If you’ve ever cooked pasta, you’ve likely added salt to your pot of boiling water. But have you ever stopped to think about what exactly is happening when you do this? In this article, we’ll explore the science behind the common kitchen practice of adding salt to boiling water.
The basics: salt increases boiling point
First things first: when you add salt to boiling water, the boiling point of the water increases. This means that you’ll need to input more energy to get the water to boil, but once it does, your pasta or egg will cook faster because the water will be hotter.
Understanding boiling point
To understand why adding salt increases the boiling point of water, we need to explore the concept of vapor pressure. The boiling point of a liquid is the temperature at which the vapor pressure of the liquid is equal to the atmospheric pressure above the liquid. Essentially, when the vapor pressure reaches a certain point, the liquid turns into a gas (i.e. boils).
How salt affects vapor pressure
So why does adding salt increase the boiling point of water? Imagine a pot of water at any given temperature (let’s say, 100 degrees Celsius). At this temperature, there will be some water molecules in the gas phase above the pot (this is the origin of the vapor pressure), and some in the liquid phase in the pot. The proportion in the two phases is determined by the interplay of lowering potential energy (by decreasing elevation in gravity, by forming hydrogen bonds, by lining up the polar ends of the molecules, etc.) and increasing the entropy (there are more accessible states in the gas phase, most liquids are incompressible, etc.).
Now, let’s toss some salt into the pot while keeping the temperature fixed. The volume fraction of the water decreases, which means there are suddenly new accessible states for the water molecules in the liquid phase. This causes the vapor pressure to decrease. If we keep adding salt, the vapor pressure will continue to decrease. Eventually, if we keep going, there will be no vapor pressure left.
So why does this matter? Remember that the boiling point is the temperature at which the vapor pressure of the liquid is equal to the atmospheric pressure above it. By decreasing the vapor pressure of the water, we are essentially increasing the boiling temperature. This means we’ll need to input more energy (i.e. wait longer) to get the water to boil.
The impact of Raoult’s law on boiling point
Raoult’s law states that the vapor pressure of a solution is proportional to the vapor pressure of the pure solvent (this is essentially the definition of an ideal solution). Real solutions have a curved functional form between the two boundary conditions, with the deviations from linearity coming from interactions between the solute (salt) and the solvent (water).
But here’s the cool part: at most temperatures and for most solvents, it doesn’t matter what solute you use (as long as the solute itself doesn’t have a vapor pressure), the vapor pressure of the solvent is still decreased by adding solute. This indicates that the entropic contribution is the most important part, and the interactions don’t play a big role.
Summing up
For a given concentration of salt dissolved in water, there are more states accessible to the water molecules in the liquid phase than there are in pure water. So at every water temperature, as we pour in energy to make it boil, there will be a lower vapor pressure than there would have been without the salt, and thus we won’t get to the boiling point until the water has reached a higher temperature (until we’ve poured in more energy than we would have had to). Salt does disrupt the network of hydrogen bonds in the water molecules, but the effect isn’t very big at reasonable concentrations of salt, and it’s never big enough to counteract the entropic effect.
Conclusion
So adding salt to boiling water does increase the boiling point, but the effect is relatively small, and the entropic contribution is the most important factor. Ultimately, adding salt to your pasta water is more about flavor than anything else!
What will happen if we add salt to boiling water?
What happens when we add salt to boiling water?
If you’ve ever cooked pasta, you’ve likely added salt to your pot of boiling water. But have you ever stopped to think about what exactly is happening when you do this? In this article, we’ll explore the science behind the common kitchen practice of adding salt to boiling water.
The basics: salt increases boiling point
First things first: when you add salt to boiling water, the boiling point of the water increases. This means that you’ll need to input more energy to get the water to boil, but once it does, your pasta or egg will cook faster because the water will be hotter.
Understanding boiling point
To understand why adding salt increases the boiling point of water, we need to explore the concept of vapor pressure. The boiling point of a liquid is the temperature at which the vapor pressure of the liquid is equal to the atmospheric pressure above the liquid. Essentially, when the vapor pressure reaches a certain point, the liquid turns into a gas (i.e. boils).
How salt affects vapor pressure
So why does adding salt increase the boiling point of water? Imagine a pot of water at any given temperature (let’s say, 100 degrees Celsius). At this temperature, there will be some water molecules in the gas phase above the pot (this is the origin of the vapor pressure), and some in the liquid phase in the pot. The proportion in the two phases is determined by the interplay of lowering potential energy (by decreasing elevation in gravity, by forming hydrogen bonds, by lining up the polar ends of the molecules, etc.) and increasing the entropy (there are more accessible states in the gas phase, most liquids are incompressible, etc.).
Now, let’s toss some salt into the pot while keeping the temperature fixed. The volume fraction of the water decreases, which means there are suddenly new accessible states for the water molecules in the liquid phase. This causes the vapor pressure to decrease. If we keep adding salt, the vapor pressure will continue to decrease. Eventually, if we keep going, there will be no vapor pressure left.
So why does this matter? Remember that the boiling point is the temperature at which the vapor pressure of the liquid is equal to the atmospheric pressure above it. By decreasing the vapor pressure of the water, we are essentially increasing the boiling temperature. This means we’ll need to input more energy (i.e. wait longer) to get the water to boil.
The impact of Raoult’s law on boiling point
Raoult’s law states that the vapor pressure of a solution is proportional to the vapor pressure of the pure solvent (this is essentially the definition of an ideal solution). Real solutions have a curved functional form between the two boundary conditions, with the deviations from linearity coming from interactions between the solute (salt) and the solvent (water).
But here’s the cool part: at most temperatures and for most solvents, it doesn’t matter what solute you use (as long as the solute itself doesn’t have a vapor pressure), the vapor pressure of the solvent is still decreased by adding solute. This indicates that the entropic contribution is the most important part, and the interactions don’t play a big role.
Summing up
For a given concentration of salt dissolved in water, there are more states accessible to the water molecules in the liquid phase than there are in pure water. So at every water temperature, as we pour in energy to make it boil, there will be a lower vapor pressure than there would have been without the salt, and thus we won’t get to the boiling point until the water has reached a higher temperature (until we’ve poured in more energy than we would have had to). Salt does disrupt the network of hydrogen bonds in the water molecules, but the effect isn’t very big at reasonable concentrations of salt, and it’s never big enough to counteract the entropic effect.
Conclusion
So adding salt to boiling water does increase the boiling point, but the effect is relatively small, and the entropic contribution is the most important factor. Ultimately, adding salt to your pasta water is more about flavor than anything else!