why do vegetables take less time to cook in salty water?

Question

If anyone know the answer, i'll appreciate it. I think it's got something to do with hydrogen bonding, but then I might be wrong.....

why is it related to hydrogen bonding though if it is related?

Answer 1

really? i never knew.

Answer 2

The fact that water boils at a higher temperature when salt is added has absolutely nothing to do with hydrogen bonding. It is all related to vapour pressure.

At the surface of any liquid, liquid molecules escape into the vapour phase, and the concentration of vapour increases until a point is reached where the number of liquid molecules escaping into the vapour phase is equal to the number of vapour molecules returning to the liquid phase. At that point, the vapour is said to be saturated, and exerts a pressure known as the saturated vapour pressure. Increasing the temperature increases the number of molecules that escape into the vapour phase at the equilibrium point, and therefore the saturated vapour pressure increases. A liquid will boil when its saturated vapour pressure is equal to the atmospheric pressure, because at that point the saturated vapour has sufficient pressure to start to be generated as bubbles within the body of the liquid rather than only at the surface.

Now, when you have a mixture of 2 liquids, the vapour pressure of each component in the vapour depends on the saturated vapour pressure of the component, and on its mole fraction in the liquid state. Thus according to Raoult's law:

Vapour pressure(A) = Mole fraction(A) x SVP(A)

From Dalton's law, the total vapour pressure will be equal to the sum of the 2 vapour pressures:

VP(Total) = P(A) + P(B)

In such a case, the combination of liquids will boil when the total vapour pressure is equal to the atmospheric pressure, but because the total vapour pressure is the sum of two vapour pressures, each individual vapour pressure doesn't need to reach the atmospheric pressure, and the mixture boils below the boiling points of the individual liquids.

Now, when you place a salt in water, the salt has a negligible vapour pressure. So P(B) is 0, assuming B is the salt and A is water, and therefore Ptotal is always equal to PA

Thus in pure water:
P(H2Opure) = 1 x SVP(H2O)

In a salt solution
P(H2Osoln) = <1 x SVP(H2O)
[The mole fraction of water in a solution is less than 1 because there is also the salt present]

Thus at any temperature the vapour pressure of a salt solution (which is equal to the PH2O because the Psalt is 0) is always less than that of pure water. Thus, to increase the vapour pressure to be equal to the atmospheric pressure (and thus have boiling), you need to increase the temperature more in a salt solution than in a water solution i.e. the boiling point is higher.

Now, the cooking process involves denaturation and hydrolysis principally of the protein and carbohydrate content of the food. The higher the temperature, the faster this will occur. However, pure water at atmospheric pressure will always boil at 100 and you cannot increase the temperature of the water phase more than this. To have the liquid phase boiling at a higher temperature, you either add salt, or you cook in a pressure cooker (that way the "atmospheric" pressure is higher and the water boils at a higher temperature).

Answer 3

The specific gravity of the medium (water) changes when you add salt. It increases the specific gravity and therefore changes the conditions of the water. Specific gravity is measured by the weight of one drop of water, compared to the changes in that medium.

A chemical example would be to determine the specific gravity of a ruby. It is 3.1 which means it is 3.1 times the weight of a drop of water for the same mass of the ruby. If you put a ruby in a solution of salt water equal to the specific gravity of the ruby, it would be invisible. This is because it would be equal to the specific gravity of the liquid.

So vegetables take less time to cook in salty water because it is heavier than water without salt. It has nothing to do with hydrogen bonding.

Answer 4

When you boil a pot of pure water at normal atmospheric pressure, it boils at 100 Celsius (212 Fahrenheit), and the temperature never changes.

However, when you add salt (or any other dissolved impurity) to water, the boiling point increases. Since the water boils at a higher temperature, the water can be a temperature higher than 100 Celsius for the entire time, cooking the vegetables faster.

Answer 5

It's because of osmosis. Vegetables don't have much salt, and osmosis causes liquid to move from the less-salty liquid to the more-salty liquid. The vegetables lose liquid more quickly, and thus cook more quickly. The classic experiment is to put a piece of raw potato in salty water for a period of time; when you take it out, it will be pretty much limp, even though it hasn't been cooked at all.

Answer 6

yeah, you're right about the h-bonding.

when you have a bit of salt in the water, the water will boil at a higher temp than 100 celcius. since the water is hotter, it cooks faster. you have a solute in the water. relate these to freezing point depression/ bp elevation problems.

Answer 7

adding salt ot water increases the temperature at which water vapourises thereby increasing the chances of water to hydrolyse the veg-molecules even beyond 100degrees which it may require. when you add salt it forms+ve and -ve poles which inturn forms hydrationshells with water which needs lot more energy to break,thus increases the temp at which water boils.thehydratedshellscan easily disrupt the veg molecules by hydrolysis.

Answer 8

adding salt makes the water temperature reach boiling point faster