Salt and Sugar in Water.
Anything dissolved in water lowers its water activity: it boils a little higher and freezes a good deal lower.
Dissolve salt or sugar in water and fewer water molecules are free to leave as vapour or to lock into ice. So the solution must get hotter before its vapour pressure matches the air (it boils higher) and colder before it freezes. Chemists call these colligative properties: they depend on how many particles are dissolved, not on what they are, which is why salt, which splits into two ions, works harder per gram than sugar.
The effects are very unequal in the kitchen. The salt in pasta water raises its boiling point by a fraction of a degree — it is there for flavour, not for heat (XXI.6). But salt lowers the freezing point by many degrees, which is how an ice-and-salt bath freezes ice cream and why brines and cures stay liquid in a freezer.
Below: the boiling point and the freezing point of salt water against concentration, with your solution marked.
- Explain boiling-point elevation and freezing-point depression.
- Size the effect of salting pasta water.
- Explain ice-and-salt freezing.
- Compare salt and sugar per gram.
A fraction of a degree up, many degrees down
Pasta water salted at 1 % — about a tablespoon in a litre and a half — boils only 0.16 °C hotter than plain water. The salt is for seasoning the pasta, nothing else; adding it before or after the boil makes no difference to the temperature. By contrast, 10 % salt water stays liquid down to −6.8 °C, and 20 % to −17.0 °C. That is why a bucket of ice with a few handfuls of salt is cold enough to freeze an ice-cream mixture: the salt melts some ice, and melting takes heat from the mixture.
Sugar works the same way but more weakly per gram, because a sucrose molecule is large and does not split: a 20 % sugar solution freezes at only −1.5 °C. In ice cream the sugar is what keeps part of the water unfrozen at freezer temperatures, so the ice cream can be scooped (X.10). In candy making the opposite effect is used: as a syrup boils, water leaves, the sugar concentration climbs, and so does the boiling point — a thermometer in a boiling syrup is a measure of its concentration (VII.6).
Make a quick ice cream bath: a bowl of ice with three or four tablespoons of salt and a little water. Put a thermometer in it and watch it fall well below zero, then sit a small metal bowl of sweetened cream in it and stir. It will begin to freeze at the edges within ten minutes.
Terms
- Colligative property
- A property of a solution that depends on how many particles are dissolved: vapour pressure, boiling point, freezing point, osmotic pressure.
- Boiling-point elevation
- The rise in boiling point of a solution compared with pure water.
- Freezing-point depression
- The fall in freezing point of a solution compared with pure water.
- Water activity
- How free the water in a solution or food is, from 0 to 1; dissolved substances lower it (III.9).
Sources
- IAPWS, Revised Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam (IAPWS-IF97), region 4 (saturation).
- R. S. Norrish, "An equation for the activity coefficients and equilibrium relative humidities of water in confectionery syrups", J. Food Technol. 1, 1966 (K = −6.47 for sucrose).