It sounds like this“A mix of ice and water is always at 0 degrees, whatever you add to it.”
Thinking ice water cannot go below 0 degrees
Why it feels right
Ice water at 0 degrees is one of the first fixed points we learn: the thermometer sits at 0 as long as there is ice in the glass. School defines the Celsius scale that way, so 0 starts to feel like a law of nature, the coldest a liquid water mix can ever be.
Why it’s wrong
0 degrees is the melting point of pure water. Dissolved stuff changes it. With salt in the water, ice melts, and water stays liquid, at temperatures well below 0, so an ice and salt mix can sit at -10 or even -20 degrees.
A worked example
The 0 degrees is true for pure water, and only for pure water. It is the temperature at which ice and pure liquid water are in balance: just as many molecules leave the ice as join it.
Dissolve something in the water, and the balance moves. Dissolved particles get in the way of water molecules trying to join the ice, but they do nothing to stop molecules leaving it. So ice melts faster than it refreezes, and the balance point drops. The amount it drops is proportional to the number of dissolved particles: for salt in water, the drop is about 2 x 1.86 degrees for every mole of salt per kilogram of water.
Ten grams of table salt in 100 g of water is about 0.17 mol in 0.1 kg, so 1.7 mol per kg, and the freezing point falls by about 2 x 1.86 x 1.7, about 6.4 degrees. Ice in that water melts until the mixture cools to about -6 degrees. Zero is the floor only for water with nothing in it.
How to catch yourself
- You expect any ice and water mix to read 0 degrees.
- You are surprised a thermometer can read -10 with liquid water in the glass.
- You think the freezing point of water is a fixed property, whatever is in it.
What to hold instead
Dissolved particles lower the freezing point of water in proportion to how many there are. With salt, ice and liquid can balance well below 0 degrees, so an ice and salt mix can read -10 or colder, with water that is still liquid.
Where you’ll meet it
- Reading why the sea freezes at about -2 degrees, not 0.
- Understanding antifreeze in a car's radiator.
- Wondering how an ice cream maker gets below freezing.
Questions people ask
- Why does salt lower the freezing point of water?
- Dissolved ions get in the way of water molecules joining the ice, but don't stop molecules leaving it. The melting side wins until the temperature falls far enough to balance them again, which is a lower freezing point.
- Why does sea water freeze below 0 degrees?
- Sea water holds about 3.5% dissolved salt, which lowers its freezing point to about -1.9 degrees. Pure water freezes at 0, salty water has to get a little colder first.