Question

Some cooks add salt to a kettle of water in which they are boiling peeled corn or unpeeled potatoes. In addition to improving the flavor, this practice is reputed to cook and soften the food better.

–Is there any scientific justification for this? Explain.

– What is the concentration of NaCl in grams of salt per kilogram of water needed to obtain a significant rise in the boiling point, say 5◦C?

–Would this concentration of NaCl render the food too salty to the taste?

Answer 1

The scientific justification behind this practice is the Elevation of boiling point which is one of the colligative properties of solutions. We know that vapour pressure of a pure liquid increases with rise in temperature. Again, a liquid boils at a temperature at which its vapour pressure equals atmospheric pressure.

For example, water boils at 100⁰C or 373.15 K, because at this temperature vapour pressure of water is 1.013 bar (1 atm)

We also know that vapour pressure of a liquid decreases in the presence of non-volatile solute. This means that boiling point of a solution is always higher than that of the pure solvent.

The salt thus added actually increases the boiling point of water to decrease the cooking time.

This elevation in the boiling point depends on the concentration of the solute in the solution and not in their nature. The mathematical expression for this concept is given by

where = elevation of the boiling point

= constant called boiling point elevation constant

= molality of the solution i.e moles of the solute dissolved per kg of the solvent.

for water is o.52 K Kg mol^-1

required is 5 ⁰C OR 278 K

Or   

1 mole of NaCl = 58.5g

This amounts to 31,275.27 g of NaCl to be added in order to increase boiling point of water by 5 degrees.

obviously this quantity of NaCl will render the food too salty to the taste.