The boiling point of water increases when salt is dissolved in it. This phenomenon is known as boiling point elevation.
When a solute, such as salt (sodium chloride, NaCl), is added to a solvent, such as water (H2O), it disrupts the normal boiling process. In a pure solvent like water, boiling occurs when the vapor pressure of the liquid equals the atmospheric pressure. However, when a solute is dissolved in the solvent, the vapor pressure of the solution decreases, requiring a higher temperature to reach the same vapor pressure as the pure solvent.
The addition of salt particles to water disrupts the hydrogen bonding between water molecules. The ions from the salt (Na+ and Cl-) interact with the water molecules, reducing their ability to escape from the liquid phase and enter the vapor phase. As a result, a higher temperature is required to increase the vapor pressure of the solution and reach the boiling point.
The boiling point elevation is a colligative property, meaning it depends solely on the concentration of particles in the solution, not the nature of the solute itself. In the case of saltwater, the number of salt particles dissolved in the water determines the extent of boiling point elevation.
The magnitude of the boiling point elevation can be calculated using the following equation:
ΔTb = Kb * m * i
Where:
- ΔTb is the change in boiling point
- Kb is the molal boiling point elevation constant (specific to the solvent)
- m is the molality of the solution (moles of solute per kilogram of solvent)
- i is the van't Hoff factor, which represents the number of particles formed from each solute molecule (in the case of NaCl, i = 2 since it dissociates into Na+ and Cl- ions)
The boiling point elevation is typically a few degrees Celsius for a typical concentration of saltwater. For example, seawater, which has a salt concentration of around 3.5%, boils at a slightly higher temperature than pure water due to the presence of dissolved salts.
It's important to note that boiling point elevation is a colligative property, and it applies to other solutes as well, not just salt. The principle behind it remains the same – the presence of solute particles in the solvent disrupts the normal boiling process, requiring a higher temperature for the liquid to boil.