When salt (NaCl) dissolves in water, its sodium (Na+) and chlorine (Cl-) ions leave the salt crystals and mix separately among the water molecules. These ions affect the water molecules and their freezing and boiling temperatures in different ways:
1. Lower freezing point
Water molecules form crystals when freezing. Na+ and Cl- ions from the salt get in the way of the water molecules, making it harder for them to become re-arranged into crystals. This means that salt water remains in a liquid state for longer as the temperature reduces.
2. Higher boiling point
The answer to the question why do bubbles form when water is boiled? explains how, as the temperature of water rises, its molecules move around faster, collide more often and release more water vapor gas molecules. When the temperature reaches boiling point - about 100°C (212°F) - the pressure from the release of these molecules (the vapor pressure) becomes greater than atmospheric pressure and water vapor starts to escape as bubbles.
In salty water, Na+ and Cl- ions occupy some of the space between the water molecules. As temperature increases, although the water molecules are moving faster, there are less of them, so there are fewer collisions, less release of water vapor molecules and lower vapor pressure compared to pure water at the same temperature. It takes more energy (temperature) for the vapor pressure of salt water to reach and exceed atmospheric pressure and start to boil.
To raise the boiling point of one liter (34 ounces) of water by 1°C (1.8°F) requires about 58 grams (2 ounces) of salt. This is much more that the amount of salt typically added to boiling vegetables, which is done primarily for taste.