Freezing Point: Definition & Equation

Instructor: Catherine Hagandora

Catherine has a doctorate in bioengineering.

This lesson describes what is required on the molecular level for a substance to freeze. The equation for the amount of energy needed for this change to occur is presented along with an example.

Freezing point

The freezing point is defined as the temperature when a liquid turns solid. The freezing point of water is 0 degrees Celsius. So why does water turn to solid ice when it is put in a cold freezer? It all comes down to energy.

To examine why this occurs, we need to take a close look at the role of energy on the atomic scale.

Energy and Heat

On the molecular scale, internal energy is used to describe the energy associated with the random motion of molecules and the bonds between them.

Heat can be defined as the transfer of internal energy from one entity to another. When you take a hot drink out on a cold day, the drink loses heat to the surrounding air, resulting in a decrease in temperature and cold coffee (yuck).

Phase Change

Depending on the level of energy of the atoms, matter can take on different phases, or physical states. Three phases of matter include solid, liquid, and gas. For example, let's consider water. The atoms in water stay close together but have enough energy to slide around one another giving water its characteristic 'flow' property. If we take that energy away by decreasing the temperature the atoms become closely packed and rigid, explaining why water placed in the freezer becomes hard ice.

The change in the physical state of matter (i.e. from liquid to solid) is referred to as a phase change. When a substance changes phases, there is a change in the internal energy but no change in temperature. The energy required to change the phase of a pure substance (Q) can be described using the following equation:

Q = +/- mL

Where m is the mass of the substance and L is the latent heat of the specific substance. The units of L are Joules per kilogram (J/kg).

Freezing Water Example

The latent heat of fusion (turning solid) of water is 3.33 x 105 J/kg. So, how much energy is needed to freeze 2.0 kg of water?

Q = +/- mL

Since we are taking energy out of the water to freeze it, we use the negative sign:

Q = -m/L

Q = -(2.0 kg)(3.33 x 105 J/kg)

Q = -6.66 x 105 Joules

So, from this example we determined that 6.66 x 105 J of energy must be removed to freeze 2 kg of water. As you can see, the greater the mass, the more energy is required for a phase change to occur.

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