A solute raises the boiling point of water. A solution with any solvent and non-volatile solute has a higher boiling point and a lower freezing point than the pure solvent. The amount by which the boiling point increases depends on the concentration of particles but not on the identity of the solute. This is called a colligative property. Water has a boiling point elevation constant of 0.512 degrees C/molal, where molality is the concentration unit of moles solute particles per kilogram of solvent. When a non-volatile solute is added to a solvent the vapor pressure of the solution decreases because, due to the presence of the non-volatile solute particles, there are fewer solvent particles at the surface of the liquid and therefore fewer entering the vapor phase above the surface. With a lower vapor pressure the solution has to be heated to a higher temperature to boil since boiling occurs when vapor pressure reaches atmospheric pressure.