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?
