boiling temperature of water with salt

[curlyfilm]

does adding salt to water lower or higher the boiling temperature, because when i add salt too nearly boiling water it comes too the boil straight away, i know the boiling point is affected by air pressure and the amount of salt added

[gen2]

Adding salt to water RAISES its boiling point. However, In order for water to convert to steam, it also needs a nucleating centre. The water usually rises slightly above 100 degrees C because of a lack of such centres. Adding the salt initially provides such nucleation and the boiling commences. Adding sand would have the same effect. (If you did chemistry at school you may have added inert 'anti bumping granules' to a boiling flask to make the boiling process occur more gently). Once dissolved, the salt will cause the boiling point to be raised slightly, but considering the volume of water usually involved and the mass of salt usually added, the rate of flow of heat into the pan is usually fast enough to cause the water to reach its new higher boiling point almost straight away.

[Gef]

Excellent answer from gen. If you want to know more then do a google search for "colligative propeties" or have a look at http://en.wikipedia.org/wiki/Colligative_prope rties

[shammydodger]

I would make the point that sand ought not to have the same effect (i.e. raise the boiling point) because sand is not soluble in water. It is the interference of the dissolved solute ions with the solvent molecules that causes the raised boiling point. Since sand is insoluble in water, this cannot be the case. Also, one of the important things to remember about colligative properties is that the laws of proportional elevation of boiling point and depression of freezing point only hold for dilute solutions. And you would have to have a fairly concentrated solution of salt water before you would notice the change in boiling point on a standard thermometer. So, for example, adding a couple of teaspoons of salt to your pan of spuds for Sunday lunch would not have an effect on the boiling point that you could detect with a kitchen thermometer.

[brachiopod]

Disagree, Shammy - a couple of teaspoons of salt does make a measurable (in my kitchen) difference to the BP of the water in the pan.

Secondly (or firstly), the 'boiling effect' when you chuck a handful of salt into a near-boiling pan is more to do with superheating of the water at the base of the pan (closest to the heat source) and the nucleation effect of the salt grains, rather than the boiling point elevation caused by dissolved salt. (The salt grains do not dissolve straight away, so the initial "boiling surge" could just as well be achieved by a hadful of sand - as the salt dissolves, the water quickly stops boiling - this is where 'boiling point elevation' kicks in.)

See also;

Salt & Boiling Water

[gen2]

shammydodger, you have misread my answer. Nowhere did I claim that sand would elevate the boiling point. Let me quote myself to make it clear:

In order for water to convert to steam, it also needs a nucleating centre. The water usually rises slightly above 100 degrees C because of a lack of such centres. Adding the salt initially provides such nucleation and the boiling commences. Adding sand would have the same effect.

The sand simply allows the superheated water to form bubbles of steam more easily.

[shammydodger]

Gen2

Having read your initial answer again, I accept that you were referring to the nucleation effect of the sand granules, rather than any elevation of boiling point.

Brachiopod

I would still have to disagree with you. I have performed this experiment many times in the lab when teaching A level candidates and, although I completely agree that the boiling point is elevated, what I disagree with is that the extent of the elevation (unless one adds piles of salt to the water) is undetectable on a kitchen thermometer, which reads in whole degrees only.

[brachiopod]

Hi shammy - sorry for delay etc etc, -been away.

But my "kitchen thermometer" is a mercury thermometer, calibrated to 0.2�C at 200mm immersion. (honest!)

ok, not your usual kitchen thermometer I admit, but is cooking not a science? ;o)

x x b. pod