Adding Salt to Water to Cook Faster

There is something I would like to point out when cooking with salt. There is a myth out there that continues to live on and it’s getting worse. I don’t know whether to blame reality television cooking shows or maybe people don’t think through things any more.

The myth is this that if salt is added to water, the boiling point rises and you can cook your food much faster. It is true but it won’t cook that much faster. It has everything to do with how much salt you add to the saucepan of water on your stove. You don’t add enough salt to elevate the boiling point to the point where cooking would occur faster. Or at least I hope you don’t. It would be a very very salty meal.

The boiling point of a substance or an element is the temperature when the vapour pressure of the liquid equals the pressure surrounding the liquid. A liquid at atmospheric pressure will have a higher boiling point than a liquid in a lower pressure like in a vacuum. At higher altitudes, say at the top of Mount Everest, the boiling point will also be lower for that substance.

Boiling point elevation is the name to the observed raising of a boiling point of a liquid when a non-volatile solute is added. The solution that results has a higher boiling point than the pure solvent. This can happen when salt is added to water. The boiling point can be accurately measured with an Beckmann thermometer, usually when hundredths of a degree need measuring and when small changes need monitoring.

Now back to the situation at hand. Raising the boiling point of water in saucepan on a stove. There are some calculations ahead to work out how much non iodised table salt, (NaCl) needs to be added to 500mL of water to raise the boiling point by one degree celsius.

The mathematical equation used to determine boiling point elevation is:

ΔT = iK_bm

where:  ΔT = change in temperature in °C

i = van ‘t Hoff Factor

K_b = constant that relates molality of a solution to the boiling point elevation of the solvent

m = molality of solute (mol/kg)

The information that we do have is:

ΔT = 1°C, i = 2, K_b = 0.512 °C kg/mol

What we don’t know is the molality of the solution, m but we can work it out by rearranging the original equation to:

m = ΔT/iK_b

m(NaCl) = 1/(2 x 0.512)

m(NaCl) = 0.976 mol/kg

We can now begin to work out how much table salt is needed to raise the boiling point of 500mL of water 1°C. I am going to make the assumption that the starting temperature of the water being used is 25°C. The density of pure water, (I know that tap water isn’t pure water and has dissolved minerals and salts in it but I want to keep things simple), at 25°C is 0.997 g/mL.

The first thing I need to work out is how much 500mL of pure water at 25°C weighs.

m(H₂O) = 0.997 g/mL x 500 mL

m(H₂O) = 498.5 g

m(H₂O) = 0.4985 kg

Now I can work out the number of moles of table salt in 0.4985 kg of water that will raise the boiling temperature by 1°C.

n(NaCl) = m(NaCl) x m(H₂O)

n(NaCl) = 0.976 x 0.4985

n(NaCl) = 0.4865 mol

I can now convert the number of moles of NaCl to the mass in grams.

m(NaCl) = n(NaCl) x M(NaCl)

m(NaCl) = 0.4865 x 58.44

m(NaCl) = 28.43 g

I’ve calculated that 28.45g of table salt is needed to be added to 500mL of water at 25°C to raise the boiling point by 1°C. It doesn’t sound like a lot of salt. It’s roughly 30g of table salt in 500mL of water. It could even be palatable and add flavour to say vegetables or pasta being cooked.

Well that is until you compare the concentration of salt the saucepan of water to the concentration of salt in open sea water. The average concentration of salt in open water is 3.5% or 3.5 g of salt for every 100mL of sea water. What we have in the saucepan is 28.45g of salt in 500mL of water which converts to 5.70g of salt for every 100mL of water. That’s more salt than sea water. Sea water tastes really salty. The saucepan of water isn’t sounding so tasty now.

I do think that the simple experiment of adding salt to water to elevate boiling point needs to be taught to vanquish the myth that adding salt will significantly speed up cooking. It won’t. It will make some foods taste better but that’s it.

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