Understand Freezing point/Melting point Part II : Freezing point/Melting Point Depression and Effect of Impurity

Melting point or freezing point depression  is basically the reduction of melting point or freezing point due to impurity.

The presence of impurity has two effects on melting and freezing point

1. lowers melting/freezing point ( If it is in range, it lowers end of range. For example, if the melting point is 188-189C, the mixture with impurity will be lower than 188C.

2. Melting point/freezing point will be in a range now instead of a sharp peak. For example, if the melting point were 188C, now it could be in a range of temperatures.

Why does this happen?

Adding impurity to the solution basically messes up the lattice (the structure of the molecule). This causes the structure of the molecule to break easily, meaning you need LESS temperature to melt it now. If it requires 180C to melt the solid, now it requires 100C. Pretty cool isnt it?

The opposite is true for freezing point

To form a structure of molecule from liquid, the correct lattice or structure is needed. Impurity basically impedes the formation of the correct structure due to difference in size, shape or charge. To allow the formation of solid to happen, you need lower entropy = low temperature. Therefore, when you add salt to water, it will remain as liquid at temperature at or below 0C, which is the freezing point of water.

Quoted from my previous post at this link

"Many people use salt to lower the freezing point of the ice. What does it mean? Imagine the temperature for today is 0 C. This is also the temperature when ice forms. Adding salt to the ice will lower it to -6 C. This means liquid will be frozen into solid at now -6 C (used to be 0 C). Since today's temperature is 0 C, the ice on the road after addition of salt is going to melt because we haven't reached -6 C (the new freezing point) yet. Until then, everything will be stayed at liquid."

This still remain true.

Understand Freezing point/Melting point

I decide to create this blog to document things that could have been simplified even more for understanding. I have stumbled upon many things that could be explained in a different way. This blog will be mostly about science.

My first topic is about freezing/melting point. What does it even mean when people say adding salt lowers the freezing point? Do you actually know what that means?

Freezing point is defined as the TEMPERATURE that liquid begins to freeze into solid.

Here is the equation from Wikipedia:

ΔT_F = K_F · b · i

• ΔT_F, the freezing point depression, is defined as T_{F (pure solvent)} - T_{F (solution)}.
• K_F, the cryoscopic constant, which is dependent on the properties of the solvent, not the solute. Note: When conducting experiments, a higher K_F value makes it easier to observe larger drops in the freezing point. For water, K_F = 1.853 C°·kg/mol.^[5]
• b is the molality (mol solute per kg of solvent)
• i is the van 't Hoff factor (number of ion particles per individual molecule of solute, e.g. i = 2 for NaCl, 3 for BaCl₂)

Many people use salt to lower the freezing point of the ice. What does it mean? Imagine the temperature for today is 0 C. This is also the temperature when ice forms. Adding salt to the ice will lower it to -6 C. This means liquid will be frozen into solid at now -6 C (used to be 0 C). Since today's temperature is 0 C, the ice on the road after addition of salt is going to melt because we haven't reached -6 C (the new freezing point) yet. Until then, everything will be stayed at liquid.

We also can understand it in terms of melting point. By the way, freezing point is pretty much the same as melting point (by definition). Melting point is just the temperature when solid melts into liquid. If you lower the melting point, that could be interpreted as it takes less heat to melt the ice. For example, say it today's temperature is 15 C. It takes 20 C to melt the ice, but now after adding the salt, the new melting point is 15 C. You are now melting the ice, changing it into the liquid form.

This is mainly for personal use. I do not hold any responsibility for any inaccurate information.