When you add salt to water and heat it, the temperature at which the water boils increases. This phenomenon is known as boiling point elevation.
The reason behind this lies in the concept of boiling point and the properties of solutions. Boiling is the process in which a liquid changes into vapor when its vapor pressure equals the atmospheric pressure. The boiling point of a liquid is the temperature at which its vapor pressure becomes equal to the atmospheric pressure.
When you dissolve salt (sodium chloride, NaCl) in water, the salt dissociates into positive sodium ions (Na+) and negative chloride ions (Cl-). These ions are then dispersed throughout the water, forming a solution.
The presence of these dissolved ions affects the vapor pressure of the water. In a pure water solution, water molecules at the surface escape as vapor, creating a pressure that opposes the atmospheric pressure. However, when salt is added, the dissolved ions interfere with the water molecules' ability to escape as vapor. The ions disrupt the hydrogen bonding between water molecules, making it harder for them to escape the liquid phase and form vapor.
As a result, the vapor pressure of the water is reduced, requiring higher temperatures to achieve the same vapor pressure as in pure water. This means that the boiling point of the saltwater solution is elevated, and the water needs to be heated to a higher temperature to reach the vapor pressure necessary for boiling.
In summary, the addition of salt to water increases the boiling point because the dissolved ions interfere with the water molecules' ability to form vapor, requiring higher temperatures to achieve the same vapor pressure as in pure water.