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Because the temperature needed to make the drink freeze is just below minus 20°C, the temperature that a freezer usually has. How is this possible? Simple: thanks to the very low freezing point of ethyl alcohol: minus 117°C. Vodka contains between 40% and 55% alcohol and this amount is more than enough for the drink to withstand the cold of the freezer without changing its liquid state.
But alcohol doesn’t work alone. “Vodka contains substances, such as salts, which also reduce its freezing point”, says chemist Flávio Maron Vichi, from USP. What salts do is increase the degree of disorder in the binding of molecules in the Smirnoffs and Stolichnayas of life. This keeps the liquid further away from freezing, in which it would reach the most ordered state of all: the solid.
To freeze a substance messed up by salts requires more energy to be extracted from it. “Withdraw more energy”, in plain English, means having to lower the temperature even further. In other drinks that have a similar amount of alcohol and salts to vodka, such as whiskey or brandy, the process is the same. The difference is that we don’t put them in the freezer out of habit. The habit of doing this with vodka is due to the nature of the drink. The low temperature prevents us from feeling the flavor of the distillate. And the grace of vodka is precisely to have the most neutral taste possible.