point).
60 Identify a line segment in which the
average kinetic energy is increasing.
ANSWER--->
AVG. KE is temp, AB CD or
EF
61 Using "o" to represent particles of
substance X, draw at least five particles as they would
appear in the substance at point F.
ANSWER--->
The five particles would
be spread out.
62 Describe, in terms of particle behavior
or energy, what is happening to substance X during line
segment BC.
ANSWER--->
The particles are
increasing in potential
energy.
Base your answers to questions
54 and 55 on the heating curve below, which represents a substance
starting as a solid below its melting point and being heated at a
constant rate over a period of time.
54 What is happening to the
average kinetic energy of the particles during segment
BC?
ANSWER--->
Since Avg. KE is
Temperature, it remains the
same.
55 How does this heating curve
illustrate that the heat of vaporization is greater than the heat of
fusion?
ANSWER--->
Segment DE is Longer than
BC. More energy requires more time. NOT DE is at a higher
temp.
6/05
17 In which process does a solid change directly into a vapor?
