# Latent heat and molecules

Hi, everybody. Thank you for watching my video
on Physics. In this video, we shall explain latent heat
of fusion and latent heat of vaporization in terms of molecules. Recall that all matters are made of atoms
or molecules. At any temperature above the absolute zero, molecules are not at rest.
Click the link in the description below. (http://www.radian.com.hk/simulations/). Click Heat 1.4 Molecules in a Solid. In a
solid, molecules are vibrating about their equilibrium positions. They cannot move freely
from one place to another. As temperature increases, the molecules vibrate faster. Clearly,
an increase in temperature will increase the kinetic energy of the molecules. Also recall that internal energy is the sum
of kinetic energy and potential energy of the molecules. Since an increase in temperature
will increase the kinetic energy of the molecules, the internal energy U of the object will also
increase. Mathematically, it is expressed as Q=mc ∆T, where Q represents the change
in internal energy ∆U, i.e. Q=∆U, m represents the mass, c represents the specific
heat capacity and ∆T represents the change in temperature. Let’s link up all the things
together: An increase in temperature will increase the kinetic energy of the molecules
or the internal energy of the object. How about phase change? We an object melts
or vaporizes, latent energy must be supplied while the temperature remains constant at
melting point or boiling point. Since temperature remains constant, the kinetic energy of the
molecules does not change but the internal energy is increasing. What kind of energy
of the molecules is increased during melting or vaporization? Pause the video for a while
now and answer the question. Please press the space bar to pause the video for a while
now and write down your answer. Since the internal energy of an object is
the sum of kinetic energy and potential energy of the molecules, if the internal energy increases
but the kinetic energy remains unchanged, then the potential energy of the molecules
must have increased. What is potential energy? If you have two
magnets sticking to each other, you need energy to separate them. The energy supplied to the
magnets is called potential energy. After separation, when the magnets are released,
they will hit each other producing a sound. Here, the stored potential energy is converted
into sound. Similarly, when you stretch a rubber band, you increase its potential energy
so that when it is released, it will fly away. Another example is raising a book. Raising
a book from the table will also increase its potential energy, because when the book is
released, it will hit the table with a loud sound. Scientists believe that molecules attract
each other with an intermolecular force, similar to the force between two magnets. In a solid,
the molecules attract each other strongly so that they cannot separate easily. In addition
to this, molecules in a solid are arranged in a lattice structure so that the molecules
are vibrating about fixed positions. Energy is needed to break down the lattice structure,
just like energy is needed to break down a house. After the solid is melted into a liquid,
the molecules are free to move. Look at the simulation. Note that the molecules in a liquid
are still closely pack. The latent heat of fusion increases the potential energy of the
molecules. Since the kinetic energy of molecules does not change, melting takes place at a
constant temperature which we call melting point. Let’s look at the change graphically. When
a piece of ice taken out of a freezer is heated, before it melts, its temperature increases
from -8oC. The increase in temperature is explained by the increase in kinetic energy
of the water molecules. When the ice reaches 0oC, it begins to melt. The temperature remains
constant, so do the kinetic energy of the water molecules. The latent heat of fusion
is used to break the lattice structure, increasing the potential energy of the water molecules. When a liquid vaporizes, the volume increases.
During the process, molecules change from closely pack to widely separated. Energy is
needed to separate the molecules against the intermolecular force. This explains why latent
heat of vaporization is needed to vaporize a liquid. For example, a beaker of water placed
inside a pool of boiling water will not boil because the boiling water and the water inside
the beaker do not have a temperature difference large enough to provide the latent heat of
vaporization for the water inside the beaker to boil. The latent heat of vaporization is used to
separate against the intermolecular force, increasing the potential energy of the water
molecules. During boiling, the temperature of the water remains constant, so is the kinetic
energy of the molecules. Only the potential energy of the molecules increases, which is
the latent heat of vaporization. We know that when liquid water changes into
a gas, its volume increases by about 1600 times. Can we say that the size of water molecules
increases by 1600 times? If not, why is the volume increased? Pause the video for a while
now and answer the question. Please press the space bar to pause the video for a while
now and write down your answer. It is important to note that the size of molecules
is always the same and is independent of temperature. When water vaporizes, the bonds that hold
the molecules closely packed in liquid are broken. Therefore, they can move freely at
high speed. We may use a balloon to trap the molecules, but we cannot stop their motions.
The molecules keep colliding the balloon so that it has a huge volume compared to the
size of the molecules. In other words, a gas is mainly empty space with a certain amount
of gas molecules moving at high speed. The collision of the molecules with the container
gives rise to the volume of the gas which is 1600 times larger than the gas molecules.
Expansion of an object by heating is explained in a similar way. When a solid is heated,
the molecules vibrate faster. This causes the spacing between molecules to increase,
but, it is important to note that the sizes of the molecules do not increase! In summary, latent heat is involved in phase
change. During phase change, the temperature remains constant. Therefore, the kinetic energy
of the molecules remains constant. Latent heat increases the potential energy of the
molecules, because during phase change, the structure of the molecules or the separation
between the molecules will change. Question for discussion: When a solid sublimates
to form a gas, how do we calculate the latent heat? Should we use latent heat of fusion
or latent heat of vaporization? Or both? Explain your answer in terms of molecules. Be the first to write your answers in the
more about latent heat and molecules. Thank you for watching.

## 9 thoughts on “Latent heat and molecules”

1. Wai Hin Ng says:

We should use both the latent heat of vaporization and fusion as sublimation makes the molecules of an object from vibrating,forming a lattice structure, to widely separated.The energy needed should be the sum of the latent heat vaporization and
fusion.

2. C CK says:

We should use both latent heat of fusion and latent heat of vaporization.
It is because energy is needed to break down the lattice structure. When a solid sublimates into a gas at a constant, solid absorbs latent heat of vaporization while the latent heat of fusion is used to increase the potential energy of the molecules. Thus, we should use both latent heat of fusion and latent heat of vaporization.

3. Thomas Chung says:

Sublimation is the direct change from solid to gas without any intermediate phase change. When a solid sublimates into a gas, the kinetic energy of the water molecules remains unchanged and the potential energy of the molecules increases. At first, the molecules of the object(in solid state) is vibrating each other at a fixed position which forms a lattice structure.After sublimation, the molecules of the object(in gaseous state) become widely separated.Energy is needed to separate the molecules against the intermolecular force. To calculate the energy needed, we should use latent heat of fusion and latent heat of vaporization.

4. Cappi Lau says:

We should use both latent heat of fusion and latent heat of vaporization. Sublimation is actually a change from solid to liquid, then from liquid to gas almost immediately. This is because when a solid sublimates, the molecules gain potential energy to break down the lattice structure between them, but then, the molecules kept gaining potential energy to separate themselves against the intermolecular force between them, which means both melting and boiling takes place in sublimation. Therefore, we should use both latent heat of fusion and latent heat of vaporization for calculating the energy needed.

5. CM Leung says:

I think we should use both to calculate the energy needed. Since the molecules change from vibration to free to move, the latent heat of vaporization and fusion still assist

6. Chun Ho Lam says:

1)During vapourization and melting , the potential energy of molecules is increased
2)Both latent heat of fusion and vaporization should be used, it is because when an object goes through sublimation, the object goes through both melt and vaporize and therefore latent heat of fusion should be applied because the object absorb energy to melt while the latent heat of vaporization should also be applied because the object also absorb energy to vaporize.

I am Lam Chun Ho

7. Lisa Ma says:

We should use both to calculate the energy needed when a solid sublimates into a gas at a constant, because either latent heat of fusion or latent heat of vaporization only is not enough to sublimate water to gas. Which means it takes the same amount of energy that it would to first melt and then vaporize. Thus, we should add them both in order to calculate the energy needed.

8. suki hsk says:

We should use both because internal energy is the sum of p.e. and k.e.

9. Rakesh Mohanty says:

Potential energy