As previous post has mentioned, adding impurity to solid would decrease the melting point due disrupted arrangement of the particles. For example, when you add impurity (say, MgF2) to NaCl, you are disrupting the ionic bonds between Na+ and Cl- ions. Putting F- next to Cl- will cause repulsion and same result will happen to Mg2+ and Na+. Because NaCl is a solid, they cannot rearrange themselves to avoid the repulsion. Due to this, the ionic bonds of NaCl will get weaken and thus, it requires less energy to melt them.
how about boiling point?
Impurity actually increases the boiling point of the liquid.
Let's use the same example from above. Imagine NaCl is in a solution now; adding impurity to it would actually stabilize the bonds! This is because in liquid, ions are free to rearrange themselves in order to achieve a maximum stability. Now, Mg2+ and Na+ are no longer repelled against each other. Rather, Mg2+ will rearrange itself to find his partners: Cl- and F-. Because of the additional stability from impurity, it requires more heat and energy to break the bonds! Boiling point increases as a result.
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