Concentrated acetic acid freezes around seventeen degree Celsius. Water freezes at zero degree Celsius. But Vinegar has a very low freezing point , that is around minus two degree celcius.

Why is it so? How does mixing of substances change their melting/freezing points?

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    $\begingroup$ It is pretty common for a mixture to have freezing point much lower than both pure components. Read about eutectic systems. $\endgroup$ – Ivan Neretin Jan 17 '17 at 7:23

Think of ice as a highly ordered crystalline solid in which the water molecules are able to bind strongly with each other. How strongly the molecules bind to each other determines how much energy, in the form of heat, will be required to break them apart.

Whether its ice or an altogether different type of solid, what happens when you disrupt these inter-molecular forces (i.e. hydrogen bonding, van der Walls forces, etc.) by introducing a second chemical substance? It requires less energy, less heat, to break apart the solid. Thus, it’s melting point is lowered. This is commonly known as "freezing point depression".

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    $\begingroup$ A small point but depression of freezing point and elevation of boiling point only strictly apply to non-volatile solutes. $\endgroup$ – porphyrin Jan 17 '17 at 13:13
  • $\begingroup$ @porphyrin: Elevation of boiling point only applies to nonvolatile (or less-volatile) solutes, but depression of freezing point applies no matter what the volatility of the solute. $\endgroup$ – Vikki Sep 19 '18 at 21:57
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    $\begingroup$ @Sean, yes you are correct and I was wrong about the depression of the freezing point being restricted to involatile solutes. The depression is usually derived assuming an involatile solute via lowering of free energy but the final equations depend only on the solute's mole fraction not on its nature. $\endgroup$ – porphyrin Sep 20 '18 at 8:47

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