Salt doesn’t magically “cool” ice by itself – it lets the ice–water mix reach a lower temperature than plain ice water can, by changing when water freezes and melts.

The core idea: freezing point depression

In a plain ice–water mix, the system sits around 0 °C because ice is melting and freezing at the same rate. When you sprinkle salt on that ice, the salt dissolves in the thin layer of liquid water that always coats the ice surface.

  • Salt water has a lower freezing point than pure water (it might freeze at, say, −10 °C instead of 0 °C).
  • That means at 0 °C the salty water is “too warm” to freeze, so more ice must melt to reach its new, colder equilibrium temperature.
  • Melting ice is an endothermic process: it absorbs heat from its surroundings, which makes the ice–salt–water mixture get colder.

So the temperature of the slushy salt–ice bath can drop well below 0 °C, down to about −21 °C for typical table salt at the right concentration.

What’s happening at the molecular level

Water freezes when its molecules line up into a regular crystal lattice. Salt (sodium chloride) breaks into sodium and chloride ions in water, and those ions get in the way of water molecules lining up neatly.

  • Because of these “obstacles,” water must be at a lower temperature (lower energy) before it can form solid ice.
  • This colligative effect (freezing point depression) depends mainly on how many dissolved particles there are, not which specific salt you use.

That’s why some salts that break into more ions (like magnesium chloride) can push the freezing point even lower than table salt at the same concentration.

Why it feels “colder” in real life

When you pack ice and salt around something (like ice cream or drinks), two things combine to give strong cooling:

  1. The salt–ice mixture can reach temperatures well below 0 °C, unlike plain ice–water, which stays near 0 °C.
  1. The resulting salty slush is liquid, so it wraps around surfaces and transfers heat more efficiently than solid ice alone.

Example: in an ice-cream maker, the salted ice bath can sit around −10 °C to −15 °C, so it pulls heat out of the ice cream base much faster than regular ice at 0 °C could.

Why salt can melt ice on roads

On sidewalks and roads, there’s usually a thin layer of liquid water on top of the ice. When salt dissolves into that film:

  • The freezing point drops, so the existing ice starts melting into this colder brine instead of refreezing.
  • As long as the air and pavement temperatures aren’t too far below the new freezing point, you end up with liquid slush instead of solid ice.

However, if it’s extremely cold (well below the salt solution’s freezing point, roughly −7 °C for practical road use and down to about −21 °C in theory with table salt), salt stops being very effective.

“Quick Scoop” summary (ELI5-style)

  • Salt doesn’t give ice “extra coldness” on its own.
  • It makes it harder for water to freeze, so some ice must melt even when it’s already at 0 °C.
  • Melting ice absorbs heat from everything nearby, driving the temperature of the salty slush lower than 0 °C.
  • That super-chilly slush is what makes your ice cream set faster and your soda cans feel extra cold.

In short: salt forces ice to melt in a way that steals more heat from its surroundings, letting the ice–salt mix drop below 0 °C and making it seem like the salt “made the ice colder.”

TL;DR: Salt lowers water’s freezing point, so ice melts while the mix keeps getting colder, and the energy needed to melt that ice is taken from the surroundings—so the salty ice slurry can end up much colder than plain ice water.

Information gathered from public forums or data available on the internet and portrayed here.