Why Does Salt Melt Ice? A Mixture Can Melt Below Both Its Parts

A pure solid melts at one temperature because its particles sit in a regular arrangement that requires a definite amount of energy to break up. Introducing a second substance disrupts that arrangement, so the solid holds together less well and melts at a lower temperature, which is the reason any impurity lowers a melting point. Adding more of the second substance lowers it further, up to a point. Approaching from the other side, adding the first substance to the second lowers that one's melting point too. The two descending curves must meet, and where they meet is the lowest temperature at which any mixture of the two can be liquid.

The meeting point behaves unlike any other mixture of the two:

• •It melts and freezes at a single sharp temperature, like a pure substance
• •Every other composition melts over a range instead
• •That temperature is below the melting point of either component
• •Freezing produces a fine intergrown mixture rather than separate crystals
• •The composition is fixed for any given pair of substances
• •Three or more components can lower the point further still

Spreading salt on ice exploits the effect directly and the numbers explain both why it works and why it sometimes does not. Pure water freezes at zero, and adding salt lowers the freezing point of the resulting solution progressively until, at about twenty three per cent salt, it reaches roughly minus twenty one degrees, which is the lowest that mixture can go. Below that temperature salt cannot keep water liquid whatever the quantity, which is why salting fails in severe cold and why other substances with lower points are used instead. The same effect is what makes an ice and salt mixture cold enough to freeze ice cream, since melting absorbs heat and the melting continues well below zero.

The information is recorded on a diagram that plots composition along the bottom and temperature up the side, showing which phases exist under each combination, and reading one is a standard skill in metallurgy and geology. Two lines descend from the pure melting points at each edge and meet at the lowest point, and above those lines everything is liquid while below a second horizontal line everything is solid. Between them sits a region containing both crystals and remaining liquid, which is why an ordinary mixture is mushy over a range of temperatures as it cools. The diagram is built experimentally by cooling many compositions slowly and recording the temperatures at which the cooling rate changes.

Choosing a composition for its melting behaviour is standard practice in several industries. Solder for electronics was traditionally a tin and lead mixture at almost exactly the composition that melts sharply at a low temperature, which matters because a joint that passes through a mushy range while cooling cracks if anything moves, and lead-free replacements struggle partly for that reason. Casting alloys use the effect to fill moulds at lower temperatures. Safety devices in sprinkler heads and in electrical fuses use low-melting alloys chosen to give way at a specific temperature. Glass and ceramic mixtures are formulated to fire at achievable temperatures. Freezing mixtures for laboratory cooling use the same principle.

Each component of a mixture disrupts the other's regular solid arrangement, so both melting points fall, and where the two descending curves meet is the lowest temperature at which the pair can be liquid. That composition melts sharply like a pure substance while every other melts over a range. Salt and water reach about minus twenty one degrees, which is why salting roads fails in severe cold.