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When you heat a material, you are adding
kinetic energy to its molecules and usually raising its
temperature. The only exception is when the material
reaches its melting or boiling points. At those two
temperatures, the heat energy goes into changing the
state of the material. After the state has changed, the
temperature will rise again with added heat. The rate
temperature changes is the specific heat of the
material. The amount of heat required to melt the
material is called the latent heat of melting. This all
can be illustrated in showing how much heat is required
to change ice into water and then change the water into
steam.
Questions you may have include:
Units of measurement
Since we are measuring the amount of
heat required to make these changes, we need to know the
definitions of the various units involved.
Heat is the total kinetic energy of all
the molecules in an object. Although energy is typically
measured in joules, a more common unit for heat is the
calorie, which is defined as the amount of heat required
to change the temperature of 1 gram of water by 1
Celsius degree. There is a conversion factor to relate
joules to calories, but we won't worry about that here.
Calories are abbreviated as cal and
grams as g. oC means degrees Celsius.
In the English measurement system, they
use the BTU (British thermal unit), which is the amount
of heat required to change the temperature of 1 pound of
water by 1 Fahrenheit degree. The BTU is seen in the
United States when referring to the capacity of a
furnace.
Materials vary in their capacity to
store thermal energy. For example, a material like iron
will heat up much faster than water or wood. The
measurement of how much heat is required to change the
temperature of a unit mass of a substance by 1 degree is
called its specific heat.
There are charts available listing the
specific heat of various materials. The chart below
shows the specific heat of ice, water and steam in units
of calorie per gram-degree Celsius.
(Note that all items are listed with the
same number of decimal points. That indicates the same
accuracy for each. Also, the zero in front of the
decimal point assures that the reader will know it is a
decimal point and not a fly speck.)
| Ice |
0.50 |
| Water |
1.00 |
| Steam |
0.48 |
Specific heat of various states of water
In other words, it would take twice as
many calories to heat some water one degree than it
would to heat the same mass of ice one degree.
When any material is heated to the
temperature where it changes state, the temperature will
remain the same until all the material changes state.
That means ice water will remain at 0oC (32oF) until all
the ice is melted. The same thing applies when cooling
the materials.
The reason is that energy must be
expended to change the state from solid to liquid or
from liquid to gas. Likewise, energy must be withdrawn
to change the state when cooling the material. The
amount of energy required is call the latent heat of
freezing or boiling. The chart below shows the latent
heat or energy required to change the states of water.
| Melting / Freezing |
80 |
| Boiling / Condensation |
540 |
Latent heat required to change state of
water
Problem
A good way to understand the concepts is
to solve a problem.
Suppose we have 50g of ice at -10oC. We
want to heat the material until it all turns to steam at
110oC. How much heat is required?
With a problem that is complex like this
one, it is good to break it down into pieces and solve
each part individually. This also helps to explain the
logic used in the solution.
How much heat would be required to raise
50g of ice to its melting point?
The ice temperature must be raised 10
degrees to reach 0oC.
Since the specific heat of ice is 0.50
cal/g-oC, that means that 0.50 calories is needed to
raise 1g of ice 1oC. Thus, it would take 50 x 0.50
calories to raise 50g up 1oC and 10 x 50 x 0.50 = 250
cal to raise the ice to its melting point.
How much heat would be required to melt
the 50g of ice?
The latent heat for melting ice is 80
cal/g. That means that 1g of ice requires 80 cal of heat
to melt.
Thus, 50g requires 50 x 80 = 4000 cal to
melt.
How much heat is required to heat 50g of
water from 0oC to its boiling point of 100oC?
Since the specific heat of water is 1.00
cal/g-oC, that means that 1.00 calorie is needed to
raise 1g of water 1oC. Thus, it would take 50 x 1.00
calories to raise 50g up 1oC and 100 x 50 x 1.00 = 5000
cal to raise the water to its boiling point.
How much heat would be required to boil
the 50g of water?
The latent heat for boiling water is 540
cal/g. That means that 1g of water requires 540 cal of
heat to boil.
Thus, 50g requires 50 x 540 = 27000 cal
to boil.
How much heat is required to heat 50g of
steam from 100oC to 110oC?
Since the specific heat of steam is 0.48
cal/g-oC, that means that 0.48 calories are needed to
raise 1g up 1oC. Thus, it would take 50 x 0.48 calories
to raise 50g of steam 1oC and 10 x 50 x 0.48 = 240 cal
to raise the temperature of the steam to 110oC.
The total heat required to change 50g of
ice at -10oC to steam at 110oC is:
250 + 4000 + 5000 + 27000 + 240 = 36490
cal.
Heating materials like ice, water and
steam increases their temperature. The specific heat of
the material determines the calories required to heat
one unit of mass one degree. Changing the state of the
material requires extra heat energy that is not used to
change the temperature. The amount of heat required to
change the state of the material is called its latent
heat. The complex problem of determining how much heat
is required to change ice into water and then change the
water into steam should be broken into parts and solved
individually. |
